U.S. patent application number 14/799601 was filed with the patent office on 2016-03-10 for optical systems.
The applicant listed for this patent is Fianium, Ltd.. Invention is credited to John Redvers Clowes, Christophe Codemard, Pascal Dupriez.
Application Number | 20160072251 14/799601 |
Document ID | / |
Family ID | 45390067 |
Filed Date | 2016-03-10 |
United States Patent
Application |
20160072251 |
Kind Code |
A1 |
Clowes; John Redvers ; et
al. |
March 10, 2016 |
Optical Systems
Abstract
An optical system comprises an optical apparatus arranged to
direct received light to different paths and to provide a first
signal and a second signal, said first and second signals having an
optical difference therebetween sufficient for distinguishing
optical signals, an amplifier in optical communication with the
optical apparatus for amplifying the first and second signals, and
a discrimination device to receive amplified light and to provide
output light responsive to the optical difference.
Inventors: |
Clowes; John Redvers; (New
Milton, GB) ; Codemard; Christophe; (Old Bishopstoke,
GB) ; Dupriez; Pascal; (Leognan, FR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Fianium, Ltd. |
Hamble |
|
GB |
|
|
Family ID: |
45390067 |
Appl. No.: |
14/799601 |
Filed: |
July 15, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13988938 |
Nov 1, 2013 |
9158177 |
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PCT/EP2011/070989 |
Nov 24, 2011 |
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14799601 |
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61458561 |
Nov 24, 2010 |
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61444779 |
Feb 20, 2011 |
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61489241 |
May 23, 2011 |
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61489735 |
May 25, 2011 |
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Current U.S.
Class: |
359/285 |
Current CPC
Class: |
H01S 3/1302 20130101;
H01S 3/0085 20130101; G02F 1/353 20130101; H01S 3/10015 20130101;
H01S 3/10061 20130101; G02F 2201/02 20130101; H01S 3/0092 20130101;
H01S 3/005 20130101; H01S 3/2308 20130101; G02F 1/113 20130101;
G02F 1/365 20130101; H01S 3/2391 20130101 |
International
Class: |
H01S 3/23 20060101
H01S003/23; H01S 3/00 20060101 H01S003/00; H01S 3/10 20060101
H01S003/10; G02F 1/11 20060101 G02F001/11 |
Claims
1. Optical system, comprising, an optical apparatus arranged to
direct received light to different paths and to provide a first
signal and a second signal, said first and second signals having an
optical difference therebetween sufficient for distinguishing
optical signals; an amplifier in optical communication with said
optical apparatus for amplifying the first and second signals; a
discrimination device to receive amplified light and to provide
output light responsive to the optical difference.
2.-124. (canceled)
Description
FIELD OF THE INVENTION
[0001] This invention relates to optical systems, such as, for
example, laser systems based around a MOPA (Master Oscillator Power
Amplifier) architecture wherein the output of a lower power source
(e.g., a diode, mode locked laser or other oscillator) is amplified
with an amplifier to a higher power. In particular, but not
exclusively, it relates to laser MOPA systems incorporating signal
discrimination based modulation and external switching.
BACKGROUND TO THE INVENTION
[0002] Modulation of the laser beam is desirable in many laser
systems. In ultrafast lasers, it is often of interest to have an
ability to select individual pulses, burst of pulses, to reduce the
pulse repetition rate and to switch on and off the laser beam with
extremely short rise and fall times. It can also be desirable to
modify the amplitude of a pulse to a selected non-zero value, or to
otherwise affect a signal. In CW lasers, it can be of interest to
gate the output, for example to turn on and off the laser beam or
to add burst mode operation
[0003] The MOPA is an attractive configuration of laser technology
for producing high average output powers. The MOPA is often
configured as an all-optical-fiber laser, where a fiber-based
oscillator seeds a fiber-based optical amplifier. However, MOPA's
are also designed as a fiber-oscillator seeding a solid-state
optical amplifier, a solid-state oscillator seeding a solid state
amplifier or a solid-state oscillator seeding an optical fiber
amplifier. MOPA's can also exist as semiconductor oscillators and
amplifiers and combinations with fiber- and solid-state
amplifiers.
