Note: Descriptions are shown in the official language in which they were submitted.
CA 02576328 2007-01-26
1
ENHANCED SEEDED PULSED FIBER LASER SOURCE
FIELD OF THE INVENTION
The present invention relates generally to the field of laser light sources
and more
particularly concerns an enhanced seeded pulsed fiber laser source with
unfolded
cavity design which provides efficient energy extraction and optical pulses
with
pulse shape flexibility.
BACKGROUND OF THE INVENTION
1o Pulsed laser light sources are used in a variety of fields such as material
processing, dentistry, range finding, remote sensing, LIDAR (Llght Detection
and
Ranging) or communication-related applications. Different applications require
pulsed lasers with different output power; however it is usually desirable to
produce a high peak power from a pulsed laser. In general, three techniques
are
used for this purpose: Q-switching, mode-locking, and gated cascade
amplification.
The Q-switching method consists of switching from a high-loss (low quality
i.e. low
Q) to a low-loss (high quality i.e. high Q) condition in a laser cavity. A Q-
switched
laser system typically includes a gain medium, pumped by laser diodes or other
2o external pumping source, and a mirror on each side thereof to generate the
laser
oscillation. The switching between a high-loss and low-loss condition is
generally
achieved with a high-speed switching device such as an acousto-optic
modulator.
While in the high-loss condition, the gain medium is pumped and feedback of
light
into the gain medium is prevented by the modulator. After some time, the gain
medium becomes fully inverted and presents its maximum gain. At this point in
time, the switching device is used to rapidly revert to a low-loss cavity
thereby
allowing feedback of light into the gain medium and enabling the build-up of a
powerful pulse in the laser through optical amplification by stimulated
emission.
CA 02576328 2007-01-26
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The resulting peak power is fairly large, but the spectrum is often composed
of
several longitudinal modes and the repetition rate is generally low due to the
limited repetition frequency of the switching device. Moreover, the pulsewidth
is
not directly adjustable and varies with the pumping rate, repetition rate and
cavity
optical length. Another drawback is a "jitter" of the output beam, that is,
substantial
variations of the delay between the moment when the pulse is triggered and the
launching of the laser output pulse.
Mode-locking is another technique by which short pulses of high peak power are
produced by synchronizing most of the longitudinal modes of the laser cavity
with
to an internal modulator. Typically, the driving frequency of the modulator
corresponds to the round-trip time of the cavity and must be precisely tuned.
Therefore, the repetition rate of a mode-locked laser and the pulsewidth are
fixed,
since they are determined by the physics of the cavity.
In order to have control over the repetition rate and the pulsewidth, a gated
cascade amplification scheme may be used. A low-power laser diode pulsed with
the desired repetition rate and pulsewidth acts as a seed for a series of
amplifiers
which increase the pulse power. The amplifiers are usually gated with
synchronously activated switches in order to limit the self-saturation of the
gain
medium in the amplifier chain due to its own noise from amplified spontaneous
2o emission. This configuration has the advantage of separating the pulse
generation
process from the amplification process, both the spectral and temporal quality
of
the laser output pulses then depending only on the laser diode source.
Directly
pulsing the laser diode current can however generate transient effects that
can
affect both the spectrum and the noise figure of the seed source. Furthermore,
longitudinal mode beating can be an important source of high frequency noise
which consequently gives rise to peak power fluctuations in the pulse
structure.
Depending on its amplitude and frequency spectrum, this noise can severely
limit
the ability to generate stable optical pulses having special shapes with fine
structures.
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LAROSE et al in U.S. patent no. 6,148,011 teaches a self-seeded laser source
including a waveguide, an optical pump source, a gain medium for producing
seed
radiation, as well as a modulator and an array of Bragg gratings for modifying
the
properties of the seed radiation (see FIG. 1A (PRIOR ART)). Once generated by
the gain medium, the seed radiation propagates in the waveguide where it is
first
pulsed by the modulator. The resulting pulses are then selectively reflected
by the
Bragg grating, which separates different spectral components of the reflected
beam. This reflected beam then travels back to the modulator, which is timed
to let
only the desired spectral components go through. In this manner, the laser is
self-
io seeded and allows spectrum and wavelength selection from pulse to pulse.
