Note: Descriptions are shown in the official language in which they were submitted.
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ADJUSTABLE PULSEWIDTH PICOSECOND FIBER LASER
FIELD OF THE INVENTION
The present invention relates to pulsed fiber laser and more particularly
concerns a
fiber laser generating light pulses having an adjustable pulsewidth of the
order of
picoseconds.
BACKGROUND
Pulsed fiber lasers are currently of great interest for a variety of
applications. One
io such application is scribing semiconductor materials. Material ablation
with laser
pulses can be separated in two distinct regimes of operation; thermal and non-
thermal. In the thermal regime, the laser energy is transferred to the
material lattice
through electron-phonon interactions. If atoms are ejected from the lattice
before
such interactions can really take place, than the process is considered non-
thermal.
is The timescale over which the energy transferred to the electrons by the
laser pulse
is further transferred to the lattice is of the order of tens of picoseconds
(typically 5-
50 Ps depending on materials). Consequently, picosecond pulses with a duration
greater than this characteristic timescale are considered the shortest pulses
that can
still be considered to operate in the thermal regime.
Advantageously, micro-machining with the shortest pulse in the thermal regime
reduces to the minimum the size of the heat affected zone (HAZ) surrounding
the
targeted region. This is highly relevant in applications where multiple layers
are
stacked and only one of those layers is targeted, such as for example in the
drilling
of via in photovoltaic cells used in solar panels.
Picosecond pulses are characterized by high peak power (ten to hundreds of
kilowatts for micro-joule pulses) and narrow linewidth (less than 1 nm for
transform
limited pulses). This combination is very advantageous for frequency
conversion
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(second, third and forth harmonic), which opens up significantly the range of
applications a single powerful picosecond source can address.
Mode-locked femtosecond laser, bulk or fiber-based, can be modified to produce
picosecond pulses. Generally speaking, mode-locked fiber lasers are considered
particularly attractive structures for ultra-short pulse generation, via
either passive or
active mode-locking. The pulse-generation mechanism in such lasers depends on
the physics of the cavity. Known cavity configurations include linear
cavities, ring
lasers and figure-of-eight cavities. To produce picosecond pulses in such a
mode-
l() locked regime, a narrow spectral filter placed inside the laser cavity
controls the
duration of the pulses by the virtue of the Fourier transform. Those designs
are
usually not very flexible since they necessitate a tuning of the filter
bandwidth to
change the pulse duration. This tuning can necessitate moving parts.
Picosecond pulses can also be produced with gain-switched semiconductor diode
lasers, where the pulses are advantageously generated on demand by an
electrical
pulse. However there is little correlation between the electrical pulse sent
and the
received optical pulse. The optical pulse is in fact the impulse response of
the
device, and therefore has a duration which differs from chip to chip. In
addition, such
diodes offer very little control on the spectral content of the emitted
pulses, which is
usually quite broad, and the optical pulse is often followed by relaxation
oscillations.
There remains a need in the field for picosecond fiber lasers suited to the
requirements of micromachining and similar industrial applications.
SUMMARY OF THE INVENTION
In accordance with an aspect of the present invention, there is provided a
fiber laser
for generating light pulses having an adjustable pulsewidth.
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The fiber laser includes a laser cavity including optical fiber arranged in a
figure-of-
eight configuration. The laser cavity defines a non reciprocal loop having a
propagation direction therein and a reciprocal loop having opposite
propagation
directions therein. The laser cavity further includes a coupler
interconnecting the
loops.
A gain medium is provided in the reciprocal loop at an asymmetrical position
with
respect to the coupler, this position being selected so that the light pulses
counter-
propagating within the reciprocal loop have a phase relationship at the
coupler
favoring coupling into the propagation direction of the non-reciprocal loop.
The fiber laser further includes a pump source coupled to the reciprocal loop
for
launching a pump light beam in the gain medium, the pump light beam having a
pump power value. Pump control means are provided for controlling the pump
power value of the pump source within a range extending above an operational
lasing threshold of the laser cavity. Changing the pump power value of the
pump
source within this range changes the pulsewidth of the light pulses, without
modifying a peak power of the light pulses.
Advantageously, the present invention makes use a nonlinear amplifying loop
mirror
using to control the pulsewidth inside a laser cavity.