[0004] In a MOPA configuration, it is possible to modulate the pump
source of the optical amplifier in order to have some method of
switching on and off the beam and/or modulating the power of the
amplifier output. Direct modulation of a fiber or solid state
amplifier is limited in the switching speed due to carrier
lifetimes within the amplifier gain media.
[0005] For low pulse repetition rates, the duty cycle of the pulses
can be insufficient to saturate the amplifier, resulting in the
amplifier producing high noise levels in between pulses. In this
situation, the amplifier gain can become too high and unsustainable
leading to catastrophic damage typically by self-Q-switching.
SUMMARY
[0006] This disclosure provides an optical system comprising an
optical apparatus arranged to direct received light to different
paths and to provide a first signal and a second signal, said first
and second signals having an optical difference therebetween
sufficient for distinguishing optical signals. The optical system
comprises an amplifier in optical communication with said optical
apparatus for amplifying the first and second signals, and a
discrimination device. According to embodiments, the discrimination
device receives amplified light and provides output light
responsive to the optical difference.
[0007] The discrimination device may receive light from the
amplifier directly, or alternatively indirectly. In use, the
discrimination device responds to the optical difference and
produces a corresponding output. For example, where the optical
difference is polarisation difference, the discrimination device
may comprise a device (e.g: a polarizer) which responds to the
polarisation difference by splitting the received amplified beam
into two beams of differing polarisation, or alternatively by
absorbing one polarisation state and passing the other.
[0008] The first and signals may be provided at different times. In
some examples, the first and second signals are modulated out of
phase with one another.
[0009] The optical apparatus may be configured to alter the
polarization of light passing along one of said paths, thereby to
provide first and second signals having a polarization difference
therebetween sufficient for distinguishing optical signals. The
discrimination device may comprise a polarisation sensitive optical
element to provide output light responsive to said polarization
difference.
[0010] The optical apparatus may be configured to selectively
control the relative amount of received light which is directed to
said different paths responsive to a control signal.
[0011] The optical apparatus may comprise modulation apparatus. The
modulation apparatus may for example include a single modulator, or
two modulators. Each modulator may comprise an acousto-optic
modulation (AOM), or electro-optic modulator (EOM). The modulation
apparatus may include two outputs. In embodiments, the modulation
apparatus comprises a single modulator, such as an AOM or EOM,
having two outputs.
[0012] The optical apparatus may comprise a modulation apparatus
having first and second outputs, wherein the modulation apparatus
is configured to selectively direct received light to said first
and second outputs responsive to a control signal, thereby to
direct received light to different paths.
[0013] The modulation apparatus may comprise an acousto-optic
modulator (AOM) having have two outputs, which is configured to
selectively control the relative amount of received light which is
directed to different orders. One of said orders may be a zero
order of the AOM and the other of said orders may be a diffracted
order of the AOM.
[0014] The optical apparatus may comprise a second acousto-optic
modulator in optical communication with said acousto-optic
modulator.
[0015] The optical apparatus may comprise an electro-optic
modulator (EOM).
[0016] In embodiments, the optical apparatus may comprise a
splitter to split received light into a first path and a second
path; a first modulator in optical communication with said first
path and a second modulator in optical communication with said
second path.
[0017] In embodiments, a nonlinear element such as a photonic
crystal fiber may be provided downstream of the splitter to
generate light nonlinearly by one or more nonlinear processes, e.g:
by way of supercontinuum generation, four wave mixing or harmonic
generation
[0018] The first modulator may be an acousto-optic modulator, and
the second modulator may be an acousto-optic modulator.
[0019] The optical system may further comprise a combiner arranged
upstream of the amplifier to combine the first and second signals
prior to amplification.
[0020] The optical system may comprise first and second outputs.
The discrimination device may be configured to direct the first
signal to the first output and the second signal to the second
output.
[0021] The optical system may comprise a nonlinear conversion
apparatus arranged to receive the first and second signals and to
cause the first and second signals to undergo one or more nonlinear
processes to generate nonlinearly generated light.
[0022] The nonlinear conversion apparatus may comprise a
microstructured optical fiber.
[0023] The discrimination device may be configured to pass and/or
reflect amplified light responsive to the optical difference,
thereby to provide said output light.
