Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD FOR STABILIZING AN OUTPUT OF A PULSED LASER SYSTEM
USING PULSE SHAPING
FIELD OF THE INVENTION
The present invention relates to the field of pulsed lasers and more
particularly
concerns methods for pulse stabilization in tailored pulsed laser systems
employing a
directly modulated laser diode.
BACKGROUND
Seeding a pulsed fiber laser system with a directly modulated laser diode is a
simple
and cost-effective manner of generating high energy, high peak power optical
pulses
with fast rise times and fall times. In material processing applications where
a single
pulse is used to process a given structure, such as severing links for memory
repair, it
is important to keep the energy of the light pulses within a given range. If
the pulse
energy is too low, then the link may be incompletely removed. In cases where
the
energy per pulse exceeds the allowable energy process window, excess pulse
energy
may be coupled into adjacent or underlying link structures, or the substrate
itself,
thereby causing highly undesirable damage to the device. In multiple pulse
laser
processes, such as laser drilling of microvias in semiconductors, or laser
scribing of
thin film photovoltaic devices, it is important that successive pulses remain
substantially uniform in order to produce laser processed features that
possess the
desired dimensions and surface quality. High value is placed upon the
throughput of
work pieces satisfactorily produced by a laser processing system. The control
of the
pulse amplitude stability is also important in laser surgery, as the amount of
energy
deposited in living tissues must be accurately controlled in order to avoid
causing
damage to the neighbouring tissues. Therefore, methods to achieve pulse
stabilization in lasers, particularly in lasers employed in laser processing
or medical
laser systems, are highly desirable.
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Optical pulse shaping is of great interest in material processing applications
as it
offers the ability to control how the energy is delivered to the target over
time.
Industrially important laser applications, such as laser repair of dynamic
random
access memory (DRAM), laser scribing of photovoltaic cells, and laser drilling
of
microvias in semiconductor, flexible interconnects, IC packages, dielectrics,
including
glass, and metals, can benefit from a pulsed laser output characterized by
tailored
pulse shapes. For example, U.S. Pat No. 7,348,516 (SUN et al.), entitled
"Methods of
and laser systems for link processing using laser pulses with specially
tailored power
profiles" presents many arguments in favour of pulsed laser systems providing
fine
to control over the pulse temporal power profile in the nanosecond regime,
for facilitating
better link process quality and yield. Those skilled in the art will recognize
that laser
sources capable of generating sharp optical pulses with stable, tailored
amplitude
profiles characterized by fast rise times and fall times at the nanosecond
time scale
are highly desirable for such purposes. Elaborate optical pulse shapes require
a
highly responsive optical shaping mechanism. Pulsed lasers based on a directly
modulated laser diode seeding a chain of optical amplifiers in a Master
Oscillator,
Power Amplifier (MOPA) configuration are commonly employed in these and
similar
laser processing applications in which laser processing pulses with
controllable pulse
amplitude profiles are highly beneficial (See for example US Patent No.
6,281,471
(SMART), entitled "Energy-efficient, laser-based method and system for
processing
target material". Embodiments of pulsed laser oscillator platforms employing
MOPA
configurations with a directly modulated seed laser diode and providing fine
control
over the pulse parameters are presented in the international patent
application
published under WO 2009/155712 (DELADURANTAYE et al), entitled "Digital laser
pulse shaping module and system" and in international patent application
No. PCT/CA2009/00365 (DELADURANTAYE et al), filed on 20 March 2009, entitled
"Spectrally tailored pulsed fiber laser oscillator".
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For pulsed laser systems employing a directly modulated seed laser diode, a
commonly encountered difficulty is obtaining a satisfactory optical pulse
amplitude
stability on the leading edge of the pulse, as switching transients often take
place in
the diode during the transitory regime corresponding to the leading edge of
the
current pulse. The pulse amplitude stability usually worsens as the pulse rise
time is
shortened and as the pulse amplitude is increased. Similar transients can
occur at the
falling edge of the pulse, and generally, transients can be observed for each
low to
high or high to low transition present in a given pulse shape. Those
transients may
find their origin in the optical gain switching dynamics taking place in the
laser diode
to cavity as longitudinal mode competition occurs when the drive current is
suddenly
increased from zero to a value that is above the laser emission current
threshold. See
for example "Mode switching of Fabry-Perot laser diodes", by P.J. Herre and U.
Barabas, in IEEE Journal of Quantum Electronics, Vol. 25, No. 8, August 1989,
pp.
1794-1799. FIG. 1 (PRIOR ART) shows an example of the temporal shape of an
is outputted optical pulse where such transient behavior is manifested by a
spike
present at the pulse leading edge. In general, the spike amplitude varies from
pulse to
pulse, leading to poor peak power stability at the pulse leading edge.
Parasitic
capacitances and inductances associated with the details of the laser diode
physical
characteristics and packaging can also contribute to create noise at the
current
20 transitions.
