Note: Descriptions are shown in the official language in which they were submitted.
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TITLE OF THE INVENTION
Stable mode-locked laser for high repetition rate operation
FIELD OF THE INVENTION
[0001] The present invention relates to a stable mode-locked laser for
high repetition
rate operation. More specifically, the present invention is concerned with a
stable mode-locked laser
for high frequency operation based on nonlinear resonators.
BACKGROUND OF THE INVENTION
[0002] In standard high repetition rate lasers, the repetition rate is
given by the
frequency spacing of the modes of the laser cavity. Repetition rates above
100GHz can be achieved
only with cavity lengths of few millimeters or shorter, as in state-of-the-art
semiconductor lasers. The
minimum width of the lines composing the emission spectrum of those lasers is
relatively large since
this width increases with the mode separation, i.e. as the length of the main
cavity shortens.
[0003] Dissipative Four Wave Mixing Lasers (DFWM) are lasers providing
high
repetition rates. However, in their standard implementation they generate very
unstable pulse trains
that exhibit severe random amplitude modulations. Due to this inherent
instability, the width of the
lines composing the laser emission spectrum is usually very large. DFWM lasers
have presently a
negligible impact on ultrafast laser applications.
[0004] DFWM lasers are based on a long laser cavity. In a passive mode-
locking
laser system, the repetition rate is fixed by the frequency spacing, i.e. the
free spectral range (FSR)
between the spectral resonances associated with each cavity mode. In DFWM, a
resonant filter
placed intracavity is used to periodically suppress groups of cavity
resonances, thereby increasing
the frequency separation between two adjacent oscillating cavity modes. This
method can be used to
set the repetition rate to an arbitrary multiple of the main cavity FSR. A
nonlinear element placed in
the main cavity induces an energy exchange between those cavity modes, thereby
maintaining their
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mutual phase, hence mode-locking them. However, state-of-the art DWFM systems
require main
cavities consisting in 10-50 meters of nonlinear fiber and 5-10 meters of
amplification fiber to sustain
the mode-locked regime. The main cavity FSR is then very low and many cavity
modes fall within the
bandwidth of each filter resonance. In the practice, the output spectrum
consists in groups of closely
spaced oscillating lines of similar gain and random phase. The beating of
those modes produces
strong low frequency amplitude modulation of the generated laser pulsed train.
[0005] Currently, there is no laser system capable natively of stable
operation at
repetition rates above 100 GHz with the characteristic linewidth of a long
cavity laser.
SUMMARY OF THE INVENTION
[0006] More specifically, there is provided a laser system, comprising
a high-Q
nonlinear optical resonator, a cavity comprising an amplifying element and a
dispersive element, an
optical delay line adapted to tune the length of the cavity, and a large pass-
band filter adapted to
tune the cavity's central oscillation wavelength, the nonlinear optical
resonator being selected with a
linewidth (LWR) comparable to the cavity free-spectral range (FSRc).
[0007] There is further provided a method for generating highly stable
pulse streams,
comprising providing a cavity comprising an amplifying element and a
dispersive element; selecting a
high-Q nonlinear optical resonator with a linewidth (LWR) comparable to the
cavity free-spectral
range (FSRc); tuning the length of the cavity; and tuning the cavity's central
oscillation wavelength
[0008] There is further provided a method for producing a laser source
with a narrow
modal linewidth and providing highly stable trains of short optical pulse at a
repetition rate higher
than 100 GHZ, comprising providing a cavity comprising an amplifying element
and a dispersive
element; selecting a high-Q nonlinear optical resonator with a linewidth (LWR)
comparable to the
cavity free-spectral range (FSRc); tuning the length of the cavity; and tuning
the cavity's central
oscillation wavelength.