[0024] The optical system may further comprise an optical output,
and the discrimination device may be configured to provide a
selected one of said first and second signals to the output.
[0025] The discrimination device may comprise a separator.
[0026] The optical system may be a laser system.
[0027] The optical system may be a master oscillator power
amplifier (MOPA) optical apparatus.
[0028] The optical apparatus may comprise optical modulation
apparatus to provide a first modulated signal and a second
signal.
[0029] The optical apparatus may be arranged to provide said first
signal along one of said paths and said second signal along another
of said paths.
[0030] The optical apparatus may be configured to change light
passing along one of said paths to provide said distinguishable
characteristic.
[0031] The optical system may comprise an optical source, and said
optical apparatus may be arranged to receive light from said
optical source. The optical source preferably provides linearly
polarised light to the optical apparatus. The optical source may
include an oscillator.
[0032] The optical system may be a master oscillator power
amplifier (MOPA) optical apparatus having enhanced modulation
capability at higher optical powers, comprising at least one
oscillator. The optical apparatus may comprise optical modulation
apparatus in optical communication with said at least one
oscillator and comprising at least one modulator, wherein said
first signal comprises a modulated optical signal. The
discrimination device may comprise an optical beam splitter in
optical communication with said optical amplifier for separating
amplified optical signals into different optical paths responsive
to said optical difference.
[0033] This disclosure also provides a method, comprising directing
received light to different paths so as to provide a first signal
and a second signal, said first and second signals having an
optical difference therebetween sufficient for distinguishing
optical signals, amplifying the first and second signals, and
discriminating between the amplified first and second signals to
provide output light responsive to the optical difference.
[0034] This disclosure also provides apparatus comprising an
optical source apparatus comprising at least one optical source,
said optical source apparatus configured for providing first and
second signals, a modulation apparatus for modulating at least one
of the first and second signals, an amplifier downstream of the
modulation apparatus to amplify the first and second signals, and a
nonlinear element in optical communication with the modulation
apparatus for providing nonlinear generation responsive to the
first signal, wherein the nonlinear element is arranged to receive
the second signal as well as the first signal. At least part of the
second signal, or at least part of any signal generated nonlinearly
by the nonlinear element responsive to the second signal, can be
removed from an output beam. The output beam may be provided by an
output which is in optical communication with the nonlinear
element.
[0035] The apparatus may comprise an output in optical
communication with the nonlinear element for providing the output
beam.
[0036] The apparatus may include a discrimination device for
removing, from said output beam, at least part of the second signal
or at least part of any signal generated nonlinearly by the
nonlinear element responsive to the second signal. The
discrimination device may comprise a separator.
[0037] The modulation apparatus may for example include a single
modulator, or two modulators. Alternatively, or in addition,
modulation apparatus in the form of suitable electronics may be
provided to directly modulate an optical source.
[0038] The nonlinear element may provide supercontinuum generation
responsive to the first signal. Alternatively, or in addition, the
nonlinear element may provide four wave mixing, or harmonic
generation responsive to the first signal. The nonlinear element
may comprise an optical fiber, e.g: a photonic crystal fiber.
[0039] The apparatus may further comprise a length of delivery
fiber in optical communication with said nonlinear element, said
length of delivery fiber having at one end a beam delivery module
comprising said output. The beam delivery module may comprise said
separator.
[0040] The separator may be configured for removing, from the
output beam, at least part of the second signal.
[0041] In embodiments, the second signal does not substantially
contribute to said nonlinear generation.
[0042] This disclosure also provides apparatus, comprising an
optical source apparatus comprising at least one optical source,
said optical source apparatus configured for providing first and
second signals, a modulation apparatus for modulating at least one
of the first and second signals, an amplifier downstream of the
modulation apparatus for amplifying the first and second signals,
and a nonlinear element in optical communication with the
modulation apparatus for providing nonlinear generation responsive
to the first signal. The second signal may not substantially
contribute to said nonlinear generation. The apparatus may also
comprise an output for outputting nonlinearly generated light. The
apparatus may further comprise a discrimination device for removing
at least part of the second signal from the output. The
discrimination device may comprise a separator.
[0043] The apparatus may further comprise a combiner downstream of
the modulation apparatus for combining the first and second
signals, wherein the amplifier is downstream of the combiner for
amplifying the first and second signals after combination.