In a MOPA configuration (Master Oscillator Power Amplifier), any undesirable
features present at the seed level will be amplified. As those skilled in the
art will
recognize, this effect is often worsened when the pulsed oscillator output is
amplified
25 and frequency converted to one or more harmonic wavelengths using the
process of
nonlinear harmonic conversion, as is well known in the art. Other issues can
arise
from the presence of peak power instabilities. For example, when fiber
amplifiers are
used to amplify the pulsed seed signal, excessive peak power induced by the
transient behavior can trigger the onset of Stimulated Brillouin Scattering
(SBS) or
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other nonlinear processes in the fibers, which can degrade the performances of
the
laser and in some cases even cause damage to it. It is consequently of very
high
interest to have methods for controlling the switching transients of a laser
diode
operated in the pulsed mode in a very predictable way in the context of
flexible pulsed
laser oscillators based on a MOPA architecture.
Previous works have described certain methods for mitigating switching
transients in
other fields of use, such as laser diode drivers used in CD-recorders. For
example,
CLAVERIE, in European Patent No. 0,053,974, discloses a method to reduce the
to effects of the switching transients by superposing to the information-
containing
current pulses a d.c. current with a slightly smaller amplitude than the laser-
threshold
current. However this method is not very attractive for pulsed lasers having a
MOPA
architecture and relying on a directly modulated semiconductor laser, because
the
pedestal (d.c.) current would generate a continuous wave emission background
between the pulses, which in turn would deplete the population inversion in
the
subsequent amplifier stage by stimulated emission. This amplifier gain
reduction can
substantially reduce the energy delivered in each pulse, especially for
regimes of
operation corresponding to low duty cycles (e.g. 10% or less), since the
relative
proportion of energy contained in the pedestal with respect to the energy
contained in
the pulse increases as the duty cycle is reduced. Such low duty cycle
operation
regimes are not exceptional for many material processing applications, where
pulse
durations in the range of 1 to 50 ns at repetition rates of a few hundreds of
kilohertz
are commonly required by practitioners. Another drawback associated with the
continuous background is that the optical power emitted between the triggered
optical
pulses can cause damage to the work piece between the different targets. As
will be
recognized by those skilled in the art, the maximum energy impinging on the
work
piece between targets must be kept lower than a certain threshold value, above
which
scorching of the material begins to take place.
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Even in its field of use, the method of CLAVERIE suffers from the drawback of
shortening the lifetime of the semiconductor laser, as mentioned by DUFOUR in
U.S.
Patent 4,817,097, entitled "Method of and device for pulse-mode driving a semi-
conductor laser". Alleviating the lifetime issue, DUFOUR's approach consists
of using
5 two superposed current pulses. A first pedestal current pulse generated by a
first
current source is used to drive the laser diode into the LED region (that is,
using a
current lower than the laser-threshold current). A second current source is
used to
generate an information current pulse that is superposed to the pedestal
current
pulse, both current pulses being synchronized. This two-stage drive approach
allows
for a reduction of the switching transients while avoiding unnecessary power
dissipation between the information pulses, which is beneficial as it extends
the laser
diode lifetime. Although this method would certainly work better than the
method of
CLAVERIE in the context of pulsed laser oscillators having MOPA architectures
(as
the inter-pulse background would not be present), it imposes a more complex
laser
is diode driving circuit, which represents an additional cost. Furthermore,
as those
skilled in the art will recognize, an increased component count in the
vicinity of the
laser diode package usually represents additional difficulties for reaching
fast rise
times and fall times, because the complex impedance usually increases along
with
the circuit footprint.
Therefore, improved pulse stabilization methods well-adapted for flexible
laser
oscillators based on directly modulated seed laser diodes are of substantial
interest
and value to practitioners in many industrially important applications.
SUMMARY OF THE INVENTION
In accordance with one aspect of the invention, there is provided a method for
stabilizing an output of a pulsed laser system, this output being controlled
using a
pulse shaping signal directly modulating a drive current of a laser diode
within the
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laser system to obtain a desired temporal shape of the output. The method
comprises
controlling the pulse shaping signal to define, over time:
- at least one processing period wherein the pulse shaping signal has an
amplitude profile tailored so as to produce the desired temporal shape of the
output; and
- at least one conditioning period, either immediately preceding or
immediately
following one of the at least one processing periods, wherein the amplitude
profile of the pulse shaping signal is tailored so that the drive current of
the
laser diode is lower than a maximum value of the drive current during the
corresponding processing period, and is of the same order of magnitude as a
laser threshold current of said laser diode, whereby the stability of the
output
during the corresponding processing period is improved.
Advantageously, embodiments of the invention provide a solution for mitigating
is
switching transients when directly modulating a laser diode in the context of
laser
oscillators, offering pulse shape flexibility. Preferably, using a stabilizing
method
according to the present invention does not impose a continuous wave (CW)
background between the optical pulses, nor does it require additional hardware
or
circuit elements specifically dedicated to the mitigation of switching
transients. The
proposed method instead relies on pulse shaping techniques to efficiently
mitigate the
transients.
