Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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LOW-AVERAGE-POWER PARABOLIC PULSE AMPLIFICATION
TECHNICAL FIELD
The present invention relates to optical amplifiers. More
particularly, the present invention relates to parabolic pulse
amplification.
BACKGROUND ART
Optical fibre amplifiers are used to amplify either continuous
or pulsed optical signals. There are two main techniques for
amplifying ultrashort light pulses, such as femtosecond
pulses, using optical fibre amplifiers.
The first technique, called fibre chirped-pulse amplification,
uses a combination of three subcomponents, namely a stretcher,
an optical fibre amplifier and a compressor. The stretcher is
a dispersive optical element which introduces a spectral chirp
in the short pulses to be amplified so that the frequency
content of each pulse is spread over time. As a result, the
pulse duration is increased, and the peak power of the pulse
is reduced.
The chirped pulses are then injected in a standard optical
fibre amplifier. Pulse amplification takes place with low
nonlinear effects, as the peak power is reduced. After
amplification, a dispersive compressor is used to bring the
frequency components of the amplified pulses back in phase,
causing the pulse to retrieve its original (short) duration.
A second and more recent technique is called parabolic pulse
amplification (see Fermann, "Self-Similar Propagation and
Amplification of Parabolic Pulses in Optical Fibers", Physical
Review Letters 84 #26, p6010 (26 June 2000)). The origin of
this technique is the observation of an asymptotic solution to
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the NonLinear Schrodinger Equation (NLSE) for short pulses
guided in an optical fibre showing gain and normal dispersion.
The shape of the pulse corresponding to this asymptotic
solution is a parabola, hence the name parabolic pulse
amplification. A pulse being amplified in the parabolic regime
gets an increasingly broader spectrum and a linear chirp
together with a higher energy as it propagates in the optical
waveguide.
Parabolic pulse amplification is typically used in
amplification of femtosecond pulses produced by a femtosecond
laser oscillator, such as a mode-locked fibre laser. A
femtosecond fibre laser oscillator usually has a pulse
repetition frequency between 5 and 100 MHz. On the other hand,
for some applications such as material processing, a pulse
repetition frequency of about 100 kHz is desirable. In
parabolic pulse amplification, an adequate balance of
dispersion, nonlinearity and gain has to be present throughout
the length of the amplifier for the parabolic asymptotic
solution to be reached. Reducing the pulse repetition
frequency of the input signal, and consequently reducing the
average power of the input signal, results in a small signal
gain regime in the front end of the amplifier. This results in
an unbalanced gain and to improper conditions for parabolic
pulse amplification.
SUMMARY OF INVENTION
Therefore, in accordance with an aspect of the present
invention, there is provided a parabolic pulse amplifier for
amplifying a low-average-power pulse light signal. An example
application of the provided amplifier is the amplification of
a low-repetition-frequency ultrashort-pulse light signal such
as a low-repetition-frequency femtosecond pulse signal. High-
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repetition-frequency short-pulse signals with low-power pulses
are also regarded as low-average-power pulse light signal.
In accordance with an aspect, there is provided a parabolic
pulse amplifier for amplifying a pulse light signal. The
amplifier comprises an ytterbium-doped amplification waveguide
pumped using a pump source with a pump central wavelength
substantially offset from the absorption transition peak
wavelength. The pump wavelength is selected such that the
absorption coefficient of pump light and the gain coefficient
of the signal are substantially equal in the amplification
waveguide such that the amplification gain is distributed
substantially uniformly along the amplification waveguide.
Another aspect provides a method for amplifying a pulse light
signal having a signal wavelength. A rare-earth-doped
amplification waveguide is provided and has at least one
absorption transition peak wavelength and has a waveguide
length. A first pump light is provided and has a first pump
central wavelength substantially greater than the at least one
absorption transition peak wavelength, lower than the signal
wavelength and being selected such that an absorption of the
first pump light and an amplification gain of the signal are
substantially equal in the amplification waveguide. The first
pump light is propagated in the amplification waveguide.
Absorption of the first pump light in the amplification
waveguide provides the amplification gain such that it is
distributed substantially uniformly along the waveguide
length. The pulse light signal to be amplified is coupled to
the amplification waveguide. The pulse light signal reaches a
parabolic pulse asymptotic solution as it propagates along the
amplification waveguide to provide an amplified parabolic
pulse light signal.
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Yet another aspect provides a parabolic pulse amplifier for
amplifying a pulse light signal having a signal wavelength.