[0044] The apparatus may be a supercontinuum optical source. The
nonlinear generation may comprise spectral broadening.
[0045] The nonlinear optical element may be in optical
communication with and downstream of both the modulation apparatus
and the amplifier apparatus.
[0046] The modulation apparatus may be configured to modulate the
first and second signals. The first signal may be modulated out of
sync with the second signal.
[0047] The optical source may be configured such that the first and
second signals are provided to the amplifier so as to be modulated
substantially out of phase with each other.
[0048] The first and second signals may comprise the same center
wavelength. The first and second signals may have substantially the
same polarization state.
[0049] The second signal may propagate linearly, or substantially
linearly, through the nonlinear element.
[0050] The first signal may comprise optical pulses having a first
peak power and the second signal may comprise optical pulses having
a second peak power, the first peak power being substantially
higher than the second peak power.
[0051] The first signal may comprise optical pulses having peak
power sufficiently high so as to initiate nonlinear generation in
the nonlinear element. The second signal may not substantially
contribute to said nonlinear generation.
[0052] The first signal may comprise optical pulses. The second
signal may be a continuous wave (CW) signal.
[0053] The modulation apparatus may be downstream of the optical
source apparatus.
[0054] The amplifier may comprise a fiber amplifier.
[0055] This disclosure also provides a method comprising, providing
first and second signals, modulating at least one of the first and
second signals, amplifying the first and second signals, providing
nonlinear generation responsive to the first signal, providing an
output beam comprising light generated nonlinearly responsive to
the first signal, removing, from the output beam, at least part of
the second signal, or at least part of any signal generated
nonlinearly by the nonlinear element responsive to the second
signal. The second signal may not substantially contribute to said
nonlinear generation.
[0056] The disclosure also provides a method comprising: providing
first and second signals, modulating at least one of the first and
second signals, amplifying the first and second signals, and
nonlinearly generating output light responsive to the first signal,
wherein the second signal does not substantially contribute to said
nonlinear generation.
[0057] The method may further comprise removing at least part of
the second signal from the output.
[0058] This disclosure also provides a method of modulating the
output of a fiber amplifier at higher output powers for improved
performance reliability, comprising providing a modulated optical
signal and a second optical signal, delivering the signals together
as an input signal to an optical fiber amplifier that amplifies
optical signals responsive to receiving a pump signal that creates
a population inversion, wherein the modulated optical signal and
the second optical signal are delivered so as to propagate along
the fiber amplifier such that at least a portion of the fiber
amplifier receives the second signal at a time or times when the
modulated signal is not present, the second optical signal reducing
population inversion in the absence of the modulated optical
signal, the modulated optical signal being amplified during
propagation along the fiber amplifier, providing an optical output
path in optical communication with the optical fiber amplifier and
delivering an output signal thereto, the output signal comprising
the modulated optical signal after its propagation along the fiber
amplifier, and refraining, for at least certain times during the
delivery of the modulated optical signal, from delivering the
second signal after its propagation along the optical fiber
amplifier to the output optical path.
[0059] The output signal may substantially comprise an
amplification of the modulated optical signal.
[0060] The fiber amplifier may comprise an ytterbium doped fiber
amplifier.
[0061] The output signal may have pulse repetition rate of at least
1 MHz. The output signal may have an average power of at least 1
Watt or a peak power of at least 10 kW. The output signal may
comprise an average power of at least 1 Watt or peak power of at
least 10 kW.
[0062] The pump signal and the modulated optical signal may each
comprise a duty cycle and the ratio of the duty cycle of the
modulated optical signal to the duty cycle of the pump signal may
be no greater than 0.8, no greater than 0.6, or no greater than
0.4, or no greater than 0.2.
[0063] The second signal and the pump signal may each comprise a
duty cycle, wherein the ratio of the duty cycle of the second
signal to the duty cycle of the pump signal is no less than 0.2, or
no less than 0.4, or no less than 0.6, or no less than 0.8.
[0064] The modulated optical signal and the second signal may be
provided such that there is a selected optical difference
therebetween. Refraining from delivering the second signal may
comprise refraining from delivering the second signal responsive to
the optical difference.