Preferably, the fiber laser according to embodiments of the invention emits
picosecond pulses. Optionally, an amplitude or phase modulator may be placed
inside the laser cavity to help ignite the mode-locked regime and to provide
the
possibility of increasing the repetition rate of the laser by an integer
number. A
modulator may also be placed at the output of the laser to serve as a "pulse-
picker",
extracting a light pulse from the cavity at an external request.
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Other features and advantages of the present invention will be better
understood
upon reading of preferred embodiments thereof with respect to the appended
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is a schematic representation of a fiber laser according to an
embodiment of
the invention. FIG. 1B is a schematic representation of a fiber laser
according to
another embodiment of the invention.
FIG. 2 is a graph showing the temporal shape of experimentally obtained pulses
for
various pump values, measured using a fast photodiode.
FIG. 3 is a graph showing the deconvoluted autocorrelation traces of
experimentally
obtained pulses for various pump values, measured using an autocorrelator.
FIG. 4 shows the pulse spectra of the optical pulses of FIG. 3.
FIG. 5 shows the temporal shape of the longest pulse obtained using an
experimental
setup similar to the embodiment of FIG. 1A.
FIG. 6 is a graph showing results of simulated laser performance as a function
of
pump power in the reciprocal loop for a laser cavity according to an
embodiment of
the invention.
FIG. 7 illustrates the simulated pulse duration as a function of parameter CNL
FIG. 8 is a schematic representation of the propagation of pulses A and B in
the
reciprocal loop.
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FIG. 9 is a diagram of the gain in the reciprocal loop as a function of
position.
FIG. 10 is a schematic representation of the electric field components on
either sides
of the main coupler.
FIG. 11 is a graph showing the relationship between the output power and pump
power above the lasing threshold in a laser cavity.
DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION
Referring to FIGs. 1A and 1B, variants of fiber lasers 20 according to
embodiments of
the invention are shown.
Advantageously, fiber lasers of the present invention allow the generation of
light
pulses having an adjustable pulsewidth, that is, pulses having a duration in
time
which can be set or changed by a user according to a desired output. The
mechanism
allowing this adaptability feature will be explained further below.
Preferably, the
pulsewidth or pulse duration is of the order of picoseconds, for example
between 1
and 1000 picoseconds.
Throughout the description below, the expressions "pulsewidth" and "pulse
duration"
will be used interchangeably to refer to the full width at half maximum of the
optical
pulse with respect to time.
Referring more particularly to FIG. 1A, the fiber laser 20 according to the
illustrated
embodiment of the invention includes a laser cavity 22, made of optical fiber
arranged
in a "figure-of-eight" configuration. In the preferred embodiment, the optical
fiber in the
laser cavity is polarization-maintaining normal dispersion fiber. The use of
such fiber
advantageously limits undesired nonlinear effects in the laser cavity such as
nonlinear
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polarization rotation, and ensures an environmentally stable operation of the
laser,
which can be an important factor in the context of industrial applications. By
definition, the figure-of-eight configuration is generally composed of a non
reciprocal
loop 24 and a reciprocal loop 26. In the present context, the concept of
reciprocality
relates to the propagation of light in the optical fiber constituting each
loop. The non
reciprocal loop limits the propagation of light therein in a single
propagation
direction, arbitrarily chosen to be the counter-clockwise direction 28 in the
illustrated
embodiment, while the reciprocal loop allows light circulation in both
opposite
directions therein, that is, in both the clockwise direction 30 and counter-
clockwise
direction 28. Preferably, an isolator 44 is provided in the non-reciprocal
loop 24 at a
position appropriate to block light propagation in the direction opposite the
propagation direction (clockwise in the illustrated case).
The laser cavity 22 further includes a main coupler 32 interconnecting both
loops 24
and 26. Preferably, the main coupler 32 is a 50/50 PM coupler, that is, the
main
coupler 32 couples light into opposite extremities of the reciprocal loop 26
according
to a ratio of substantially 50/50. Alternatively, the main coupler 32 may be
embodied
by a coupler having a different coupling coefficient value, a WDM coupler or a
polarization combiner/splitter. Due to the bidirectional nature of the
reciprocal loop
26, light pulses circulating therein will interfere at the main coupler 32,
and the
phase difference therebetween will dictate the fraction of power that will be
coupled
in the clockwise direction of the non-reciprocal loop, where it will be lost,
and the
fraction coupled in the propagation direction of the non-reciprocal loop and
make a
round-trip in the laser cavity 22. The laser cavity 22 is designed as a
nonlinear loop
mirror to take advantage of this characteristic. A gain medium 34 is provided
in the
reciprocal loop 26, at an asymmetrical position with respect to the main
coupler 32.