The amplifier comprises a rare-earth-doped amplification
waveguide having an absorption transition peak wavelength and
a first pump source coupled to the amplification waveguide.
The pulse light signal is also coupled to the amplification
waveguide for parabolic pulse amplification. The first pump
source provides a first pump light with a first pump central
wavelength substantially offset from the absorption transition
peak wavelength and lower than the signal wavelength. An
absorption of the first pump light in the amplification
waveguide provides an amplification gain along the
amplification waveguide. The first pump central wavelength is
selected such that the absorption of the first pump light and
the amplification gain of the signal are substantially equal
in the amplification waveguide such that the amplification
gain is distributed substantially uniformly along the
amplification waveguide. The pulse light signal reaches a
parabolic pulse asymptotic solution as it propagates along the
amplification waveguide to provide an amplified parabolic
pulse light signal.
Still another aspect provides a low repetition frequency
femtosecond pulse source comprising a femtosecond pulse source
for generating a high repetition frequency femtosecond pulse
light signal having a signal wavelength; a pulse picker for
selecting part of pulses of the high repetition frequency
femtosecond pulse light signal to produce a low repetition
frequency femtosecond pulse light signal; a parabolic pulse
amplifier for amplifying the low repetition frequency
femtosecond pulse light signal; and a dispersive compressor
for compressing the amplified parabolic pulse light signal to
produce an amplified low repetition frequency femtosecond
pulse light signal. The amplifier has a rare-earth-doped
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amplification waveguide having an absorption transition peak
wavelength and receiving the low repetition frequency
femtosecond pulse light signal; and a first pump source
coupled to the amplification waveguide for providing a first
pump light with a first pump central wavelength substantially
offset from the absorption transition peak wavelength and
lower than the signal wavelength. An absorption of the first
pump light in the amplification waveguide provides an
amplification gain along the amplification waveguide. The
first pump central wavelength is selected such that the
absorption of the first pump light and the amplification gain
of the low repetition frequency femtosecond pulse light signal
are substantially equal in the amplification waveguide such
that the amplification gain is substantially uniform along the
amplification waveguide. The low repetition frequency
femtosecond pulse light signal reaches a parabolic pulse
asymptotic solution as it propagates along the amplification
waveguide to provide an amplified parabolic pulse light
signal.
Still another aspect provides a parabolic pulse amplifier for
amplifying a pulse light signal having a signal wavelength.
The amplifier comprises an in-line pump source for providing a
source pump light and an ytterbium-doped amplification
waveguide coupled to an output of the pump source waveguide.
The in-line pump source comprises a primary pump source for
providing a primary pump light having a primary pump central
wavelength; an ytterbium-doped pump source waveguide receiving
the diode pump light and the pulse light signal, and having an
absorption transition peak wavelength of about 976 nanometers
and an emission transition peak wavelength of about
1030 nanometers. The primary pump central wavelength
corresponds to the absorption transition peak wavelength and
an absorption of the primary pump light in the pump source
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waveguide generates amplified spontaneous emission at least at
the emission transition peak wavelength to produce a source
pump light. The pulse light signal propagating in the pump
source waveguide is to exit at an output of the pump source
waveguide. The ytterbium-doped amplification waveguide is
coupled to an output of the pump source waveguide such that
the source pump light produced in the pump source waveguide
and the pulse light signal exiting the pump source waveguide
are both coupled to the amplification waveguide for parabolic
pulse amplification. An absorption of the source pump light in
the amplification waveguide provides an amplification gain
along the amplification waveguide. The pulse light signal
reaches a parabolic pulse asymptotic solution as it propagates
along the amplification waveguide to provide an amplified
parabolic pulse light signal.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a block diagram illustrating a low repetition
frequency short-pulse source incorporating a parabolic pulse
amplifier;
Fig. 2 is a block diagram showing the main components of a
parabolic pulse amplifier such as the one used in the short-
pulse source of Fig. 1;
Fig. 3 is a graph showing the absorption and emission
transition cross-section spectra of the ytterbium ion in a
silica glass optical waveguide;
Fig. 4 comprises Fig. 4A, Fig. 4B and Fig. 4C which are graphs
showing the amplification dynamic of an ytterbium-doped
optical amplifier, wherein Fig. 4A shows the 976-nm pumped
amplification of a high Pulse Repetition Frequency (PRF)
signal, Fig. 4B shows the 976-nm pumped amplification of a low
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PRF signal and Fig. 4C shows the 1032-nm pumped amplification
of a low PRF signal;
Fig. 5 is a block diagram illustrating a parabolic pulse
amplifier wherein a pump source is independently provided;
Fig. 6 is a block diagram illustrating another parabolic pulse
amplifier wherein a fibre laser is used as an in-line pump
source;
Fig. 7 is a block diagram illustrating still another parabolic
pulse amplifier wherein an amplified spontaneous emission
source is used as an in-line pump source; and
Fig. 8 is a block diagram illustrating another parabolic pulse
amplifier wherein a polarisation dependent fibre laser is used
as an in-line pump source and used polarization dependent
reflective filters.