Optionally, a second gain medium may be provided between the modulator and
Bragg grating to provide further amplification of the signal.
A drawback of the self-seeded source of LAROSE et al. is that the obtained
pulse
shape includes a step or "pedestal" preceding the desired pulse associated
with
residual ASE when the second gain medium is used. This is illustrated in FIG.
1 B
(PRIOR ART). Another drawback of the self-seeded source of LAROSE et al. is
that the modulator extinction ratio must be high in order to prevent spurious
lasing
of the source due to the parasitic back reflections coming from the output
isolator
or from other components such as the pump couplers. This ultimately limits the
maximum achievable output power of the source and its stability, depending on
both the modulator extinction ratio and the back reflection level of the other
optical
components.
Unseeded or self-seeded pulsed fiber laser designs like the source of
LAROSE et al. use intrinsic fluorescence from amplifying fibers of the device
to
generate optical output pulses, i.e. pulsed laser output. This offers the
possibility to
use a minimal number of components for generating optical output pulses and to
thus keep the devices simple and low-cost.
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However, when laser diode (preferably single transverse mode) seed sources are
available with the required line-width, it is sometimes advantageous to use a
seeded geometry for generating a pulsed laser output. This is the case when
the
modulation device for generating the pulsed laser output has a low optical
power
damage threshold. Using a seeded geometry with low optical power damage
threshold components and an appropriate seed source ensures that a maximum
number of the photons which impinge onto the low damage threshold components
lie within a useable optical bandwidth.
There is therefore a need for a low-cost, stable, seeded pulsed fiber laser
which
io allows for easy control over the repetition rate and pulsewidth as well as
spectral
pulse-shape tuning.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a pulsed laser light
source that
optimises the energy extraction efficiency while providing amplified optical
output
pulses.
In accordance with one aspect of the present invention, there is therefore
provided
a pulsed laser light source for outputting amplified light pulses. The pulsed
laser
light source includes a first, a second and a third waveguide branch, an
optical
circulator having a first, a second and a third port respectively connected to
the
first, second and third waveguide branches, and a seed module for generating
light pulses and propagating the light pulses in the first waveguide branch
towards
the first port of the optical circulator for circulation to the second
waveguide branch
through the second port of the circulator. A reflector is provided in the
second
waveguide branch for reflecting the light pulses back towards the second port
of
the optical circulator for circulation to the third waveguide branch through
the third
port of the circulator. A second-branch amplifier is disposed in the second
waveguide branch between the optical circulator and the reflector for
amplifying
the light pulses circulating therethrough towards and from the reflector. A
third-
CA 02576328 2007-01-26
branch optical modulator is disposed in the third waveguide branch, the third-
branch optical modulator being operable to be opened and closed in
synchronization with the light pulses. A light output is provided in the third
waveguide branch downstream the third-branch amplifier for outputting the
5 amplified light pulses.
Preferably, the seed module comprises a seed light source generating a seed
light
beam of at least quasi-continuous radiation, and a seed light modulator
operable
to modulate the seed light beam to obtain the light pulses.
Preferably, a third branch amplifier is disposed downstream the third-branch
io optical modulator for further amplifying the light pulses.
Also preferably, the pulsed laser light source further includes a control
system for
controlling the operation of the third-branch optical modulator.
In one embodiment of the pulsed laser light source, the control system is
operable
to open the third-branch modulator before arrival of a leading edge of one of
the
light pulses coming from the circulator, and close the third-branch modulator
after
the leading edge and a portion of the light pulse corresponding to a desired
pulse
duration has gone therethrough.
In another embodiment of the pulsed laser light source, the control system is
operable to open the third-branch modulator after arrival of a leading edge of
one
of the light pulses coming from the circulator, and close the third-branch
modulator
after passage of a remainder of the corresponding light pulse therethrough.