[0065] The optical difference may comprise the modulated optical
signal and the second signal having different states of
polarization.
[0066] The optical difference may comprise the modulated optical
signal and the second signal comprising different wavelengths. The
difference between the different wavelengths of the modulated
optical signal and the second signal may comprise at least 10
nm.
[0067] The modulated optical signal and the second signal may be
substantially orthogonally polarized.
[0068] The modulated optical signal and the second signal may have
different center wavelengths.
[0069] The modulated optical signal and the second signal may have
different states of polarization. Refraining from delivering the
second signal may comprise providing in optical communication with
the fiber optical amplifier a polarization sensitive optical
element constructed so as to direct light having different
polarizations to different optical paths.
[0070] The modulated optical signal and the second signal may
comprise different wavelengths. Refraining from delivering the
second signal may comprise providing in optical communication with
the amplifier a wavelength sensitive optical component constructed
so as to direct light having different wavelengths to different
optical paths.
[0071] The second signal may comprise optical pulses.
Alternatively, the second signal may comprise a continuous wave
signal
[0072] The pump signal may comprise a continuous wave signal.
[0073] Providing the second and modulated optical signals may
comprise providing the modulated optical signal and the second
optical signal such that they counter propagate within the fiber
amplifier.
[0074] The method may comprise providing an optical pulse train.
Providing at least of one of the modulated optical signal and the
second signal may comprise modulating the optical pulse train.
[0075] The optical pulse train may be characterized by a first
pulse repetition rate (PRR), the modulated optical signal and the
second signal have, respectively, second and third PRRs that are
each less than the first PRR, and the second signal and modulated
signal considered together may propagate along the fiber amplifier
as a pulse train having a fourth PRR that is greater than the
second and third PRR's but less than or equal to the first PRR.
[0076] The method may comprise modulating the optical pulse train,
and the optical pulse train may comprise a pulse repetition rate,
and the pulse optical signal and the second signal may form a pulse
train having a pulse repetition rate that is less than the PRR of
the optical pulse train.
[0077] Modulating the optical pulse train may comprise providing an
optical modulator having an input port and two output ports, one of
said output ports providing the modulated optical signal and the
other output port providing the second signal.
[0078] Modulating the optical pulse train may comprise selecting
certain pulses of the optical pulse train for delivery as pulses of
the modulated optical signal and other pulses of the optical pulse
other than the certain pulses for delivery as the second
signal.
[0079] This disclosure also provides a method of modulating the
output of a fiber amplifier at higher output powers for improved
performance reliability, comprising: (a) providing a desired
modulation of an output signal, (b) modulating, responsive to (a),
an input signal by providing an optical difference between
different time parts of the input signal, (c) delivering the
modulated input signal to an optical fiber amplifier, (d)
separating out time parts from the output signal of the fiber
amplifier responsive to the optical difference so as to provide a
modulated output signal.
[0080] The method may comprise using the output signal in the
processing of a material. In some embodiments, the output signal
may not comprise an information bearing signal.
[0081] This disclosure also provides a method of modulating high
power output signals from an optical amplifier, comprising:
providing an amplifier, providing a first modulated signal having a
first optical property, providing a second signal having second
optical property that is different from the first optical property,
amplifying both of the first and second signals with the amplifier
to provide an amplified signal having both optical properties,
providing an optical output in optical communication with, and
optically downstream of, the optical amplifier, and amplifying the
first and second signals with the amplifier but for at least for
certain times refraining from outputting from the output one of the
first and second amplified signals such that the output signal
comprises signals having one but not the other of the first and
second optical properties.
[0082] Within the amplifier, the other of the signals may propagate
for at least certain locations along the amplifier without overlap
with the one signal and may serve to reduce energy storage in the
amplifier.
[0083] This disclosure also provides a master oscillator power
amplifier (MOPA) optical apparatus having enhanced modulation
capability at higher optical powers, comprising, at least one
oscillator, optical modulation apparatus in optical communication
with said at least one oscillator and comprising at least one
modulator, said optical apparatus providing first and second
optical signals, said first signal comprising a modulated optical
signal and said first and second signals having an optical
difference therebetween sufficient for distinguishing optical
signals, an amplifier in optical communication with said optical
apparatus for amplifying the signals; and an optical beam splitter
in optical communication with said optical amplifier for separating
amplified optical signals into different optical paths responsive
to said optical difference.