A pump source 38 is coupled to the reciprocal loop 26 for launching a pump
light
beam in the gain medium 34. As shown in FIGs. 1A and 1B, the pump signal may
be coupled on either side of the gain medium 34. A WDM pump coupler 40 or any
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other appropriate coupling means is provided for connecting the optical fiber
from
the pump source 38 to the optical fiber of the reciprocal loop 26.
The nonlinear amplifying loop mirror (NALM) defined by the reciprocal loop 26
will
have a maximum transmission for a certain value of pulse peak power which is
dependent on the available gain in the loop. To produce picosecond pulses the
loop
needs to be asymmetrical, meaning that the available gain must be concentrated
at
one end of the loop, preferably favoring low values of peak power (tens of
watts). In
practice, this is accomplished through an adequate positioning of the gain
medium
34 in the reciprocal loop 26. Varying the pump power will affect the laser
dynamic in
such a way that the pulse duration will vary as explained hereinbelow.
The position of the gain medium 34 along the reciprocal loop 26 with respect
to the
main coupler 32 is further selected so that light pulses counter-propagating
within
is the reciprocal loop 26 have a phase relationship at the main coupler 32
favoring
coupling into the propagation direction 28 (counter-clockwise) of the non-
reciprocal
loop 24. The relevance of the position of the gain medium 34 along the
reciprocal
loop 26 is best understood through the following theoretical analysis of the
accumulated nonlinear phase shift in a nonlinear loop mirror, with reference
to FIGs.
8, 9 and 10.
With particular reference to FIG. 8, consider a light pulse 36 guided along
the
propagation direction of the non reciprocal loop 24. The main coupler 32, here
assumed to be a 50/50 coupler, separates the power in pulse 36 equally into
two
counter-propagating pulses 36A and 36B respectively coupled into opposite ends
of
the reciprocal loop 26. At the beginning of the propagation within the
reciprocal loop
26, immediately after the main coupler 32, pulses 36A and 36B are identical.
The accumulated nonlinear phase shift through the propagation in the
reciprocal
loop is given by the B-integral:
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=
8
B(t) = yp(t)dz
(1)
where 7 is the nonlinear coefficient, which is fiber dependent, and P1(t) is
the peak
power along the pulse.
Referring to FIG. 9, let L be the length of the fiber inside the reciprocal
loop mirror, P
the central position of the gain inside the loop, A the width of the gain
section and G
the maximum gain value (in dB/m). The gain will be assumed constant over the
doped fiber length and the effects of gain saturation will be neglected.
The B-integral for pulse A or B can be expressed as the sum of three
contributing
terms: the nonlinear phase shift accumulated in the passive fiber before
amplification, the nonlinear phase shift accumulated in the gain medium, and
the
is nonlinear phase shift accumulated in the passive fiber after
amplification.
For pulse A, this can be expressed as:
GA/ 1(
A
GA/
B A (t) = 7PA(t) P A + 10 log(e) 10 10 ___ 1 +10 '10 L ¨ P ¨ ¨
(2)
2 G 2)
and for pulse B :
A 10A , GA/ Gzy
B(t) = 7PB(t) L ¨ P ¨ G + __ log(e) 10 '10 _1 +10 10 ¨
(3)
2 2
_)
As mentioned above, the parameters of the reciprocal loop should be selected
so as to
favor coupling of the light power returning to the main coupler 32 for both
pulses 36A
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and 36B towards the propagation direction of the non-reciprocal loop 24.
Referring
to FIG. 10 for the identification of each branch of the cavity on either sides
of the
main coupler, the electric field associated with the light coupling from the
non
reciprocal loop to the reciprocal loop is governed by the following equations:
E3= aEl+i[1- 41' E2
(4)
E4 = - af2 a'12 E2
(5)
E1 p = al/2 E p - a r2 E3p
(6)
E2p = - E4p + aV2E3p
(7)
where E, is the electric field in branch i and a is the coupling ratio of the
coupler. For
the returning pulses from the reciprocal loop at the coupler, we have:
E 0 20 e
"
3p 3
(8)
E4p =10 20 ew+841E4
(9)
where 0 is the linear phase shift associated with the propagation inside the
loop.