In yet another embodiment of the pulsed laser light source, the control system
is
operable to open the third-branch modulator after arrival of a leading edge of
one
of the light pulses coming from the circulator, and close the third-branch
modulator
after a portion of the light pulse corresponding to a desired pulse duration
has
gone therethrough.
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In another embodiment, the control system is operable to open the third-branch
modulator before arrival of a leading edge of one of the light pulses coming
from
the circulator, and close the third-branch modulator after passage of the
corresponding light pulse therethrough.
The objects, advantages and other features of the present invention will
become
more apparent and be better understood upon reading of the following non-
restrictive description of the preferred embodiments of the invention, given
with
reference to the accompanying drawing. The accompanying drawing is given
lo purely for illustrative purposes and should not in any way be interpreted
as limiting
the scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A (PRIOR ART) is a schematic illustration of a self-seeded light source
according to the prior art of LAROSE et al.; FIG. 1 B(PRIOR ART) illustrates
the
temporal shape of a pulse generated by the source of FIG. 1A.
FIG. 2 is a schematic illustration of the pulsed laser light source according
to one
embodiment of the invention.
2o FIG. 3 is a schematic illustration of the pulsed laser light source
according to
another embodiment of the invention.
DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
In the following description, the term "light" is used to refer to all
electromagnetic
radiation, including but not limited to visible light. Furthermore, the term
"optical" is
used to qualify all electromagnetic radiation, that is to say light in the
visible
spectrum and light in other wavelength ranges.
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A pulsed laser light source (10) for producing amplified light pulses is shown
in
FIGs. 2 and 3 according to two preferred embodiments of the invention. As will
be
apparent from the description below for one skilled in the art, the pulsed
laser light
source of the present invention provides great versatility in shaping the
temporal
and spectral profile of the light beam while using readily available and
relatively
inexpensive components. The temporal profile of the light beam is defined as
its
intensity as a function of time and defines the width, repetition rate and
amplitude
shape of the light pulses. The spectral profile of the light beam is defined
as its
intensity as a function of wavelength.
to The pulsed laser light source (10) includes three waveguide branches,
namely a
first (12), a second (14), and a third (16) waveguide branch. Preferably, each
of
the waveguide branches (12, 14, and 16) is embodied by a length of optical
fiber.
The optical fiber may be a standard fiber or a polarisation maintaining (PM)
fiber,
preferably with a single mode core. It may be single-clad or double-clad (clad-
pumped).
In addition, the pulsed laser light source (10) includes a three-port optical
circulator
(18). The optical circulator (18) has a first (20), a second (22) and a third
(24) port
connected respectively to the first (12), second (14) and third (16) waveguide
branches. It is preferably made out of, or pigtailed with, optical fiber that
guides a
single transverse mode at the operating wavelength. For example, at
wavelengths
around 1 pm, integrated circulators pigtailed with PM980 or H11060 fibers are
readily available. While the optical circulator (18) induces low losses for
light at the
operating wavelength traveling from the first port (20) to the second port
(22) and
from the second port (22) to the third port (24), it induces high losses for
light
circulating from the second port (22) to the first port (20) and for light
circulating
from the third port (24) to the second port (22).
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First wavepuide branch
A seed module (28) for seeding the downstream components is provided. The
seed module generates a light beam composed of input light pulses having an
initial temporal shape. The accompanying drawings show two different
embodiments of such a seed module.