It will be noted that throughout the appended drawings, like
features are identified by like reference numerals.
DETAILED DESCRIPTION
Referring now to the drawings, Fig. 1 shows a low-repetition-
frequency short-pulse light source system 10 incorporating a
parabolic pulse amplifier 12. The system 10 is used to produce
a low-Pulse-Repetition-Frequency (PRF) femtosecond pulse light
signal having a wavelength of 1064 nm, from a high-PRF pulse
laser source 16. The system 10 comprises a low-PRF pulse light
source 14 which is amplified using a parabolic pulse amplifier
12. In this example, the low-PRF pulse light source 14 is
provided by cascading a high-PRF pulse light source 16 and a
pulse picker 18 for selecting a fraction of the pulses
produced by the high-PRF pulse light source 16 in order to
reduce the PRF.
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In this case, the high-PRF pulse laser source 16 is a
commercially available mode-locked fibre laser manufactured by
Calmar OptcomTM. Femtosecond fibre laser oscillators usually
produce a high-PRF pulse signal 50 having a PRF of the order
of 5 to 100 MHz. For many applications, a PRF below 5 MHz, and
typically in the order of 100 kHz, is more desirable. In this
case, the high-PRF pulse laser source 16 produces a high-PRF
pulse signal 50 having a PRF of 23.5125 MHz with an output
power of 29 mW and pulse duration of 1.08 ps.
One possible solution for producing a low-PRF signal is to
first amplify a high-PRF signal and then reduce the PRF using
a pulse picker placed at the output of the amplifier. This
scheme is not ideal for various reasons including the fact
that it is not quite efficient to use the amplification gain
to amplify pulses that are then discarded.
In the system 10, the pulse picker 18 is placed before the
parabolic pulse amplifier 12 to receive the high-PRF pulse
signal 50 and select some of the pulses to generate a low-PRF
pulse signal 52. A pulse picker 18 is usually made of an
acousto-optic modulator or an electro-optic modulator. In this
case, the pulse picker 18 is a Photline TechnologiesTM NIR-MX-
LN03-00-P-P electro-optic modulator having an extinction ratio
of 30 dB and insertion losses of 5 dB. Placing the pulse
picker 18 before the parabolic pulse amplifier 12 is more
efficient as pulses are selected before extracting energy from
the parabolic pulse amplifier 12. Moreover, the optical
damage threshold requirements of the pulse picker 18 are
relaxed when the blocked pulses are low-energy pulses.
The parabolic pulse amplifier 12 receives the low-PRF pulse
signal 52 for amplification. The low-PRF pulse signal 52
reaches a parabolic pulse asymptotic solution as it propagates
along the parabolic pulse amplifier 12 to provide an amplified
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parabolic pulse signal 54. According to the asymptotic
solution of the nonlinear Schrodinger equation, the high-
energy pulses of the resultant amplified parabolic pulse
signal 54 has a broad spectrum and linear chirp, the pulses
are thus stretched over time. The linear chirp therefore needs
to be compensated for using a dispersive compressor 20 having
an opposite chirp. The parabolic pulses are thus recompressed,
which result is an amplified short pulse signal 56. The
dispersive compressor 20 consists, for instance, of a portion
of anormal dispersion compensating optical fibre, a pair of
diffraction gratings or dispersive prisms or a combination of
the last two, namely a pair of grisms. In this case, the
dispersive compressor 20 consists of a pair of 715.703.060 PC
0900 30x30x6 holographic diffraction gratings from
SpectrogonT"', having 900 lines per millimetres optimized at
1064 nm. The pair of gratings is arranged for double
propagation in the compressor using a hollow roof mirror. The
deviation angle on the gratings is of 12 degrees.
Fig. 2 shows the main components of a parabolic pulse
amplifier 12 such as the one used in the system 10 of Fig. 1.