Other features and advantages of the present invention will be better
understood
upon a reading of the preferred embodiments thereof, with reference to the
appended
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 (PRIOR ART) is a graphical representation of the temporal shape of an
optical
pulse affected by a switching transient at its leading edge.
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FIG. 2A is a schematic illustration of a pulsed laser system which may be used
to
embody the present invention. FIG. 2B is a schematic representation of the
digital
pulse shaping module part of the pulsed laser system of FIG. 2A.
FIG. 3A (PRIOR ART) shows the amplitude profile of an exemplary pulse shaping
signal according to prior art. FIG. 3B (PRIOR ART) is a graph representing the
typical
output power curve as a function of the drive current curve for a
semiconductor laser
diode, along with the current pulse applied to the diode and resulting from
the pulse
io shaping signal of FIG. 3A. FIG. 3C (PRIOR ART) is a graph showing the
power of the
outputted waveform from a semiconductor laser diode, to which has been applied
the
pulse shaping signal of FIG. 3A.
FIG. 4A shows the amplitude profile of an example of a pulse shaping signal
according to an embodiment of the invention. FIG. 4B is a graph representing
the
output power curve as a function of drive current curve for a semiconductor
laser
diode, along with the current pulse applied to the diode and resulting from
the pulse
shaping signal of FIG. 4A. FIG. 4C is a graph showing the power of the
outputted
waveform from a semiconductor laser diode, to which has been applied the pulse
shaping signal of FIG. 4A.
FIG. 5A (PRIOR ART) illustrates the pulse shaping signal applied to a laser
system
provided with pulse shaping capacity; FIG. 5B (PRIOR ART) shows the resulting
output of this laser.
FIG. 6A illustrates the pulse shaping signal applied to a laser system
provided with
pulse shaping capacity, in accordance with one embodiment of the invention;
FIG. 6B
shows the resulting output of this laser.
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FIG. 7A is a schematic illustration of a pulse system associated with a
retroaction
loop; FIG. 7B is a flow chart illustrating steps of a method using the
retroaction loop of
FIG. 7A.
DESCRIPTION OF PREFFERED EMBODIMENTS OF THE INVENTION
The present invention generally provides methods for stabilizing an output of
a pulsed
laser system. Embodiments of the present invention may advantageously be
applied
in the context of applications such as memory repair, milling, micro-
fabrication, drilling
and other material processing applications. It will be understood that
embodiments of
the present invention may also be used in other contexts, such as medical
laser
devices, remote sensing or any other application which may benefit from
optical
pulses having good optical characteristics.
Pulsed laser system
The expression "pulsed laser system" is understood to refer to any grouping of
components outputting an optical beam having a varying intensity as a function
of
time. Generally, the outputted light defines a succession of high power
"peaks", or
"pulses", separated by gaps where the outputted optical power is low or
absent. It will
be understood by one skilled in the art that the consecutive pulses of a given
outputted light beam may vary in duration and intensity, and that the
repetition rate of
the optical beam need not be constant. As a matter of fact, embodiments of the
present invention may be particularly useful in industrial applications, where
the
temporal profile of the outputted beam is often customized according to the
particular
needs of a given process.
Referring to FIG. 2A, there is shown an example of a laser system 20 to which
a
method according to embodiments of the invention may be applied. Such a system
is,
for example, shown in the international patent application published under WO
2009/155712 (DELADURANTAYE et al).
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The pulsed laser system 20 includes a laser diode 22. The expression "laser
diode" is
understood to refer to a laser with a semiconductor-based gain medium. The
laser
diode 22 may be embodied by various types of diodes such as Fabry-Perot laser
diodes, DFB (distributed feedback) laser diodes, DBR (distributed Bragg
reflector)
laser diodes, external cavity laser diodes and the like. The laser diode 22
may be
wavelength-locked or wavelength-tunable, that is, the wavelength of spectral
profile of
the outputted light may be fixed or controllable. An example of an appropriate
laser
diode for use in flexible pulsed laser systems is the laser diode model
11064.3SB0050P from Innovative Photonics Solutions, which is an external-
cavity
laser diode including a wavelength selective feedback element.
Of course, the pulsed laser system 20 may include any number of components
amplifying, shaping, focusing or otherwise acting on the optical pulses
outputted by
the laser diode 22. By way of example, in the illustrated embodiment of FIG.
2A, the
pulsed laser system 20 has a MOPA configuration and includes two or more
optical
fiber amplifiers 58, each pumped by a pump P. Other components may be
included,
such as amplitude modulators, phase modulators, circulators, reflectors such
as
Bragg gratings or the like, isolators, etc. One skilled in the art will be
readily familiar
with a number of possible configurations for the pulsed laser system 20 and
may also
refer to WO 2009/155712 (DELADURANTAYE et al) for additional examples.