In the embodiment of FIG. 2, the seed module (28) includes a seed light source
(26) generating a light beam of continuous wave or quasi-continuous wave
radiation. The expression "quasi-continuous" is understood herein to designate
a
io light beam having optical pulses with a pulse width which is long when
compared
to the desired pulse width of the light pulses outputted by the pulsed laser
light
source (10). The seed light source (26) may be embodied by a laser, an optical
source of amplified spontaneously emitted radiation, or any continuous wave
(CW)
or quasi-continuous wave (quasi-CW) source of radiation be it coherent or
is incoherent. A preferred seed light source (26) is a single transverse mode
laser
diode with narrow output linewidth. The light beam generated by the seed light
source (26) has a spectral profile which preferably corresponds to a gain
spectrum
of the gain section of the pulsed laser light source (10) or at least includes
within
its wavelength range a wavelength overlapping a gain spectrum of this gain
20 section. (The gain section of the pulsed laser light source (10), that is
to say the
amplifier (40), is described in more detail hereinbelow.) The seed light
source (26)
may emit linearly polarized light, in which case the three waveguide branches
(12,
14, and 16) are preferably embodied by polarization-maintaining fiber. Still
in
respect of the embodiment of FIG. 2, the seed module also preferably includes
a
25 seed light modulator (38A) providing an initial spectral and temporal
modulating of
the continuous light beam generated by the seed light source (26) into pulses
with
an initial temporal profile. Preferably, the seed light modulator (38A) is an
optical
modulator which has high transmission losses when closed but low transmission
loses when open. It is preferably fiber pigtailed with single mode fiber to
the optical
30 fiber of the first waveguide branch (12). It is preferably embodied by an
electro-
CA 02576328 2007-01-26
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optic modulator but any other modulation scheme, such as one based on an
acousto-optic modulator, an electro-absorption modulator, etc., could also be
considered within the scope of the invention.
According to another preferred embodiment shown in FIG. 3, the seed module
may include a pulsed seed light source (126) generating the input light pulses
directly. The initial temporal profile of the input light pulses is preferably
controlled
through a pulse format generator (27) incorporated into or associated with the
driver of the pulsed seed light source (126).
The seed module (28) is optically connected to the first waveguide branch (12)
so
io that the generated light beam propagates therein towards the first port
(20) of the
optical circulator (18). As explained above, the optical circulator (18) is
such that
the light received at the first port (20) is circulated to the second
waveguide branch
(14) through the second port (22) of the circulator (18).
Second waveguide branch
is A reflector (36) is provided in the second waveguide branch (14) for
reflecting the
light beam back towards the second port (22) of the optical circulator (18)
for
circulation to the third waveguide branch (16) through the third port (24) of
the
circulator (18). As shown in FIGs. 2 and 3, the reflector (36) is preferably a
fiber
Bragg grating which has a reflection profile selected so that it reflects only
20 wavelengths corresponding to the desired spectral profile of the output
pulses. The
fiber Bragg grating may be single- or multi-wavelength and may be chirped,
sampled, or of any appropriate design. In the case where the pulsed light beam
generated by the seed module (28) already has a spectral profile corresponding
to
the desired output spectral profile, the reflector (36) could be wideband or
of a less
25 discriminatory reflection profile. Alternatively to a Bragg grating, the
reflector could
for example be embodied by a reflective coating deposited on a facet of the
fiber,
a bulk mirror butt-coupled to the end of the fiber, a fiber loop mirror, a
cascade of
CA 02576328 2007-01-26
fiber Bragg gratings or any other appropriate component or combination of
components.
A second-branch amplifier (40) is disposed in the second waveguide branch (14)
between the optical circulator (18) and the reflector (36). The input light
pulses will
5 therefore encounter the amplifier twice during their trip forward and back
in the
second branch (14). The second-branch amplifier (40) amplifies the input light
pulses a first time after they exit the second port (22) of the optical
circulator (18)
and a second time after they have been reflected by the reflector (36) and
travel
back towards the circulator (18). In the preferred embodiments of FIGs. 2 and
3,
io the second-branch amplifier (40) is a length of optical fiber, either
single clad or
double-clad, preferably with a single mode core doped with a rare earth
element,
such as Er, Yb, Nd, etc. In the former case, pump radiation is introduced
directly to
the gain medium of the fiber core. In the latter case, the pump radiation is
introduced first into the inner cladding surrounding the core and is then
absorbed
by the core - the core acts as the gain medium and the inner cladding acts to
carry the pump light that maintains the population inversion in the core. The
pumped radiation is produced using an appropriate pump source (41). The
pumping energy propagates backwards or forwards or both through the second-
branch amplifier (40) to maintain the required population inversion therein.