[0084] The optical modulation apparatus may include a second
optical modulator in optical communication with said optical
modulator and said optical amplifier.
[0085] The modulation apparatus may provide said first and second
signals as a pulse train with pulses of the first signal interposed
between pulses of the said second signal.
[0086] The oscillator may provide optical pulses characterized by
an oscillator pulse repetition rate, and said pulse train may be
characterized by a pulse repetition rate that is less than said
oscillator pulse repetition rate.
[0087] The oscillator may provide a first pulse train of pulses
characterized by an oscillator pulse repetition rate. The pulse
train may include consecutive pulses separated by a time period
that is greater than the reciprocal of the oscillator pulse
repetition rate.
[0088] The modulation apparatus may be adapted and constructed to
provide said first pulse, wherein the amplitude of the pulse may be
modulated to have different non-zero values.
[0089] The modulation apparatus may include a second modulator in
optical communication with said modulator and a fiber coupled
polarization maintaining fiber coupler in optical communication
with said first and second modulators.
[0090] The optical modulation apparatus may include an optical path
along which said first and second modulator are arranged optically
in series.
[0091] The first and second optical modulators may be optically
arranged in parallel. The first modular may modulate said first
signal independently of said second signal and said second
modulator may modulate said second signal independent of said first
signal.
[0092] The MOPA apparatus may comprise a non-linear optical element
in optical communication with one of said different optical paths
for receiving pulses optical signals.
[0093] Said optical difference between said first and second
signals may comprise said first signal having a first center
wavelength and said second signal having a second center wavelength
that is different than said first center wavelength.
[0094] Said at least one oscillator may comprise a first oscillator
providing an output signal having said first center wavelength and
a second oscillator providing an output signal having said second
center wavelength.
[0095] Said at least one modulator may comprise a first modulator
in optical communication with said first oscillator for providing
said first signal.
[0096] Said at least one modulator may comprise a first modulator
in optical communication with said first oscillator providing said
first signal and a second modulator in optical communication with
said second oscillator for providing said second signal.
[0097] This disclosure also provides a master oscillator power
amplifier (MOPA) optical apparatus having enhanced modulation
capability at higher optical powers, comprising, at least one
oscillator, optical modulation apparatus in optical communication
with said at least one oscillator and comprising at least one
modulator, said optical apparatus providing first and second
optical signals, said first signal comprising a pulsed optical
signal and said first and second signals having an optical
difference therebetween sufficient for distinguishing optical
signals, an amplifier in optical communication with said optical
apparatus for amplifying the signals, and a non-linear optical
element in optical communication with said amplifier for generating
spectrally broadened optical pulses responsive to receiving optical
pulses amplified by said amplifier.
[0098] This disclosure also provides a method of generating
supercontinuum pulses, comprising providing a first pulsed signal
having a first center wavelength, providing a second signal having
a second center wavelength that is different than said first center
wavelength, amplifying both the first and second signals with an
amplifier so as providing first and second amplified signals,
providing, optically downstream of the amplifier, a non-linear
optical element for generating a supercontinuum responsive to at
least one of the first and second signals amplified by the optical
amplifier and an output for outputting a supercontinuum signal
generated by the non-linear optical element, and wherein for at
least certain times the supercontinuum signal output from the
output is generated responsive to one but not the other of the
first and second amplified signals.
[0099] In embodiments, first and second center wavelengths of the
first and second signals may differ by at least 1 nm. The first and
second center wavelengths may differ by at least 5 nm. The first
and second center wavelengths may differ by at least 25 nm.
[0100] Providing a first pulsed signal may comprise providing a
first source and providing a second signal may comprise providing a
second source that is a different source than the first source.
[0101] Providing the first and second signals may comprise
providing a source that is designed to provide output signals
having different center wavelengths.
[0102] Amplifying both the first and second signals may comprise
amplifying one but not the other of the first and second signals at
certain times and amplifying the other of the first and second
signals at other times different than the certain times. Amplifying
both of first second signals may comprise refraining from
amplifying the one of the first and second signals at the other
times.