In the laser configuration of embodiments of the invention, light from the non-
reciprocal loop propagates in a single direction, and the input in the
reciprocal loop
at the coupler is at E2 only. Consequently, the analysis can be simplified by
assuming that El = 0, and a round-trip inside the laser cavity implies that
the output
of the loop mirror is at Eli, :
25E GV õ = E =10 '" ek'+3"aE -10GA/ " e"'B" - et)E2
out ip 2
(10)
E2
(11)
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In the case of a 50/50 coupler, the coupling coefficient a is 0.5. In
addition, the phase
shift between the returning light pulses at the coupler can be expressed as Ag
where
B4 = B3+ AB. With these considerations, the power outputted by the coupler
after
the interference between the returning pulses can be expressed as:
1 Gy
Pout = E,P E = ¨10 1 cos(AB)) (12)
- IP 2
=EE; (13)
from which it is apparent that the output power will be maximal for A8=71-.
Returning to
the expression of the accumulated nonlinear phase shift for pulse A and B,
this
imposes the condition:
B AO¨ BB(t)=-. 7Z" (14)
As BA(t) and BB(t) depend on the length L of the reciprocal loop and the
position P of
the gain medium therealong, these parameters can be jointly selected so that
the
condition of equation (14) is met.
Of course, one skilled in the art will understand that in different
embodiments, for
example if the main coupler is not 50/50 as above, the optimal value for 6.8
could
differ from 7c. Indeed, studies of the effect of gain saturation on the laser
cavity
described above seem to indicate that the optimal ratio to obtain maximum
coupling
into the propagation direction of the non-reciprocal loop could be up to
60/40, and
may vary with the choice of laser components and operating conditions. It is
to be
noted that the coupling coefficient may also be wavelength-dependent, as one
skilled
in the art will know to take into account in the design of the laser cavity
and selection
of the main coupler.
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In addition, it will be further understood that the position of the gain
medium in the
reciprocal loop could be chosen so as to offset the value for which the
outputted
power in the propagation direction of the non reciprocal loop is maximum, if
resulting
losses from the light coupled into the opposite direction of the non-
reciprocal loop
are acceptably low for the application considered.
Referring back to FIGs. 1A and 1B, the fiber laser 20 further includes pump
control
means 42 for controlling the pump power value of the pump source within a
range
extending above a lasing threshold of the laser cavity 22. By changing the
pump
power value of the pump source 38 within this range, the pulsewidth of the
light
pulses generated by the fiber laser 20 can also be changed.
From equations (2), (3) and (14) above, in the case where a = 0.5 the input
power
PA and PB are the same, which brings :
GA/
7PA(2P - L) 1-10 /1 = 71"
(15)
Using equation (15) to deduce PA(t), and a, = 2PA(t), equation (12) becomes:
GA/
GA/ 27z-10 /10
Poõ,(t)= 2PA (010 /10 = ________________________________________
GA/
(16)
y 2P - L[10 /10_i
Introducing by definition the asymmetry factor As :
As = ¨ 2P- L
(17)
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which can vary between 0 and 1, The optimal output power can then be expressed
as:
cv
2n-10 /1
P -- _________________
out,optimal GA/ (18)
y L As [10 /1 ¨11
it can be seen from equation (18) that the peak power of the pulses out of the
reciprocal loop 26 will be dependent on the gain inside the loop (GA), the
length of
the loop L, and the position P of the gain medium 34 relative to the length L
of the
fiber inside the reciprocal loop 26.
The basic principle of a laser configuration is that the gain is equal to the
losses
inside the cavity:
GA=cavity losses (dB) (19)
If the reciprocal loop 26 is placed inside a laser resonator, and lasing
action is
achieved, the gain will be fixed and its value will be independent of the
pumping
conditions. Therefore an optimal value for the peak power will exist and it
will only be
dependent on the geometrical configuration of the loop 26 (length and
asymmetry
factor).