The parabolic pulse amplifier 12 comprises an ytterbium-doped
amplification waveguide 30 to which the low-PRF pulse signal
52 to be amplified, or a low-average-power input pulse signal,
is coupled for parabolic pulse amplification. In the
illustrated example case, the low-PRF pulse signal 52 to be
amplified has a wavelength of about 1064 nm but it is noted
that the signal wavelength can be varied. The amplification
waveguide 30 is pumped using a pump source 32 having a central
wavelength selected to be of about 1032 nm as will be
explained hereinbelow, for providing the proper parabolic
pulse asymptotic conditions in the case of a low-average-power
input pulse signal. An absorption of the pump light 58 in the
amplification waveguide 30 provides an amplification gain
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which is distributed along the amplification waveguide 30. The
result is an amplified parabolic pulse signal 54 that can be
recompressed using a dispersive compressor 20 as shown in
Fig. 1 to provide an amplified short-pulse signal.
As a consequence of their duration, short pulses are usually
characterized by a high peak power. When such pulses are
guided in an optical waveguide, this high peak power spurs
nonlinear optical effects, such as self-phase modulation.
Propagation of a pulse signal 52 through an amplification
waveguide 30 is governed by the nonlinear Schrodinger
equation, which takes into account self-phase modulation,
group-velocity dispersion, background loss, and amplification
gain. As a rule of thumb, nonlinear effects become significant
once the pulse has been guided over a fibre length
corresponding to the nonlinear length, LNL, given by:
Ae~. ~,
LN` 2;rn2 Po
wherein Aeff i.s the effective area of the waveguide core, X is
the wavelength, n2 is the nonlinear index, and Po is the pulse
peak power. Since the nonlinear index n2 is a constant for the
optical waveguide material, nonlinear effects are significant
after propagation over a short waveguide strand when the core
effective area Aeff is small, and when the peak power Po is
large.
Parabolic pulses correspond to an asymptotic solution to the
nonlinear Schrodinger equation, which describes the
propagation of light in optical waveguide amplifiers; for
short pulses propagating in an optical waveguide showing gain
and normal dispersion. According to the nonlinear Schrodinger
equation, a combination of normal dispersion, self-phase
modulation and distributed gain in an amplification waveguide
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(i.e. a rare-earth doped waveguide) results into a self-
similar asymptotic solution wherein the pulse acquires a
parabolic shape in time and a linear chirp. In these
conditions, the parabolic pulse propagates in the
amplification waveguide, and gets amplified and acquires a
linear chirp, while maintaining its parabolic shape. The
parabolic pulse is the asymptotic solution of the nonlinear
Schrodinger equation for long propagation lengths. The pulse
characteristics are determined by the energy of the incoming
pulse signal 52 and the parameters of the parabolic pulse
amplifier 12, and are independent of the shape of the incoming
pulse signal 52. This means that any pulse propagated over a
sufficient length of amplification waveguide under the above-
mentioned conditions will reach the parabolic pulse asymptotic
solution, independently of the input shape of the pulses.
The parabolic asymptotic solution is characterized by the
following first equation which links the energy of the
parabolic pulse E and its overall half duration Tp with the
amplification waveguide constants:
E_ 2g2
T3 27y,82 (2)
P
and by the following second equation describing the temporal
frequency spread of the pulse by linking the instantaneous
angular frequency winst(t) of the pulse to amplification
waveguide characteristics:
Winst \t ) - CO 0 + g t
3,82
(3)
wherein g is the linear gain coefficient in m-1, y is the non-
linear coefficient which is inversely proportional to the
effective area of the mode, and P2 is the dispersion
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coefficient of the optical waveguide. Equation (2) defines the
specific asymptotic solution corresponding to given
amplification conditions.
Since this amplification principle leans on an asymptotic
solution, short pulses need to travel over a certain fibre
length before being qualified as parabolic. An adequate
balance of dispersion, self-phase modulation and amplification
gain has to be present throughout the length of the
amplification waveguide 30. Consequently, one has to optimize
the gain per unit length together with the total gain of the
amplifier. Studies show that performances of a parabolic pulse
amplifier having a gain per unit length too low will be
limited by stimulated Raman scattering. On the other hand,
parabolic pulse amplification in an amplification waveguide 30
having a gain per unit length too high will be limited by the
gain bandwidth.