2o Alternatively, the second-branch amplifier (40) may be a fiber-pigtailed
semiconductor optical amplifier (SOA).
Following the second pass of the input light pulses through the second-branch
amplifier (40), the amplified input light pulses enter the second port (22) of
the
optical circulator (18), exit the third port (24) of the circulator (18) and
enter the
third waveguide branch (16).
Third waveguide branch
In the third waveguide branch (16), the input light pulses encounter an
optical
modulator (38B). The third-branch optical modulator (38B) is preferably fiber-
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pigtailed with the optical fiber embodying the third waveguide branch (16). It
may
be an electro-optic modulator but any other modulation scheme, such as one
based on an acousto-optic modulator, an electro-absorption modulator, etc., is
possible. In the case of the preferred embodiment of FIG.2, the optical
modulator
(38B) may or may not be of the same type as that of the seed light modulator
(38A).
The third-branch optical modulator (38B) is opened and closed in
synchronization
with the light pulses to either let through or adjust the temporal shape of
the light
pulses coming from the third port (24) of the circulator (18), as will be
further
lo explained below. It preferably has high transmission losses when closed and
low
losses when open. In addition, a control system (37) is preferably provided
for
controlling the operation of the optical modulator (38B). The control system
(37)
may be embodied by any device or combination of devices appropriate for this
purpose, as well known to those skilled in the art.
A light output (34) is provided in the third waveguide branch (16) for
emitting the
pulsed laser light. An isolator (not shown) may be provided at the light
output (34)
for preventing parasitic light to enter the device.
Further amplifiers may be provided for increasing the power of the pulsed
laser
light coming out of the third port (24) of the optical circulator (18). As
shown in the
preferred embodiments of FIGs. 2 and 3, a second amplifier (42) is preferably
disposed in the third waveguide branch (16). As with the first (second-branch)
amplifier (40), this second (third-branch) amplifier (42) is preferably single
mode
and consists preferably of a length of optical fiber doped with a rare earth
element,
such as Er, Yb, Nd, etc., which is pumped with an appropriate pump source
(43).
In operation, in the embodiment of FIG. 2, the seed light source (26) of the
seed
light module (28) emits a CW optical signal, i.e. a light beam. The light beam
travels to the entrance of the seed light optical modulator (38A), which is
optically
CA 02576328 2007-01-26
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connected to the seed light source (26), for appropriate pulse generation and
shaping. In the embodiment of FIG. 3, the seed light source (126) directly
produces a pulsed light beam.
The pulsed light beam exits the seed module (28), travels along the first
waveguide branch (12) into the first port (20) of the optical circulator (18)
and out
the second port (22) of the optical circulator (18) with low losses. Most of
the light
beam impinging onto the second port (22) is prevented from being transmitted
back through the first port (20) given that the second port (22) is isolated
from the
first port (20) through high insertion losses. In this way, the optical
circulator (18)
io prevents detrimental optical feedback into the seed module (28).
Following the transmission through the optical circulator (18), the modulated
pulsed light beam coming from the second port (22) goes through the second-
branch amplifier (40) a first time as it travels along the second waveguide
branch (14). The reflector (36) (embodied by a fiber Bragg grating in FIGs. 2
and 3) placed downstream the amplifier (40) reflects the light beam back along
the
second waveguide branch (14) and through the amplifier (40) a second time. The
peak reflectivity and the optical bandwidth of the reflector (36) are chosen
so as to
achieve high-reflectivity of the seed light source optical signal, i.e. the
light beam
generated by the seed light source. The pulsed light beam undergoes a back-and-
forth trip, i.e. a double-pass, through this first second-branch amplifier
(40), which
thereby increases the energy extraction efficiency of the design.