Referring to FIG. 11, the average output power of a laser is defined by its
lasing
threshold and its slope efficiency:
output power = slope efficiency X (pump power¨ lasing threshold) (20)
Equation (20) being valid only above the lasing threshold. If the laser is
pulsed, its
pulse energy is defined as follow:
1
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output power
Pulse energy = _________________________________________________ (21)
repetition rate
For square optical pulses, the pulse energy can also be expressed as:
Pulse energy = peak power x pulse duration (22)
Combining equations (21) and (22), we obtain finally:
slope efficiency x (pump power ¨ la sin g threshold)
pulse duration = (23)
peak power x repetition rate
If the configuration of the reciprocal loop is such as the laser cannot
achieve
threshold in CW mode because the optimal peak power is too high, it will then
need to
operate in pulsed mode. In equation (23) the peak power is fixed, the slope
efficiency
is fixed, the lasing threshold is fixed, and the fundamental repetition rate
is fixed since
it is related to the length of the laser cavity. The only parameter that can
vary with the
pump power is therefore the pulse duration. The square pulse is the natural
pulse
shape associated with this kind of laser since it is a shape with a constant
peak
power.
The analysis above neglects gain saturation effects inside a same pulse and
cross
effect between the two pulses propagating in the reciprocal loop. The effects
of gain
saturation are of second-order. Depending on the application, it may be
preferable for
the width of the gain medium A to be negligible with respect to the length L
of the
reciprocal loop in order for the asymmetry factor As to be sufficiently
independent of
the pump power inside the loop mirror.
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As one skilled in the art will readily understand, the analysis above
demonstrates that
in a laser configuration according to embodiments of the invention, there is a
direct
relationship between the value of the pump power launched into the gain medium
and
the pulsewidth of the resulting pulses generated within the cavity. This
results into an
advantageous mechanism for controlling the pulsewidth of the output pulses
through
a simple control of the pump power, within a range extending above the
operational
lasing threshold.
From the reasoning above, the expression "operational lasing threshold" is
understood to refer to the state where lasing is achieved through the
balancing of
cavity losses and gain. As one skilled in the art will readily understand,
lasers of the
type described above may present hysteresis with respect to pump power, that
is, the
pump must be increased above a first threshold in order to achieve lasing, but
once
this is done, the pump can be lowered to a second threshold just above a value
for
which lasing would stop. In such circumstances, the second lower threshold
would be
the parameter above which the pump power value is changed in order to obtain
the
desired pulsewidth adjusting effect.
The pump control means may be embodied by a control of the current feeding the
pump source, which can for example be a laser diode. Alternatively, the
attenuation
between the pump source and the fiber laser may be varied through the use of
one or
more attenuation component therebetween.
Referring back to FIGs. 1A and 1B, according to an embodiment of the invention
the
fiber laser 20 further includes an additional gain medium 46 provided in the
non
reciprocal loop 24. An additional pump source 48 is coupled to the non-
reciprocal
loop 24, preferably through a WDM additional pump coupler 50, to launch an
additional pump light beam therein. Advantageously, varying the pump power
value of
CA 02763828 2011-11-29
the additional pump source 48, through appropriate additional pump control
means
52, modulates the amount of losses associated with the non-reciprocal loop 24
of
the laser cavity 22. As will be understood from the analysis above, changing
cavity
losses will fix the gain in the reciprocal loop to a different value (equation
(19)),
5 therefore impacting the peak power of the pulses generated. Controlling
the pump
power value of the additional pump source 48 of the non-reciprocal loop 24
will
therefore have no impact on the pulsewidth, but offers a means of controlling
the
peak power of the light pulses. The additional gain medium and its associated
pumping apparatus are optional to the operation of the laser cavity 22, but
give
10 added flexibility to control the output power of the fiberlaser 20 and
extend the
available pulsewidth range in the low values as it keeps the laser above the
lasing
threshold.
FIG. 6 illustrates the simulated laser performance as a function of pump power
in
15 both gain media, using as an example a cavity having a repetition rate
of 16 MHz
with a single pulse circulating per round-trip.
An empirical analysis of these simulation results brings the introduction of
the
parameter CNIL, which is defined as follow:
, As[ [m [W
[W
114 I (24)
Where As is the asymmetry factor, L is the length of the reciprocal loop,
Preop is the
pump power in the reciprocal loop and M is the number of pulses per round-
trip.