The present parabolic pulse amplifier 12 is illustrated herein
using a silica ytterbium-doped optical waveguide, which ion
transition cross-sections are shown in Fig. 3. Ytterbium-doped
optical waveguide amplifiers are generally pumped using a pump
source having a central wavelength around 976 nm which
corresponds to the emission (E curve) and to the absorption
(A curve) transition cross-section peak P. The emission cross-
section at 1064 nm is about 0.25 pm2, whereas the peak
absorption cross-section at 976 nm is about 2.65 pm2.
The amplification gain coefficient in an optical amplifier is
defined as follows:
g- rs (N2 6e,s - N16a,s ) i (4)
wherein N1 and N2 are respectively the ground state population
density and the metastable state population density in the
amplification waveguide, 6a,g and 6e,s are respectively the
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absorption and the emission cross-sections at the signal
wavelength, and rs is the confinement factor between the
signal field and the rare-earth population. The pump
absorption coefficient in an optical amplifier is defined as
follows:
a rP (N2 6e,P - Ni 6a,P ) (['J )
wherein aa,p and 6e,P are respectively the absorption and the
emission cross-sections at the pump wavelength, and rp is the
confinement factor between the pump field and the rare-earth
population.
As in the embodiment of Fig. 2, pumping around 1032 nm
provides a pump absorption coefficient a which is similar to
the gain coefficient g at the signal wavelength 1064-nm. It is
consequently possible to design an amplifier having a gain
which is substantially uniform over its length, while
absorbing most of the available pump power. Suitable
conditions for parabolic pulse amplification can then
consequently be reached.
It is noted that the pump wavelength can be varied within
certain conditions. First, the pump wavelength should be
sufficiently offset from the cross-section peak P such that
the pump absorption cross-section aa,P is substantially reduced
compared to the peak absorption cross-section 6a,976 at peak P
corresponding to the absorption transition wavelength
(976 nm). The pump wavelength should of course be lower than
the signal wavelength to impart gain at the signal wavelength.
In order to avoid high generation of ASE at the cross-section
peak P, the pump wavelength is also selected above the cross-
section peak P at 976 nm. A pump wavelength between about 1015
and 1045 nm is typically suitable to provide a pump absorption
coefficient a which is similar to the signal gain coefficient
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g. An appropriate pump wavelength may be selected as a
function of the signal wavelength and average power.
Fig. 4 compares the amplification dynamic of three ytterbium-
doped optical amplifiers through numerical simulations.
Fig. 4A shows the 976-nm pumped amplification of a high-PRF
signal of 20 MHz. The average signal input power is 20 mW in
this case. It can be appreciated that the gain coefficient of
the signal is substantially uniform over the first 6.5 meters
of the optical waveguide. These amplification conditions
lo should then be proper for parabolic pulse amplification by
reducing the optical waveguide length to 6.5 meters.
Fig. 4B shows the amplification dynamic obtained with the
same ytterbium-doped optical waveguide but when a pulse picker
is used to reduce the PRF of the signal to 300 kHz. The pump
wavelength is still 976 nm. The average signal input power is
95 uW in this case. Fig. 4B shows that in these small signal
conditions the gain coefficient of the signal is substantially
uniform over the first 11.5 meters of the optical waveguide.
However, it is also shown that, due to the small signal
conditions, the Amplified Spontaneous Emission (ASE) generated
over this length is substantially more important. The high ASE
level at the output of the amplifier is an important drawback.
Whereas adequate amplification at 1064 nm can nonetheless be
carried out, ASE at 1032 nm would use a significant part of
the pump power, therefore curtailing the potential gain at the
signal wavelength and spoiling the signal-to-ASE ratio of the
amplifier. It is noted that in the case of small signal
amplification in a non parabolic pulse amplifier, the optical
waveguide could be made longer (more than 20 m in the present
conditions) so that the ASE generated around 1030 nm is
reabsorbed in favour of the signal to be amplified. This is
however not applicable to a parabolic pulse amplifier as the
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signal gain should be substantially uniform along a parabolic
pulse amplifier.
Fig. 4C shows the amplification dynamic obtained with a
similar ytterbium-doped optical waveguide using a pulse picker
to reduce the PRF of the signal to 300 kHz but when the pump
wavelength is 1032 nm rather than 976 nm. The average signal
input power is also 95 uW. Even in these small signal
conditions, it can be seen that the gain coefficient of the
signal is substantially uniform over the first 12.5 meters of
the optical waveguide. This results from the lower absorption
cross-section at 1032 nm which stretches the pump power
absorption, and consequently the signal gain, over a longer
portion of optical waveguide. Less ASE is also generated as
the pump wavelength is above the second emission cross-section
peak located which is located at about 1030 nm. It is noted
that the optical waveguide used in this case was more strongly
doped in ytterbium than in the two other simulations, in order
to increase absorption at 1030 nm. The ytterbium-doped optical
fibre used in this simulation has a maximum absorption of
530 dB/m at 976 nm.