In the illustrated embodiment, the output of the pulsed laser system 20 is
passed
through an optical harmonic converter 90, which converts the fundamental
wavelength ko of the output to a harmonic kh thereof. The resulting wavelength
kh is
shorter than the fundamental wavelength, as will be well understood by one
skilled in
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the art. The optical harmonic converter may for example be based on a non-
linear
crystal, and may be internal or external to the pulsed laser system 20. As the
harmonic conversion process can worsen the impact of the transients in the
shape of
the optical pulses subjected thereto, such a system can greatly benefit from
mitigation
5 techniques as disclosed herein.
The output of the pulsed laser system 20 is controlled using a pulse shaping
signal 24
directly modulating the drive current of the laser diode 22; by changing the
value of
the drive current fed to the laser diode, the amplitude of the outputted light
beam is
10 changed accordingly, thereby obtaining the desired temporal shape of the
output.
According to one embodiment of the invention, the pulse shaping signal 24 is
generated by a digital pulse shaping module 26. The digital pulse shaping
module 26
controls the laser diode 22 according to a digital input waveform. The digital
input
waveform is preferably a sequence of digital samples which may be defined or
selected by a user, and which determine the resulting temporal shape of at
least one
pulse to be outputted by the laser diode 22.
In the embodiment of FIG. 2A, The pulsed laser system 20 is preferably adapted
to
interact with user equipment 54, which enables a user to interact and control
the laser
system according to desired operation parameters. A connector interface 56
preferably interfaces communication between the digital pulse shaping module
26
and the user equipment 54. The user equipment 54 may be embodied by any
appropriate device or combination of devices such as, for example, a memory
repair
system, a photovoltaic cell scribing system, a micro-via drilling system or a
laser
surgery system.
Each electrical shape output by the digital pulse shaping module 26 may be
triggered
either by an internal or an external trigger signal. Preferably, the connector
interface
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56 provides an external trigger signal EXT_TRIGGER to the digital pulse
shaping
module 26 in response to a trigger command from the user equipment 54. The
connector interface 56 also optionally outputs a trigger synchronization
signal
SYNC OUT, having a predetermined timing relationship with respect to a light
pulse
emitted by the laser oscillator responsive to the external trigger signal
EXT_TRIGGER.
In one embodiment of the invention, the pulsed laser system allows switching
between two input waveforms pre-selected by the user, hereinafter referred to
as
to SHAPE_A and SHAPE_B. The connector interface 56 preferably provides a shape
switching signal SHAPE_A/B for dynamically switching the output from one of
the
preselected waveforms to the other, in response to a shape selection command
from
the user equipment 54.
Is In the illustrated embodiment, the shape or amplitude profile of the
optical pulses can
be programmed in a straightforward manner using a computer 38. To program a
pulse shape in the pulse shaping signal 24, the system user preferably enters
a
series of amplitudes (amp1, amp2,...,ampn) corresponding to a series of
concatenated temporal "bins", or time slots, of equal duration (typically 2.5
ns or less).
20 A high speed digital to analog converter (DAC) then converts this
digital shape into an
analog pulse having the desired shape. The analog pulse is then used to drive
the
laser diode with a current pulse having the desired shape. Additional
components
may be further provided to adapt the analog pulse for feeding into the laser
diode,
such as buffers, amplifiers and the like. One example of a drive circuit
adapted for this
25 purpose is shown in co-assigned U.S. patent application 12/780,556
(DESBIENS et
al) filed on May 14, 2010. The laser diode then emits an optical pulse having
an
amplitude profile (over time) substantially faithful with respect to the
digital (or
programmed) shape.
1
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12
Of course, the pulsed laser system may have a configuration different than the
one
illustrated in FIG. 2A and include any additional digital, analog or optical
components
as appropriate for a given application, as would be readily devised by one
skilled in
the art.
Stabilizing method
In general, the optical pulse shape outputted by the laser system 20 will
exhibit some
distortions with respect to the programmed shape. Several factors can
contribute to
this distortion, such as the limited bandwidth of the laser diode current
driver, the
limited bandwidth of the laser diode itself, the optical gain saturation
taking place in
the different optical amplifier stages, the nonlinear transfer function of
some
components such as nonlinear crystals, etc. In order to distinguish between
the
optical pulse shape and the programmed pulse shape, the latter is denoted
herein as
an oscillator preform seed pulse (OPSP).
In accordance with an aspect of the present invention, there is provided a
method for
stabilizing the output of a pulsed laser system, such as the one of FIG. 2A or
the like,
by mitigating the effect of switching transients on the temporal shape of the
outputted
pulses.
The method includes controlling the pulse shaping signal to define, over time,
two
types of periods which arise sequentially: processing periods and conditioning
periods.