The amplified light beam leaves the second-branch amplifier (40), enters the
circulator (18) via the second port (22) and is circulated out the third port
(24) to
the second modulator (38B) disposed in the third waveguide branch (16).
One function of this third-branch optical modulator (38B) is to prevent
Amplified
Spontaneous Emission (ASE) from the first gain section, i.e. the second-branch
amplifier (40), from reaching subsequent gain sections, i.e. subsequent
amplifiers,
CA 02576328 2007-01-26
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when the generation of pulses is not in progress. By isolating the amplifying
sections, the energy stored in the third-branch amplifier and each of the
subsequent amplifiers is increased which promotes high pulse energies of the
output light pulses.
Another function of this second optical modulator (38B) is to further refine
the
shape of the pulses of the light beam in the case where the seed module (28)
is
used to generate input light pulses with a pulse shape that is only
approximately
defined: i.e. approximate pulse width, exact pulse repetition rate, and
approximate
pulse amplitude shape.
io In the case where further refining of the pulse shape is needed, the
amplified light
beam enters the second (third-branch) optical modulator (38B) for further
modulation. The generation of the refined output light pulses with the desired
temporal and spectral profile as well as amplitude is accomplished through the
synchronized use of the seed light modulator and the third-branch optical
ts modulator (38B), i.e. through the opening and closing of the third-branch
optical
modulator (38B) in synchronization with the light pulses. As explained
hereinbelow, the opening and closing of the third-branch optical modulator
(38B) is
synchronized with, that is to say coordinated with or maintained in step with,
the
light pulses, and not necessarily with the leading or trailing edge of the
pulses.
20 Preferably, the synchronization is carried out using the control system
(27).
The control system (27) may be used to adjust each input light pulse by
opening
the third-branch optical modulator (38B) before arrival of a leading edge of
one of
the light pulses coming from the circulator (18), and closing the third-branch
optical
modulator (38B) after the leading edge of the light pulse and a portion of the
light
25 pulse corresponding to a desired pulse duration of the desired pulse
profile has
gone therethrough. Alternatively, in another case, the control system (27) may
be
used to adjust each input light pulse by opening the third-branch optical
modulator (38B) after arrival of a leading edge of the light pulse coming from
the
CA 02576328 2007-01-26
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circulator (18), and closing the third-branch optical modulator (38B) after
passage
therethrough of the remainder of the light pulse. In yet another case, the
control
system (27) may be used to adjust each input light pulse by opening the third-
branch optical modulator (38B) after arrival of a leading edge of one of the
light
pulses coming from the circulator (18), and closing the third-branch optical
modulator (38B) after a portion of the light pulse corresponding to a desired
pulse
duration of the desired pulse profile has gone therethrough.
In addition to using the control system (27) to adjust the temporal profile of
the
light pulses, as described in the cases above, the control system (27) may
also be
1o used to adjust the amplitude of the modulation of the third-branch optical
modulator (38B) for further adjustments of the pulse shape.
The control system may also be used to adjust the spectral profile of the
light
pulses. For example, in the case where the reflector (36) consists of a
cascade of
fiber Bragg gratings, a delay may be induced between different spectral
components of the light pulse corresponding to the difference in the time it
takes
for the different spectral components of the light pulse to reach the second
(third-
branch) optical modulator (38B) after being reflected from their respective
fiber
Bragg gratings. By synchronizing the opening of the second optical modulator
(38B) with the time it takes for a particular spectral (wavelength) component
of the
light pulse to reach the second optical modulator (38B), it is possible to
select a
particular spectral (wavelength) component and thereby adjust the spectral
profile
of the light pulse. This concept of wavelength selection is described by
LAROSE et al in U.S. patent no. 6,148,011. As such, the second (third-branch)
optical modulator (38B) may be opened and closed several times in order to
obtain
the desired spectral profile of the light pulses.
Fine pulse-shape control can thus be accomplished using the second optical
modulator, i.e. the third-branch optical modulator (38B), through timing
and/or
modulation amplitude adjustments.