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[0009] Other objects, advantages and features of the present invention
will become
more apparent upon reading of the following non-restrictive description of
embodiments thereof,
given by way of example only with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] In the appended drawings:
[0011] Figure I a is a schematic view of a laser system according to
an embodiment
of an aspect of the present invention; Figure lb shows the output in time,
consisting of a train of
pulses with a repetition rate defined by the free spectral range of the
nonlinear resonator, of the
system of Figure la;
[0012] Figure 2a is a schematic view of an integrated ring resonator,
assembled
using coupled single mode waveguides, according to an embodiment of the
present invention; Figure
2b shows the typical transmission spectrum of the ring resonator of Figure 2a
used for two
orthogonal input field polarizations (TE and TM); Figure 2c shows a detail of
the transmission
spectrum of Figure 2b around a single TM ring resonance;
[0013] Figure 3 is schematic view of a laser system according to an
embodiment of
an aspect of the present invention;
[0014] Figures 4a-c show a definition of control parameters in a
numerical analysis
according to an embodiment of an aspect of the present invention;
[0015] Figures 5, 6 and 7 show numerical simulation and predictions of
output
identifying the general condition of stable operation according to an
embodiment of an aspect of the
present invention;
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[0016] Figure 8 shows a typical output spectrum (a) and
autocorrelation (b) for a
comparable system (unstable) obtained using a long main cavity (unstable
configuration), the main
cavity having FSRe6MHz (33 meter of SMF), for different resonator input
powers, from 5.5 to
67.8mW (input powers and the amplifier pump currents are indicated in the
inset);
[0017] Figure 9 shows a typical output spectrum and
autocorrelation obtained with a
short main cavity (stable configuration), for different input power in the
resonator (7, 11.4, 14,
15.4mVV), the main cavity having FSRc=67MHz (3 meter of SMF) according to an
embodiment of an
aspect of the present invention; and
[0018] Figure 10 shows a consistent comparison of the stability
of a system
according to an embodiment of an aspect of the present invention with long
cavity systems of the
prior art: (a, d): stable case, FSlic=67MHz, in the stable case the DC line
exhibits Sub-Hz width at -
60dB and 150Hz width at -80dB; (b, e): unstable case, FSRe=67MHz and different
phase delay; (c,
f): unstable case, FSRe=6MHz; (d): PDC/noise 41dB (8bit-sampling limited); and
(f):
DC/noise-- 13dB.
DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0019] There is generally provided a method and a system to
produce a laser with
very narrow modal linewidth providing highly stable trains of short optical
pulse at a very high
repetition rate.
[0020] In a nutshell, there is provided a dispersive active
cavity in which a pass-band
filter and a high quality factor (Q) nonlinear resonator are inserted. The
cavity length is accurately
tuned, with a delay line, so that the one main cavity mode oscillates centered
within each resonator
band. A pass-band filter shapes the main cavity gain and selects the central
oscillation frequency.
The linewidth of the resonator is narrow enough to let only one main cavity
mode per resonator
resonance to oscillate, hence achieving stable operation.
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[0021] Figure 1 shows a dispersive active cavity 12 comprising a pass-
band filter 16
and a high-Q nonlinear optical resonator 14. The nonlinear optical resonator
14 is selected with
linewidth (LWR) comparable to the main cavity free-spectral range (FSRc) and a
free spectral range
(FSRR), defining the repetition rate of the mode-locked laser.
[0022] The cavity 12 may consists of a standard single-mode fiber (SMF-
28), which
provides the dispersion and connects the different elements, and an Erbium-
doped fiber amplifier 18,
which provides both dispersion and optical amplification. The length of the
cavity is accurately tuned
with a delay line 20.
[0023] The high-Q nonlinear optical resonator 14 may be a ring
resonator, as shown
in Figure 2a for example providing Q= 1.2x106, and FSRR=200.8GHz, i.e. a
resonator bandwidth
LWR <160MHz and a cavity field enhancement factor greater than 17, constituted
by a waveguide
with effective Kerr nonlinearities per unit of resonator loop length 7=0.2W-1m
(see Figures 2). Figure
2b shows the typical transmission spectrum of the ring resonator used for the
demonstration for two
orthogonal input field polarizations (TE and TM). Figure 2c shows a detail of
the transmission
spectrum around a single TM ring resonance.
[0024] The nonlinear resonator has a very high-Q, hence a very narrow
linewidth
LWR. Such a high Q greatly enhances the internal optical field intensity at
the resonance frequencies.
This corresponds to an enhancement of the nonlinearity and in a lower input
power required to
induce an effective four-wave-mixing. Hence, the energy in the laser cavity
can be smaller, and
consequently the external cavity length required to properly amplify the
signal can be shorter. In case
of a short external cavity, but still of order of magnitudes longer than the
nonlinear resonator optical
cavity, the modal separation in frequency is sufficient to let oscillate only
one mode for each
resonator resonance. This condition allows stable operation. In the case of
stable operation, the
tunability of the external cavity length allows operations having one external
cavity mode within each
resonator resonance. The optical delay line controls both the phase and group
delay in the external
cavity. A stable pulse train is obtained when the pulse coupled in the
nonlinear resonator overlaps
exactly the pulse circulating inside it. This condition occurs when the
external cavity group delay is
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approximately a multiple of the nonlinear resonator optical round trip time.
It is noted that, in similar
devices with long main cavity, the tunability of the main cavity length is
irrelevant as the emission is
always unstable.
[0025] In an embodiment illustrated in Figure 3, the system comprises
a fiber optical
cavity of a total approximated length of L=3m embedding a nonlinear resonator
14, a pass-band
optical filter 16, an Erbium doped fiber amplifier 18 and an optical delay
line 20 of length Ld. The
amplifier 18 is pumped with a standard 980 nm fibered optical pump 17 isolated
from the laser cavity
using two standard WDM filters 19. The laser output is realized by sampling
the optical field within
the delay line 20. A polarization controller 22 is used to optimize the
polarization state at the input of
the nonlinear ring resonator 14 and a Faraday isolator 24 is used to define
the direction of circulation
of the optical pulse within the main fiber cavity.