During the processing periods, the pulse shaping signal has an amplitude
profile
tailored so as to produce the desired temporal shape of the output, as is
generally
known in the art and taught, for example, by DELADURANTAYE et al. in
WO 2009/155712. As mentioned above, different outputted pulses may have a
different intensity, width and shape. The pulse shaping signal may define a
single
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processing period, the laser system thereby outputting a single pulse.
Alternatively,
the pulse shaping signal may define a succession of processing periods, which
may
be identical or different, and which may be generated at a constant or
irregular
repetition rate. During any given processing period, the pulse shaping signal
may
have a constant amplitude, thereby defining a square pulse, or may
advantageously
have a tailored amplitude profile defining a more complex pulse shape.
The expression "processing period" is meant to refer to portions of time where
the
laser system outputs a light beam that can be used for carrying out a specific
duty. In
m several applications, the optical pulses outputted during the processing
periods are
used to "process" a target, such as performed, for example, in the laser
drilling of
microvias in semiconductors, or the laser scribing of thin film photovoltaic
devices.
However, it will be understood that any use of a pulsed light beam may be
considered
a "process", and that embodiments of the present invention are not limited to
applications generally referred to as "material microprocessing".
In accordance with an aspect of the invention, at least one conditioning
period is
provided. Each conditioning period either immediately precedes or immediately
follows one of the processing periods. By "immediately" it is understood that
the
conditioning period is close enough to the corresponding processing period
that the
current provided to the laser diode during the former has a significant impact
on the
laser dynamics during the latter.
It is further understood that the method of the present invention encompasses
any
pulse shaping signal where at least one processing period is preceded and/or
followed by a corresponding conditioning period. Of course, in a given
application, a
pulse shaping signal may define a plurality of processing periods, and each of
these
processing periods may be associated to a conditioning period either before,
after or
both. It is also possible that some of the processing periods within a given
pulse
1
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14
shaping signal may not be associated with a conditioning period at all. One
skilled in
the art will understand that a given pulse shaping signal may include any
appropriate
combinations of processing and conditioning periods.
During the conditioning period, the amplitude profile of the pulse shaping
signal is
tailored so that the drive current of the laser diode has the following
characteristics: it
is lower than the maximum value of the drive current during the corresponding
processing period, and is of the same order of magnitude as the laser
threshold
current of the laser diode. Preferably, the amplitude profile of the pulse
shaping signal
to is such that the drive current of the laser diode is lower than twice
the laser threshold
current of the laser diode. It has been found that in such conditions, the
stability of the
output during the corresponding processing period is improved.
In embodiments of the invention, the method above may be accomplished by
tailoring
OPSPs so that the amplitudes of a number of bins preceding a leading edge or
following a falling edge in a given pulse amplitude profile are adjusted to
values
corresponding to current amplitudes that are close to the laser threshold
current.
Preferably, the one or more conditioning periods are provided right before a
low to
high transition, or right after a high to low transition contained in the same
OPSP. The
expression "high" is understood to refer to a programmed amplitude
corresponding to
a drive current higher than or equal to the threshold current, whereas "low"
is
understood to refer to a programmed amplitude corresponding to a current lower
than
the laser threshold current.
A conditioning period may be embodied by a single bin. Advantageously, it may
contain several bins, which allows for a lot of flexibility, as tailored
waveforms can be
used in a given conditioning period to mitigate the switching transients. This
flexibility
is quite beneficial, as no hardware modifications are required when variations
(e.g. in
I
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the laser diode manufacturing process) leading to different transient
responses occur,
or when different models of laser diodes having different transient response
characteristics are employed for addressing different material processing
applications. The programmable aspect of a preferred embodiment of the method
5 allows the adjustment of the number and amplitude of the bins in the
conditioning
periods, so as to efficiently improve the pulse amplitude stability in the
processing
periods without adding circuit elements and without changing any part of the
system
design.
to One skilled in the art will readily understand that any pulsed laser
system providing an
appropriate measure of pulse shaping may be adapted to enact the method of the
present invention. By way of example, the section below describes such a
pulsed
laser system incorporating an appropriate pulse shaping module.
Is Example of pulse shaping module
Referring to FIG. 2B, a digital pulse shaping module 26 is shown, which may be
used
to embody the invention.
The digital pulse shaping module 26 generally includes a clock generator 28
generating a plurality of phase-related clock signals at a same clock
frequency, and a
shape generator 30 outputting a digital shape signal DAC_D corresponding to
the
digital input waveform. The phase-related clock signals from the clock
generator 28
are used as timing signals in the shape generator 30, preferably in a double
data rate
configuration, hereinafter referred to as "DDR". In one embodiment, the
digital shape
signal DAC_D from the shape generator 30 is either pulse shape data or quasi-
continuous data. The pulse shaping module 26 further includes a Digital-to-
Analog
Converter 32, hereinafter DAC, receiving the digital shape signal DAC_D from
the
shape generator 30 and converting it into an analog shape signal DAC_OUT. In
the
illustrated embodiment, the digital pulse shaping module 26 further includes a
shape
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buffer driver 34 receiving the analog shape signal DAC_OUT from the shape
generator 30 through the DAC 32 and generating the pulse shaping signal 24
adapted for controlling the laser diode. A microcontroller 36 is preferably
used for
controlling the communications with the internal sub-systems of the laser
diode (pump
drivers, etc.) and with the host computer 38 (shown in FIG. 2A).