Referring to FIG. 7, the pulse duration obtained in the simulation mentioned
above
was plotted as a function of the characteristic parameter CAL. This graph may
serve
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as an interesting design tool for the laser cavity according to embodiments of
the
invention, as the parameter CNL is dependent on characteristics of the
reciprocal loop.
Referring back to FIGs. 1A and 1B, preferably, the fiber laser 20 includes a
narrowband filter 54 within the laser cavity 22 which selects the spectral
band of
operation of the laser 20. Typical spectral bands can be selected in the range
of
1032, 1050, 1064, 1080, 1530, 1550, 1560 nm, etc. The narrowband filter 54 may
for
example be embodied by an interferometric filter or a thin-film filter,
preferably
pigtailed. By "narrowband", it is understood that filter 54 may cover any
range of
wavelengths appropriate for a target application, for example 5 nm around the
central
wavelength. It is of interest to note that the narrowband filter 54 does not
play a
significant role in the propagation dynamic of the light pulses within the
laser cavity.
In the illustrated embodiments of the invention, the laser cavity 22 is
operated in a
passive mode-locked regime. Any appropriate means of mode-locking the laser
cavity
22 on the desired operation parameters may be provided. Referring still to
FIGs. 1A
and 1B, a mode-locking modulator 56 is disposed within the non-reciprocal loop
24
and is actuated at a modulator frequency defining the repetition rate on the
light
pulses in the laser cavity 22. The modulator frequency corresponds to an
integer
multiple of a fundamental repetition rate of the laser cavity 22. The mode-
locking
modulator 56 may for example be embodied by a pigtailed electro-optic phase or
amplitude modulator. A repetition rate controller 64 is preferably coupled to
the mode-
locking modulator 56 for setting the repetition rate.
The mode-locking modulator 56 advantageously facilitates the ignition of the
mode-
locked regime and its synchronization, but could potentially be switched off
if the laser
20 is operated at the fundamental repetition rate, defined by the round-trip
time
around the laser cavity 22. The fundamental repetition rate may not, however,
be
suitable for all embodiments and applications. For example, supposing a laser
cavity
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22 having 20 m of fiber, the fundamental repetition rate of the laser design
would be
around 10 MHz. If one would select one pulse through a pulse selection
mechanism
for a particular application, the jitter time could be up to 100 ns between a
pulse
request and the emission of an optical pulse. This temporal jitter is
considered too
high for industrial micro-machining applications where high-speed translation
stages
are used and precise synchronization with the laser source are necessary. To
decrease this jitter time down to a more acceptable 2 ns, with a laser
operating at the
fundamental repetition rate of the cavity, would require a 500 MHz oscillator,
which
translates to a cavity length 40 cm long. This would be very difficult to
achieve using
present day optical fiber technology. The operation of the laser on a multiple
of the
fundamental repetition rate is therefore a requirement for some of the
targeted
applications.
As one skilled in the art will readily understand, increasing the repetition
rate inside
the cavity will decrease the pulse energy (the average power being held
constant),
and since the optimal peak power is fixed by the cavity losses, the pulse
duration will
have to decrease. Consequently, the tunability range of the pulse duration
will depend
on the chosen repetition rate.
A feedback loop 58 may also be provided for locking the modulator frequency on
the
repetition rate of the light pulses. This ensures stable operation in
environmentally
varying conditions which can affect the fundamental repetition rate. The
feedback
loop 58 preferably includes a modulator tap 60, for example embodied by a
coupler,
extracting a fraction of the pulse energy circulating in the non-reciprocal
loop 24. A
repetition rate analyzer 62 measures the repetition rate of the light pulses
and
forwards the information to the repetition rate controller 64. In the
embodiment of FIG.
1B, a bias controller 68 receives the tap signal from the modulator tap 60 for
adjusting
the bias point of an electro optic, Mach-Zehnder, amplitude modulator for long-
term
stability without drift of the amplitude modulator operating point.
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A pulse extraction assembly 70 is connected to the non-reciprocal loop 24 for
extracting a number, or a pattern of light pulses from the laser cavity 22.