[0026] Numerical simulations analysis are performed by solving the
coupled
equations (1), shown in Figure 4, of the field evolution f(z,t) in the
amplifying fiber and of the field
a(z,t) evolution in the resonator (see Figure 4a), where c is the speed of
light, nF, nR and I32F, 02R are
the group indices and second order dispersion in the fiber (F) and in the
resonator (R), respectively; y
is the Kerr nonlinear coefficient in the resonator; and a is the linear
absorption for the fiber (F). In
equation (2), shown in Figure 4, g(f) is the saturable gain, Go and Pa
representing the fiber low-signal
gain and the amplifier saturation power, which controls the energy in the
laser cavity, and L being the
main cavity length. 0 regulates the fiber gain bandwidth. t and r define the
coupling between the
fields a and fat the input (in) and the output (out) of the ring resonator.
The equations are solved in
time with a pseudo-spectral method and are coupled at the ring ports according
to the relation in
equation 3 (see Figures 5, 6 and 7).
[0027] Figures 4a-c show a definition of the control parameters in a
numerical
analysis according to an embodiment of an aspect of the present invention;
here and anywhere in the
text, z and t are the spatial and temporal propagation coordinate
respectively; a(z,t) is the field inside
the ring resonator and f(z,t) is the field inside the main laser cavity. The
optical output is defined as
f(0,t). The radio frequency (RF) output is defined as the power If(0,t)I2
operatively detected using an
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electronic photodetector (i.e. its spectrum being limited to radio
frequencies).
[0028] As shown in Figures 4b and 4c, 41)mc dictates the initial
position of the main
cavity modes with respect to the ring modes, with Omc=0 indicating the initial
perfect alignment
between them. Note that the nonlinear interaction introduces a power dependent
phase contribution.
Simulations shows that stable operating condition can be achieved also for
main cavity modes
initially not centered with the resonator resonance. OK fixes the position of
the ring modes with
respect to the center of the gain bandwidth.
[0029] The simulations were performed for a 200GHz repetition rate
system, starting
from noise and letting the system reach the stationary state. The operation is
investigated for a ring
line LWR =12GHz. The dispersion is scaled accordingly to obtain a total
dispersion corresponding to
the physical setup. The results are presented in terms of oscillation optical
bandwidth measured in
unit of the ring resonator FSRR (RC mode) and amplitude low frequency
bandwidth (RF BVV)
expressed in unit of the main cavity FSRe (MC mode). The output is stable when
the amplitude
exhibits a very narrow peak around the 0 frequency, i.e. the amplitude of the
pulse train does not
exhibit significant low-frequency modulation.
[0030] Figures 5 show the case of an even number of modes within the
gain
bandwidth. When FSRc is significantly larger than LWR, the RF BW is always
close to zero (Figure
5a). When FSIRc is comparable to LWR, Ow, i.e. the main cavity length, and the
resonator input
power affect the stability (Figure 5b).
[0031] Figures 6 show a comparable case for an odd number of modes
within the
gain bandwidth. For this latter case, Figures 7 present a detail of the output
spectrum and the
temporal evolution of the output power, the central resonator line being
presented magnified in the
inset. As the laser becomes unstable, each resonator resonance is filled with
multiple oscillating main
cavity lines.
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[0032] Figure 8a shows the typical output spectrum and Figure 8b shows
second-
order non-collinear autocorrelation for a comparable system of the prior art
with a long main cavity
(unstable configuration). Those measurements have been performed using a
similar set up with a
very long external cavity having FSRe6MHz (33 meter of SMF), i.e. by removing
the required
stability condition, and for different resonator input powers, from 5.5 to
67.8mW (input powers and
amplifier pump currents are indicated in the inset). The mode-locked pulsed
emission is highlighted
by the spectrum shape and autocorrelation profile. Note that the stability
cannot be consistently
addressed in those measurements.
[0033] Figure 9a shows the typical output spectrum and Figure 9b shows
autocorrelation for a system according to an embodiment of the present
invention with a short main
cavity (stable configuration), for different input power in the resonator (7,
11.4, 14, 15.4mW). The
mode-locked pulsed emission is highlighted by the spectrum shape and
autocorrelation profile. Note
that the stability cannot be consistently addressed in those measurements.