[0103] The amplifier may amplify the signals to provide an average
optical power of at least 1 Watt, at least 2 Watts, at least 5
Watts, at least 10 Watts, or at least 100 Watts.
[0104] The amplifier may amplify the first signals to have a peak
pulse power of at least 10 kW, at least 20 kW, at least 35 kW, at
least 50 kW or at least 75 kW.
[0105] The first signal may have a pulse width of no greater than
250 ns, no greater than 200 ns, no greater than 100 ns, no greater
than 20 ns, or no greater than 1 ns. The first signal may have a
pulse width of no greater than 250 ps, no greater than 200 ps, no
greater than 100 ps, no greater than 20 ps, or no greater than 1
ps.
[0106] The second signal may be a substantially continuous wave
signal.
[0107] This disclosure also provides a method of generating
supercontinuum pulses, comprising [0108] providing a first pulsed
signal having a first center wavelength; [0109] providing a second
signal having a second center wavelength; [0110] amplifying both
the first and second signals with an amplifier so as providing
first and second amplified signals; [0111] providing, optically
downstream of the amplifier, a non-linear optical element for
generating a first supercontinuum responsive to at least one of the
first and second signals amplified by the optical amplifier and an
output for outputting a supercontinuum signal generated by the
non-linear optical element; and [0112] wherein for at least certain
times the non-linear optical element receives a signal but does not
generate supercontinuum light responsive thereto or for the certain
times generates a supercontinuum that is substantially different
than the first supercontinuum.
[0113] The first and second signals may have respective first and
second center wavelengths which are different. Alternatively, the
first and second center wavelengths may be substantially the same.
The first and second signals may comprise substantially the same
polarization.
[0114] The first and second signals may be provided to the
amplifier so as to be substantially out of phase.
[0115] Overlap of one of the first and second signals to the other
of the first and second signals along the nonlinear element may, in
some embodiments, not be greater than 35%.
[0116] The nonlinear optical element may receive and propagate the
first and second signals as amplified by the amplifier and may
generates the first supercontinuum responsive to one of the signals
as amplified and wherein any supercontinuum generated responsive to
the other of the optical signals as amplified may be substantially
different than the first supercontinuum.
[0117] This disclosure also provides a method of generating a
supercontinuum signal, comprising: providing a first pulsed signal
and a second signal to a nonlinear optical element for generating a
supercontinuum signal having selected characteristics responsive to
at least one of the first and second signals, and wherein for at
least certain times the supercontinuum signal having the selected
characteristics is generated responsive to one but not the other of
the first and second signals.
[0118] The first and second signals may be provided to the
non-linear optical element so as to be substantially out of
phase.
[0119] One of signals may not have sufficient optical power for the
nonlinear element to generate a supercontinuum signal responsive
thereto.
[0120] This disclosure also provides a supercontinuum optical
source, comprising: a laser source apparatus comprising at least
one laser, said laser source apparatus configured for providing
first and second signals; a modulator apparatus downstream of the
laser source apparatus for modulating at least one of the first and
second signals, an amplifier downstream of the modulator apparatus,
said supercontinuum optical source configured such that amplifier
amplifies the first and second signals; a nonlinear element in
optical communication with and downstream of both the modulator
apparatus and the amplifier apparatus for providing selected
spectral broadening responsive to the first signal, said
supercontinuum optical source configured such that said non linear
element receives the second signal as well as the first signal; a
length of delivery fiber in optical communication with said
nonlinear element, said length of delivery fiber having at one end
a beam delivery module for providing an output beam from the
supercontinuum optical source; and wherein said beam delivery
module comprises a separator for removing at least part of the
second signal or at least part of any signal generated nonlinearly
by the non-linear element responsive to the second signal from the
supercontinuum optical source output beam.
[0121] This disclosure also provides a supercontinuum optical
source, comprising: [0122] a laser source apparatus comprising at
least one laser, said laser source apparatus configured for
providing first and second signals; [0123] a modulator apparatus
downstream of the laser source apparatus for modulating the first
and second signals, said modulator apparatus including at least one
modulator; [0124] a combiner downstream of the modulator apparatus
for combining the first and second signals, [0125] an amplifier
downstream of the combiner for amplifying the first and second
signals after combination, [0126] a nonlinear element downstream of
the amplifier for receiving the first and second signals after
amplification, the nonlinear optical element providing spectral
broadening responsive to the first signal and wherein the second
signal does not substantially contribute to spectral broadening;
and [0127] an output for outputting spectrally broadened light from
the optical supercontinuum source.