CA 02576328 2007-01-26
In the case where no refining of the pulsed light beam is necessary, the
second
(third-branch) optical modulator (38B) is opened for a time which allows the
pulses
of predefined shape generated by the initial seed light modulator to be
transmitted
with low losses through the optical modulator (38B). The optical modulator
(38B) is
5 then closed after the passage of the pulse.
It should be noted that in the preferred embodiment of FIG. 3, the generation
of
the seed light beam is accomplished by the seed light source (126) and the
pulse
format generator (27) associated with the driver of the seed light source
(126). For
high efficiency, the seed light source (126) cannot be operated in continuous
wave
lo (CW) mode. As such, the fine adjustments regarding the pulse shape of the
pulsed
light beam are preferably carried out by the seed light source modulator, that
is to
say, by the pulse format generator (27). Of course, minor refinement of the
pulsed
light beam may be carried out by the second (third-branch) optical modulator
(38B)
as described above.
15 After exiting the optical modulator (38B), the generated optical pulse is
preferably
further amplified by additional fiber amplifiers, for example by the third-
branch
amplifier (42) located in the third waveguide branch (16) according to the
preferred
embodiment of FIGs. 2 and 3.
Finally, the pulsed light beam with the desired pulse shaping exits the pulsed
laser
light source (10) through a light output (34) provided in the third waveguide
branch (16).
Advantageously, the proposed geometry of the pulsed laser light source (10)
shown in FIG. 2 allows using a continuous wave (CW) or quasi-continuous wave
(quasi-CW) seed light source (26) to generate arbitrary temporal pulse shapes
out
of the CW or quasi-CW seed light beam through the use of two optical amplitude
modulators (38A and 38B). Moreover, the position of the optical amplitude
modulators (38A and 38B) in conjunction with the use of the three-port optical
CA 02576328 2007-01-26
16
circulator (18) allows the first modulator (38A) to be used to produce the
required
pulse shape and to isolate the first gain section (i.e. the first amplifier
section) from
the seed light beam when pulses are not required and the second optical
modulator (38B) to be used to further shape the light pulses and to promote
higher
energy extraction efficiency in the subsequent amplifiers (e.g. third-branch
amplifier (42)) by isolating them from the ASE generated in the second-branch
amplifier (40).
Given the non-negligible optical losses in the light beam as it travels from
the
second port (22) to the third port (24) of the circulator (18), the geometry
of the
1o pulsed laser light source (10) as illustrated in FIGs. 2 and 3 allows for
the
amplification of the optical pulses generated using a double-pass
configuration into
at least one gain (amplifier) section thereby advantageously increasing the
energy
extraction efficiency of the design. Moreover, the disposition of the optical
modulator (38B) in the third waveguide branch (16) in this geometry offers the
possibility to optimize the energy in the pulsed light beam before the second
gain
section (i.e. the section in the third waveguide branch (16) in which the
second
amplifier (42) is disposed). For a fixed optical damage threshold of the
optical
modulator (38B), the geometry of FIGs. 2 and 3 allows amplifying the light
beam
exiting the second port (22) to a level practically equal to the damage
threshold of
the optical modulator (38B) disposed in the third waveguide branch (24) plus
the
amount of the losses incurred by the light beam as it passes from the second
port (22) to the third port (24) of the circulator (18) on its way to the
optical
modulator (38B). More importantly, the position of the second modulator (i.e.,
that
of optical modulator (38B) disposed in the third-waveguide branch (24)) is
such
that the second modulator is the very last component before the second gain
section, thus allowing for the injection of a maximum pulse energy - a pulse
energy that is practically equal to the modulator damage threshold minus the
modulator insertion losses - into the second gain medium (i.e., the third-
branch
amplifier (42)) and thereby providing enhanced energy extraction efficiency in
the
third-branch amplifier (42).
CA 02576328 2007-01-26
17
Numerous modifications could be made to any of the embodiments described
above without departing from the scope of the present invention as defined in
the
appended claims.