[0034] Figures 10 show the stability of the system according to an
embodiment of the
present invention in comparison with the stability of a similar system with a
long external cavity as
known in the art. The laser amplitude noise is used to assess the stability
and was determined by
measuring the spectrum of the electrical radio-frequency (RF) output collected
using a fast
photodetector (bandwidth - 200MHz). For the stable case (a, d), the RF signal
exhibited a dominant
DC component with a bandwidth < 0.25Hz, which corresponds to the resolution of
the measurement
system, as well as an out-of-band noise 55dB lower than the DC peak. The ratio
rs between the
power of the DC component and spectral noise (within the 200MHZ bandwidth) is
estimated to be
above 41dB, again limited by the sensitivity of the measurement, which in the
present system is
dictated by detection and sampling noise. Those results are compared with the
present system with a
wrong main cavity delay (b, e) and with a design having a long cavity (c,f),
both being unstable. For
the case presented in (b) and (c), the RF bandwidth is very large and the DC
component brings a
quite weak spectral contribution, the ratio Fs always being << OdB. For the
case (c), this ratio Ts is
estimated to be lower than -13dB.
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[0035] In high-quality-factors resonators, optical fields having
wavelength matching a
resonance condition undergo severe intensity enhancement due to the
constructive interference
between the field circulating in the resonator and the field coupled at the
input. Intensity dependent
optical nonlinearities are then greatly enhanced. In stark contrast with the
standard DFWM method
and system, the present invention provides embedding the nonlinear element in
the passive mode
locking scheme directly into the filter.
[0036] Previous long cavity DFWM mode-locking systems for very high
repetition rate
are not stable, as described hereinabove; the pulse train at the output is
usually characterized by
severe low frequency amplitude modulation, strongly limiting the general
interest of those devices to
deploy industrial applications. In addition, this effect significantly
broadens the width of the laser
spectral lines, which therefore cannot be used as frequency reference in
metrological application for
example.
[0037] In stark contrast with regular lasers, and not only fiber based
lasers, the
present invention allows to obtain very high repetition rate without the
requirement of very short laser
cavities. The present stable system hence is capable to operate with long
laser cavity, potentially
providing significantly narrower laser spectral lines in principle not related
to the desired repetition
rate.
[0038] As people in the art will appreciate, the present invention
presents a mode-
locked laser based on a nonlinear high-Q resonator that achieves extremely
stable operation at high
repetition rates while maintaining very narrow linewidths, thus leading to a
high quality pulsed
emission.
[0039] As people in the art will appreciate, the present system uses a
resonator with
a quality factor high enough i) to enable an enhanced nonlinearity, which in
turn requires lower
external gain, and allow a shorter external cavity; and ii) to exhibit narrow
bandwidths. Both features
contribute to a resonator bandwidth comparable with, or smaller than, the main
cavity mode
separation, and enable stable operations.
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[0040] Moreover, the present system and method provide an
adjustable/matched
main cavity length to achieve stable operation.
[0041] As people in the art will now be in a position to appreciate,
the present
invention provides a stable mode-locked laser with the characteristic
repetition rate of a very short
cavity with the narrow oscillation lines bandwidth characteristic of a much
longer cavity, i.e. 3000
times longer for example. The bandwidth of these oscillation lines does not in
principle depend on the
laser repetition rate. As long as the resonator resonances have bandwidth
comparable to the main
cavity free-spectral range, the repetition rate is simply the free-spectral
range of the resonator. The
present system can oscillate at an arbitrary wavelength. As long as the
external cavity provides the
required dispersion, the central oscillation wavelength can be tuned with the
passband filter in the
main cavity and a stable operating condition can be always found adjusting the
main cavity delay
line. The high nonlinearity of the high-Q nonlinear resonator reduces the need
of a long amplification
path in the external cavity. The high-Q of the nonlinear resonator corresponds
to a narrow bandwidth
of its resonances. These two factors enable the external cavity free-spectral
range to be comparable
to the bandwidth of the nonlinear resonator opening the access to stable
operating regimes.
[0042] The present method allows the realization of a stable-mode-
locked laser
having very high repetition rate (>100GHz) in principle not dependent on the
main cavity length that
can have free-spectral-range (FSR) orders of magnitude lower than the
repetition rate.
[0043] The present system and method provide highly stable pulse
streams, which
are required for time division multiplexing in optical telecom channels and in
optical sampling
systems. Moreover, high repetition rate lasers are used for high speed optical
clock distribution for
the synchronization of different optical devices. With respect to high
repetition rate multiplexed
source, an inherently mode-locked laser is characterized by a phase relation
between pulses and
can be applied in phase-keyed optical telecommunication channels. Optical
frequency comb
generation are devices where the creation of a series of equally spaced
spectral lines enables direct
measurement of the optical spectrum on an unknown source with high spectral
accuracy. This
accuracy is related to a particularly narrow bandwidth of the comb line hence
this application is
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particularly suited for the present source.
[0044]
Although the present invention has been described hereinabove by way of
embodiments thereof, it may be modified, without departing from the nature and
teachings of the
subject invention as defined in the appended claims.