The plurality of phase-related clock signals generated by the clock generator
28
includes a Clock_O signal, and Clock_90, Clock_180 and Clock 270 signals
respectively lagging a quarter of a period, half a period and three-quarters
of a period
io behind the Clock_O signal. In one embodiment, the phase-related clock
signals are
driven at 200 MHz. The clock generator 28 further preferably outputs a slow
clock
signal CLKD4_0 at 50 MHz, used mainly for clocking the slower elements in the
design.
A frequency measurement module 40 is provided for measuring the frequency of
the
external trigger signal EXT_TRIGGER from the connector interface.
The DAC 32 may, for example, be embodied by the AD9736 model (trademark) from
Analog Devices. The interface to the DAC 32 is preferably differential LVDS,
10-bit
source-synchronous with the 200 MHz clock signal, DAC_CLK. The data is
inputted
into the DAC 32, in double data rate, on each rising edge and each falling
edge of
DAC CLK.
The analog output DAC_OUT of the DAC 32 is either 100 MHz QCW or an analog
pulse shape. This signal is inputted to the shape buffer driver 34, which is
selected
amongst devices appropriate for the laser diode. As such, the shape buffer
driver 34
preferably has a very high slew rate, and is capable of driving a 50-Ohm load.
This
driver can use the Texas Instruments THS3102 amplifier (trademark), or a
parallel
combination of a few such amplifiers. If desired, the output of the shape
buffer driver
,
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34 may be transformer-coupled to the laser diode. The resulting output signal
is the
pulse shaping signal 24 which is inputted to MOD1 of the laser diode.
A low-jitter frequency synthesizer 42, such as the IDT ICS8442 (trademark)
with a 10
MHz crystal, outputs a 400 MHz clock signal to the DAC 32. The DAC 32 divides
this
frequency down to 200 MHz to output the FPGA_CLK signal used by the clock
generator 28.
In one embodiment, the shape generator 58 also outputs the PREDR_GATE signal,
to either in QCW or pulse mode. This output has a fixed amplitude and is
also buffered
similarly to the DAC_OUT, with a gate buffer driver 44, for example a THS3102
type
amplifier, to drive the laser source. The resulting buffered output is the
GATE signal
that is inputted to MOD2 of the laser source.
is Peripherals such as a timer 46, read/write registers 48, dual-port shape
memory
buffers 50 and the like may additionally be provided as would be readily
understood
by one skilled in the art, all of which are preferably mapped on the bus of
the
microcontroller 36.
20 Preferably, the microcontroller 36, clock generator 28, shape generator
30 and
related peripherals are embedded on a high speed digital logic circuit 52. In
the
different embodiments of the present invention, high speed digital logic
circuits
available in technologies such as ASIC or FPGA or off-the-shelf digital ICs
and high
speed Digital-to-Analog Converters (DAC) may be used to implement the desired
25 pulse shaping capability.
Examples
The section below provides examples of pulse shaping signals incorporating
processing and conditioning periods according to embodiments of the invention.
Of
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course, it will be readily understood that these examples are given for
illustrative
purposes only, and are in no way considered limitative to the scope of the
present
invention.
FIG. 3A (PRIOR ART) shows an example of an OPSP containing 43 temporal bins,
as could be readily programmed as explained above. The amplitude profile is
characterized by six main intervals in the time domain with uneven durations,
denoted
by Ato, Ati, At2, At3, Att, At5 and At5, the programmed amplitude being
adjusted to a
specific and constant value for each interval. There are four non-zero
distinct
amplitude values, denoted by V1, V2, V3 and V5, corresponding to the time
slots Ati,
At2, At3 and At5, respectively. In the given example, the time slots Ato, At4
and At6
contains 7,8 and 8 bins respectively, with all amplitudes set to zero. In FIG.
3A, Vt
denotes the amplitude corresponding to the seed laser diode laser-threshold
current.
Therefore, all non-zero programmed amplitudes in this OPSP correspond to drive
currents that are higher than the laser-threshold current.
FIG. 3B (PRIOR ART) illustrates the seed laser diode output power levels P1,
P21 P3
and P5 corresponding to the non-zero currents 11, 12, 13 and 15 flowing
through the
junction as the current pulse generated from the OPSP shown in FIG. 3A is used
to
drive the laser diode. The upper part of the graph shown in FIG. 3B represents
the
typical output power vs drive current curve for a semiconductor laser diode.