Fig. 5 shows an example parabolic pulse amplifier 112 for
amplifying a low-average-power pulse signal 152 with a signal
wavelength of about 1064 nm, and which can be used in the
system 10 of Fig. 1. An ytterbium-doped amplification optical
fibre 130 consists of a 65-m-long ytterbium-doped optical
fibre and is pumped in copropagation with the low-average-
power pulse signal 152 to be amplified using an independent
pump source 132 at about 1032 nm. The pump source 132 and the
low-average-power pulse signal 152 are coupled to the
amplification optical fibre 130 using a multiplexer 134, for
providing an amplified parabolic pulse signal 154 at the
output of the amplification fibre 130.
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The multiplexer 134 is a dichroic coupler receiving the signal
152 at 1064 nm and the pump light 158 at 1032 nm using two
separate optical fibre inputs and combining both into a single
output optical fibre 156 which is fusion spliced with the
input of the amplification optical fibre 130. It is however
pointed out that combining two singlemode signals respectively
at 1032 nm and 1064 nm is technically challenging. It is
possible to use a dichroic coupler or wavelength-division
multiplexer such as a thin film filter, a Fibre Bragg Grating
(FBG) filter with a circulator, a free space grating or an
arrayed waveguide grating. However, since the difference
between the wavelengths to be combined is only 32 nm, such a
dichroic coupler is likely to show a narrow bandwidth, a high
cross-talk, and strong polarization dependency.
The pump source 132 is an ytterbium-doped Fabry-Perot fibre
laser pumped at 976 nm and uses FBG filters at 1032 nm to
produce a stable 1032-nm pump light. It is however noted than
a semiconductor laser may also be used as a pump source 132.
It is noted that the amplification fibre 130 could also be
pumped in a conterpropagation configuration, as opposed to the
copropagation configuration of Fig. 5, by providing the
multiplexer 134 at the output of the amplification fibre 130.
A wavelength selective filter may also be used if necessary at
the output of the amplification fibre 130 to filter out the
residual pump light exiting the amplification fibre 130.
Fig. 6 shows another example of a parabolic pulse amplifier
212. The parabolic pulse amplifier 212 comprises two stages,
i.e. a 1032-nm pump source 232 arranged in-line with an
amplification optical fibre 230. The pump source 232 is used
to produce a 1032-nm pump light which is used to pump the
second stage for parabolic pulse amplification of the low-
average-power input pulse signal 252. The pump source 232 is a
Fabry-Perot ytterbium-doped fibre laser consisting of an
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ytterbium-doped pump source waveguide 236 pumped by a laser
pump diode 238 providing pump light 258 with a power of about
400 mW at 974 nm. The optical cavity of the pump source fibre
laser is created by placing a reflective filter 240, 242
having a peak reflectivity at 1032 nm at both ends of the pump
source waveguide 236, which is, in this illustrative example,
a 1.5 m-long ytterbium-doped optical fibre having a maximum
absorption of 75 dB/m at 976 nm. The 974-nm pump light 258 and
the pulse light signal 252 are combined and coupled to the
input of the pump source waveguide 236 using a multiplexer
234. The output of the pump source 232 is coupled to the input
of the second stage, i.e. the amplification waveguide 230,
such that pulse light signal propagated in the pump source
waveguide 236 and exiting at an output of the pump source
waveguide 236, and a 1032-nm pump light produced by the 1032-
nm pump source 232 are both coupled to the amplification
waveguide 230 for parabolic pulse amplification.
The pump source 232, which consists of a Fabry-Perot
ytterbium-doped fibre laser, is placed in the optical path of
the incoming input low-average-power pulse signal 252. As the
pump source waveguide 236 is pumped in copropagation using the
pump laser diode 238 at 974 nm, the pumped ytterbium-doped
optical fibre produces ASE in the wavelength region between
about 1000 and 1150 nm and with a maximum power density around
1030 nm. The ASE produced and propagating in the pump source
waveguide 236 is filtered and reflected back in the pump
source waveguide 236 at both of its ends, thereby defining the
optical cavity and producing a laser emission at 1032 nm. The
input reflective filter 240 placed at the input of the pump
source waveguide 236 is a highly reflective FBG (nearly 100%
reflective)and reflects the conterpropagating laser emission
produced in the pump source waveguide 236 at 1032 nm, back
into the pump source waveguide 236. The output reflective
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filter 242 is a 4% reflective FBG 242 that reflects part of
the copropagating laser emission at 1032 nm back into the pump
source waveguide 236. Laser emission not reflected by the
output reflective filter 242 is coupled to the second stage,
i.e. the amplification optical fibre 230.