The pulse
extraction assembly 70 preferably includes an output fiber 72 and an output
coupler
74 disposed in the non-reciprocal loop 24 downstream the main coupler 32 along
the propagation direction. The output coupler 74 therefore couples optical
pulses
into the output fiber 72. Preferably, the pulse selection is controlled by an
output
amplitude modulator 76 disposed in the output fiber 72, which can select one
pulse
or a pattern of pulses at the request of an external trigger. An external
trigger signal
66 may be provided to the repetition rate controller 64 to synchronize the
pulse
extraction pattern signal sent to the modulator 76 with the circulating
optical pulses
in the cavity. The output modulator may for example be a phase or amplitude
modulator. It may be advantageous to operate the laser at a high repetition
rate, as
it diminishes the maximum jitter between a trigger request and the produced
pattern,
is this maximum jitter being equal to the period of the laser pulse train.
If an electro
optic amplitude modulator is used in the pulse extraction assembly, an output
modulator tap loop 80 and bias controller 82 may be necessary for long-term
drift-
free stability of the system. Advantageously, the configuration of the pulse
extraction
assembly shown in FIGs. 1A and 1B provides a "pulse-picking" capability which
can
be user controlled. Other configurations could however be considered without
departing from the scope of the invention.
As mentioned above, the entire laser cavity 22 is preferably made of PM fiber
in order to keep only the interferometric aspect of the nonlinear mirror for
the
generation of square picosecond pulses. It is known in the art that nonlinear
loop mirror made with non-PM components and fibers can demonstrate
nonlinear polarization rotation which complicates the short pulse dynamic as
it can act as a fast saturable absorber for femtosecond pulse generation. A
polarizer 78 is preferably provided in the cavity 22 and fixes a linear state
of
polarization aligned with either the slow or fast axis of the
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PCT/CA2009/000938
19
PM fiber. This device is optional as other components in the cavity could be
polarization dependent.
Experimental demonstration
The design of FIG. 1A was fabricated in one exemplary embodiment of the
invention,
omitting the mode-locking modulator 56 and associated components. The laser
cavity
was operated at 1064 nm, using components compatible with this wavelength, and
both gain medium 34 and 46 were embodied by Yb-doped fiber. The selection of
these parameters was related to the dispersion requirements of the design.
If the laser was operated in the 1550 nm window with Er-doped fibers, the
dispersion
of the fibers could be mostly anomalous. In that case soliton formation would
be
possible. The threshold for soliton formation is:
y/30 T 02
N = __ . 0.5 _____________________ (25)
\ A
The energy of a soliton is given by:
E = 2P01o (26)
Consequently the soliton formation threshold in energy is :
1
E 0.5fi2 (27)
yTo
For typical values associated with standard telecom fibers, the maximum energy
attainable is 800 fJ for 10 ps pulses and 80 fJ for 100 ps pulses. For
targeted
applications those values are extremely low. Soliton shaping mechanisms are
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WO 2011/003166 PCT/CA2009/000938
detrimental in the picosecond regime as pulse break-up could occur, and
consequently normal dispersion fiber should be employed throughout the laser
cavity
if high energy pulses are desired.
Referring to FIGs. 2 to 5, preliminary results obtained experimentally with
the
proposed configuration are shown to illustrate the kind of performance which
can be
expected.
FIG. 2 depicts the measured temporal pulse shape obtained with a fast
photodiode
(risetime of 35 ps) as a function of the pump power inside the NALM. Measured
pulse
durations vary from 50 Ps to 270 Ps. The energy per pulse is around 1 nJ,
typical
linewidth is 0.3 nm and the fundamental repetition rate is 8 MHz. referring to
FIG. 3,
other pulses measures using an autocorrelator are shown. Pulse durations as
low as
17 Ps could be measured in this case. It can clearly be seen from FIG. 3 that
the peak
power is held constant while the pump power inside the reciprocal loop is
changed to
vary the pulse duration.
Using the fundamental repetition rate, the highest pulse energy achieved was
near 20
nJ and the associated linewidth was around 0.3 nm.
Typical pulses spectra are shown on FIG. 4. It is to be noted that the shorter
the
pulse, the wider the spectrum. The longest pulses which could be generated in
this
experiment had a duration of 500 Ps (see Fig. 5), with a linewidth less than
0.06 nm
(limited by the resolution of optical spectrum analyzer).
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.