It and Pt
denote the laser-threshold current and the emitted optical output power at the
laser
threshold respectively.
FIG. 3C (PRIOR ART) illustrates an example of optical waveform exhibiting
switching
transients that could be obtained with the OPSP of FIG. 3A and corresponding
to the
current pulse of FIG. 3B. As shown in FIG. 3C, optical amplitude instabilities
can
occur at each transition of an OPSP for which one amplitude (initial or final)
is set to a
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value corresponding to a current higher than It while the other amplitude
(final or
initial) is set to a value lower than It.
FIGs. 4A to 4C illustrate how the present invention may be used to improve the
stability of the optical pulses generated in the example of FIGs. 3A to 3C.
In both FIGs. 3A and 4A, the OPSP may be said to include two processing
periods, a
first processing period 60 corresponding to time intervals Litt, At2 and At3,
and a
second processing period 62 being embodied by interval At6. As seen in FIG.
3C,
switching transients would normally appear at the onset and at the end of each
of
these conditioning periods.
FIG. 4A shows the addition of conditioning periods 64 immediately prior and
immediately following each processing period. The conditioning bins are
represented
with a crosshatched background. The first conditioning period 64 corresponds
to the
end of interval Ato, in which a single bin immediately preceding the low to
high
transition occurring between intervals Ato and Ati has been used to mitigate
the pulse
amplitude instability on the leading edge of the optical pulse corresponding
to the first
processing period 60. In that case, the programmed amplitude is slightly
higher than
Vt. The other conditioning periods 64 occur within intervals At4 and At6,
where the
same method has been applied with different numbers of conditioning bins and
different programmed amplitudes for those bins. Two bins of equal amplitude
have
been programmed to mitigate the transient at the high to low transition
occurring
between intervals At3 and At4. For the transition At4
At6 , three consecutive bins
have been used right before the transition, with increasing amplitudes; the
amplitude
of the first bin is smaller than Vt, the amplitude of the second bin is equal
to Vt,
whereas the amplitude of the third bin is slighty higher than V. Finally, in
the last
conditioning period, corresponding to beginning of interval At6, a single bin
has been
programmed with an amplitude lower than Vt, right after the transition At6¨>
At6. FIG.
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4B and FIG. 4C are modified versions of FIG. 3B and FIG. 3C, respectively,
where
the conditioning bins defined above have been added with a crosshatched
background. As can be seen in FIG. 4C, as compared with FIG. 3A, the optical
pulse
amplitude profile obtained following the application of the present method is
free of
5 amplitude noise at the critical transitions.
Those skilled in the art may note that a side effect of the method according
to
embodiments of the invention is the addition of small pedestals in the optical
pulse
amplitude profile emitted by the laser diode near the critical amplitude
transitions. It
10 has been found, however, that the impact of such features can usually be
kept
negligible at the output of a MOPA system, as long as the pedestal current
remains in
the same order as It. Typically, very high peak amplitude to pedestal
amplitude ratios
(e.g. > 1000:1) can be obtained because of optical gain saturation effects,
nonlinear
conversion processes (for lasers employing harmonic modules for example), or
is spectral filtering by selective elements, as the spectral bandwidth
corresponding to
the pedestal is usually broader than the linewidth of the main pulses.
Alternatively, the
pedestal can be removed by gating the optical pulse with an amplitude
modulator
located downstream to the seed laser diode, this modulator being driven in
synchronization with the processing periods of the pulse shaping signal, at
least
20 partially, so as to be closed when the output generated during the
conditioning
periods impinge thereon and therefore block the unwanted pedestals while
transmitting the power emitted during the processing periods. The GATE signal
shown in FIG. 2B could be used for this purpose.
The OPSPs appearing in the examples of FIG.3 and FIG.4 are arbitrary, and are
presented purely for the sake of explanation. It will be understood by those
skilled in
the art that the method can be applied to any OPSP in which switching
transients are
to be mitigated.
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FIGs. 5A and 5B and FIGs. 6A and 6B present an experimental demonstration of
the
efficiency of the method for improving the pulse amplitude stability. The
results were
obtained with a 1.5 watt MOPAW Green fiber (trademark) laser from INO, Quebec,
Canada. The laser architecture is based on a directly modulated semiconductor
laser
diode seeding a fiber amplifier chain, using a second harmonic generation
(SHG)
module to convert the 1064 nm fundamental wavelength to 532 nm. The system
provides pulse shaping flexibility, with rise times and fall times typically
smaller than
ins and 2 ns, respectively. Up to 64 temporal bins with 2.5 ns duration per
bin can be
used to define tailored OPSPs having total durations in the range of 2.5 ns to
160 ns.