The pump light 258 and the input pulse signal 252 are combined
and are both coupled to the pump source waveguide 236 using
the multiplexer 234, i.e. a 976-1064 dichroic fibre coupler.
The multiplexer 234 may also consist, for instance, of a thin
film filter, a Fibre Bragg Grating (FBG) filter with a
circulator, a free space grating or an arrayed waveguide
grating. Accordingly, both 974-nm pump light 258 and input
pulse signal 252 propagate in the pump source waveguide 236.
While the pump light 258 is mostly absorbed to produce the
1032-nm pump light, the input pulse signal 252 is only
slightly amplified due to the high gain depletion resulting
from the 1032-nm lasing. In this illustrative embodiment, the
pump source 232 produces a 1032-nm pump light having an
optical power of about 200 mW, while the pulse light signal
sees a low amplification gain of about 3 dB.
The second stage consists of an amplification waveguide 230,
more specifically, a 65-m long ytterbium-doped optical fibre
having a maximum absorption of 75 dB/m at 976 nm.
Absorption of the 1032-nm pump light provides an amplification
gain that is distributed substantially uniformly along the
length of the amplification waveguide 230. Consequently, the
input pulse signal can be amplified such that it reaches the
parabolic pulse asymptotic solution as it propagates along the
amplification waveguide to provide an amplified parabolic
pulse light signal 254.
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Fig. 7 shows a parabolic pulse amplifier 312 according to
another example embodiment. The amplifier 312 of Fig. 7 is
similar to the parabolic pulse amplifier 212 of Fig. 6 but the
input and output reflective filters 240 and 242 are omitted in
the pump source 332. The pump source 332 is an ASE pump source
around 1030 nm instead of a Fabry-Perot fibre laser. As the
amplifier 312 and the amplifier 212 are similar in
construction and use similar components, the description of
like elements and connections will not be repeated.
Differences between both will only be described.
The pump source 332 of the amplifier 312 consists of an
ytterbium-doped optical fibre pumped by a pump laser diode 238
at 976 nm. The input pulse signal 252 and the pump light are
combined using a multiplexer 234 before being injected in the
pump source waveguide 336. Absorption of the 976-nm pump light
in pump source waveguide 336 produces ASE in the wavelength
region between about 1000 and 1150 nm and with a maximum power
density around 1030 nm. The copropagating ASE produced is
directly coupled to the amplification waveguide 330 for
pumping and amplifying the input pulse signal. While the pump
light 258 is mostly absorbed to produce the ASE pump light,
the input pulse signal 252 experiences only a moderate gain
while propagating in the pump source waveguide 336 but ASE
generation is significant in due to the small signal regime.
Similarly to the parabolic pulse amplifier 212 of Fig. 6, the
input pulse signal is amplified such that it reaches the
parabolic pulse asymptotic solution as it propagates along the
amplification waveguide 330 to provide an amplified parabolic
pulse light signal 354. In order to provide a suitable
amplification gain coefficient at the signal wavelength when
pumping at 1030 nm, the amplification waveguide 330 is more
strongly doped than the pump source waveguide 336. For
instance, the maximum absorption at 976 nm of the
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amplification waveguide 330 can be ten times larger than that
of the pump source waveguide 336, for the gain coefficient in
the amplification waveguide 330 to be substantially equivalent
to that of the pump source waveguide 336 at the signal
wavelength.
Fig. 8 shows another example of a parabolic pulse amplifier.
The amplifier 412 of Fig. 8 is similar to the parabolic pulse
amplifier 212 of Fig. 6 but the pump source 432 is a
polarization maintaining pump source 432 in which the
generated 1032-nm pump light propagates along one first
polarization axis of the pump source waveguide 436 while the
pulse light signal at 1064 nm propagates along the other, i.e.
the second, polarization axis. This configuration is used to
easily separate the amplified signal from the residual pump
light at 1032 nm at the output of the parabolic pulse
amplifier 412, using a polarisation splitter. As the amplifier
412 and the amplifier 212 are similar in construction and use
similar components, the description of like elements and
connections will not be repeated. Differences between both
will only be described.