The amplitude of each bin can be adjusted finely, as 1024 levels are available
for this
instrument. More details about the programmable pulse shaping platform
embedded
in the MOPAW Green laser can be found in WO 2009/155712 (DELADURANTAYE et
al.).
is FIG. 5A (PRIOR ART) shows an OPSP similar to the OPSP of FIG. 3A, that was
programmed using the MOPAW Green laser. In FIG. 5B (PRIOR ART), an
oscilloscope screenshot shows the corresponding pulse amplitude profile
detected at
the output of the system. As can be seen in FIG. 5B, pulse amplitude
instabilities
occur at two critical leading edges in the pulse waveform, corresponding to
the
transitions At At1 and At4 At5, by analogy with FIG. 3A,
whereas no transients
are observed at the critical falling edges for this specific laser. Pulse
amplitude
instabilities of the order of 20%, as shown in FIG. 5B, are not acceptable
for many
industrially important applications, as explained above. Practitioners often
require
pulse amplitude instability levels smaller than 5% for satisfactory
processing. FIG.
6A presents a modified OPSP that was employed for improving the pulse
amplitude
stability of the pulse presented in FIG. 5B. As can be seen in FIG 6A,
additional bins
(appearing with a crosshatched background) have been added in two different
conditioning periods of the OPSP, in a way similar to the case presented in
FIG. 4A,
except that no conditioning bins were required following the high to low
transitions. As
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can be seen in FIG. 6B, a fourfold improvement of the optical pulse amplitude
stability
was obtained with respect to FIG. 5B. It is believed that the improvement
factor is
even higher, given the fact that most of the remaining amplitude instability
appearing
in FIG. 6B is due to detection noise, as those skilled in the art will
appreciate by
comparing the oscilloscope trace thickness of the zero amplitude level with
that of the
peak of the pulse.
One skilled in the art will also understand that an optimal waveform for a
given
conditioning period may be determined using a trial-and-error technique. For
example, in the example of FIG. 4A, attempts were made with a single
conditioning
bin immediately preceding the Litt Lit 5 transition. However, as opposed to
the Ato
bat transition, it was noted that occasional transients events (perhaps 1
pulse over
100 000) still occurred with the bin amplitude set to a value more than twice
as high
as Vt, in which case the associated pedestal became significant in the
outputted
optical pulse. Using three bins with amplitudes set close to Vt solved the
issue, as
shown in FIG. 6. Alternatively, a model based on details of the seed laser
diode
internal dynamics in the context of tailored amplitude profiles at the
nanosecond time
scale could be built to predict this transient response for a given laser
system. Such a
model could predict the dynamics of longitudinal mode competition occurring
during
the shaped pulse, and would preferably include dominant factors such as the
time-
dependent wavelength chirp and the seed laser diode chip operating
temperature.
In accordance with another embodiment of the invention, with reference to
FIGs. 7A,
there is preferably provide a retroaction loop 66, to provide an error signal
to the
digital pulse shaping module 26 generating the pulse shaping signal 24. The
retroaction loop preferably includes a tap coupler 70, or other appropriate
sampling
device apt to extract a portion of the output of the pulsed laser system. The
extracted
portion is detected by a photodetector 72, providing a feedback electrical
signal 68
which is preferably forwarded to the digital pulse shaping module 26 for
processing.
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Referring to FIG. 7B, the retroaction loop preferably performs the following
substeps:
i. detecting 74 a portion of the output of the pulsed laser system. This is
preferably achieved through the use of a tap coupler and photodetector are
illustrated in FIG. 7A, although other arrangements could also be considered.
ii. processing the signal detected at i. This is preferably performed by
appropriate
components in the digital pulse shaping module or connected thereto. This
processing allows extracting 76 information related to pulse amplitude
stability,
and generate 78 a corresponding error signal which is representative of the
to difference between the measured pulse amplitude stability and the
targeted
pulse amplitude stability. The expression "pulse amplitude stability" is
understood to refer to the relative pulse to pulse variation of the amplitude
of
the optical pulses for a given number of detected pulses. For example, this
parameter can be measured by acquiring the optical shape using a
photodetector and an oscilloscope in infinite persistence for an acquisition
time
corresponding to the desired number of pulses (e.g. 10 000 pulses). The pulse
amplitude stability is calculated from the ratio of the oscilloscope trace
thickness with respect to the average pulse amplitude. Of course, other
calculation methods may also be considered.
iii. the controlling of the pulse shaping signal is adapted in view of the
error signal.
In practice, the digital pulse shaping module may for example compare 80 the
error signal with a pre-programmed error threshold. If the error signal is
below
this threshold, then the output is considered sufficiently stable and there is
no
need for action. If the error signal is above the error threshold, then the
conditioning bins are modified 82, for example by adding or removing bins
and/or changing their amplitude. The pulse shaping signal incorporating the
modified conditioning bins is then used to drive 84 the laser diode.
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Of course, numerous modifications could be made to the embodiments described
above without departing from the scope of the invention as defined in the
appended
claims.