The input and output reflective filters 440 and 442 are fibre
Bragg gratings written in a polarization maintaining fibre,
and the pump source waveguide 436 is a 1.5 m-long polarization
maintaining ytterbium-doped optical fibre. Accordingly, each
reflective filter 440 or 442 show two different reflection
wavelengths, respectively corresponding to the two
polarization axes. In this illustrative embodiment, the two
reflection wavelengths have a difference of 0.3 nm. The
reflective filters 440 and 442 are produced so that the
reflection wavelength along the slow propagating axis of one
filter corresponds to the reflection wavelength along the fast
propagating axis of the other filter. A polarization-crossed
fusion splice 452 is introduced in the pump source 432
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somewhere between the input and the output reflective filters
440 and 442 so that the slow polarization axis of one
reflective filter is aligned with the fast polarization axis
of the other reflective filter, such that laser emission at
said reflection wavelength and propagating along the proper
propagation axis is reflected at both input and output
reflective filters 440 and 442. A second polarization-crossed
fusion splice 450 is introduced before the input reflective
filter 440so that the input pulse signal 252 propagates along
the same polarization axis before and after the pump source
432. The second polarization-crossed fusion splice 450 may
also be introduced after the output reflective filter 442
depending on the polarization axis of the reflective filters
440 and 442. Pump light generated at 1032 nm and reflected on
the reflective filters 440 and 442 propagates on the
polarization axis which is perpendicular to that of the signal
along the pump source waveguide 436. All other fusion splices
are standard "polarization-aligned" fusion slices. The
amplification waveguide 430 is also a polarization-maintaining
ytterbium doped optical fibre so that the pulse signal and the
1032-nm pump light also propagate along different polarization
axes in the amplification waveguide 430. In this
configuration, the pulse signal and the 1032-nm pump light may
be more easily split apart, using a polarization splitter or a
polarizer for instance, at the output of the amplifier 412 in
order to remove residual pump light at 1032 nm.
In this illustrative embodiment, the pump source waveguide 436
is a 1.5-m-long INO Yb-100 optical fibre which has a numerical
aperture of 0.15, a mode field diameter at 1060 nm of about
5.6 pm, a cutoff wavelength of 950 nm and a maximum absorption
at 976 nm of 75 dB/m. The amplification waveguide is also an
INO Yb-100 optical fibre, but is 65-m long.
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It is noted that the parabolic amplifiers 12, 112, 212, 312
and 412 are also useful for amplifying other low-average-power
pulse sources such as a low-power high-PRF pulses.
Accordingly, the low-PRF pulse source 14 of the system 10 of
Fig. 1 may be replaced by any low-average-power pulse source.
It is also noted that the amplification waveguides 30, 130,
230, 330 and 430 may use different materials. In the
illustrated case, the amplification waveguides 30, 130, 230
and 330 are ytterbium-doped silica optical fibres but it is
io noted that ytterbium-doped chalcogenide optical fibres may
also be used and that the waveguide may include other dopants.
The concentration of ytterbium may also vary.
It is also noted that planar rare-earth-doped waveguides may
also be used for one or both of the amplification waveguide
and the pump source waveguide. Since planar waveguides are
typically shorter, ytterbium concentration may be increased to
achieve a similar amplification gain.
Increasing/reducing ytterbium concentration in the
amplification waveguide may also be used to reduce/increase
the length of the amplification waveguide. The same also
applies to the pump source waveguide.
Furthermore, it is noted that ytterbium may be replaced by
another rare-earth dopant such as erbium for parabolic
amplification of a low average power pulsed signal at a
different wavelength, such as a wavelength greater than
1600 nm. For example and referring to the configuration of
Fig. 5, the pump source 132 provides a pump light having a
wavelength around 1550 nm in order to provide a parabolic
pulse amplification at a wavelength around 1600 nm in the
amplification waveguide 130. Referring to Fig. 8, the pump
diode 238 produces pump light at 1480 nm to pump the pump
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source waveguide 436 of the in-line laser in order to generate
a pump source at a wavelength around 1550 nm. The 1550-nm pump
source is then used to pump the amplification waveguide 430
for parabolic pulse amplification of the signal around
1600 nm. It is noted that, in order to produce parabolic pulse
amplification, the erbium-doped amplification waveguide should
have a normal dispersion at the signal wavelength.
The embodiments described above are intended to be exemplary
only. The scope of the invention is therefore intended to be
limited solely by the appended claims.
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