[0128] The supercontinuum optical source may comprise a separator
for separating at least part of the second signal from the
spectrally broadened light output from the output.
[0129] In one aspect, the present invention provides a method of
having a MOPA laser system with an extremely large range of
selectable modulation rate. The modulation can be achieved without
high levels of amplified noise (Amplified Spontaneous Emission)
and/or without the need for external modulation. In certain
practices, the approach can ensure that the amplifier is seeded at
a relatively constant power level, sufficient to prevent
Q-switching or excessive ASE development, and/or operates at a
relatively constant gain (so constant output peak power) yet
enables the combined laser MOPA to deliver variable repetition
rate, user selectable from single shot to the fundamental frequency
of the oscillator without having to rely on pump power modulation.
Note--constant gain and seed level are not obligatory but a
satisfactory seed and gain level can usually be maintained to avoid
the effects of self-Q-switching and excessive ASE.
[0130] Embodiments of the present invention allow the fast
automatic selection of two different beam paths. The selection can
be non-mechanical, and can reduce or eliminate the need for any
external mechanical components.
[0131] In one aspect, the invention describes a means for
modulating laser (MOPA) systems by other than external modulation.
The modulation can be achieved prior to at least the final stage of
amplification and involves establishing at least two signals,
individually and separately modulating the signals and recombining
the signals into an amplifier seed, whereby the two or more signals
have one or more different optical parameters or characteristics,
allowing them to be distinguished and separated following
amplification. The configuration enables the output of even the
highest power amplifier system to be modulated on demand with user
defined modulation profile and duty cycle.
[0132] Optical systems, (e.g: laser systems) according to the
invention preferably generate linearly polarised light. This is
further preferable where polarisation is employed to differentiate
between signals.
[0133] As used herein, the term "light" refers to electromagnetic
radiation of any wavelength, whether visible or not. Light
generated in embodiments of the invention may be infrared, e.g:
near infra-red.
BRIEF DESCRIPTION OF THE DRAWINGS
[0134] In order that the invention may be more fully understood,
embodiments thereof will now be described by way of example only,
with reference to the accompanying drawings, in which:
[0135] FIG. 1 shows a first example of a MOPA system according to
an embodiment;
[0136] FIG. 2a is a schematic illustrating operation of an
acousto-optic modulator (AOM);
[0137] FIG. 2b shows a fiber coupled AOM with fiber coupled output
of the diffracted and zero orders.
[0138] FIG. 3 is an example timing diagram for the first example
with a pulsed oscillator and a specific set of selected pulses, and
illustrates pulse repetition rate reduction;
[0139] FIG. 4 is a timing diagram for an alternative set of
selected pulses according to the first example, showing pulses on
demand;
[0140] FIG. 5 is a schematic of an electro-optic modulation (EOM)
pulse picker based on a Mach Zehnder interferometer with dual fibre
output;
[0141] FIG. 6 is a schematic of a second example system using two
modulators according to an embodiment;
[0142] FIG. 7 is a timing diagram for a system according to the
second example using a specific set of pulses, and illustrates
pulse repetition rate reduction;
[0143] FIG. 8 is an alternative use of the second example system
showing an exemplary timing diagram for a specific case of pulse
selection on-demand;
[0144] FIG. 9 is an exemplary timing diagram showing fast pulse
amplitude control;
[0145] FIG. 10 is a schematic of an example system implemented
within a fibre-MOPA architecture;
[0146] FIG. 11 is an optical schematic according to another
example.
[0147] FIG. 12 is a schematic of an exemplary fibre MOPA system
employing two modulators and capable of preferentially switching a
free-space beam between two outputs without the need for external
switching of the beam.
[0148] FIG. 13 is a timing diagram for an exemplary pulsed MOPA,
showing switching the pulses of a laser from a first to a second
output using polarisation modulation;
[0149] FIG. 14 shows an example of a multi-output laser system
using external modulation of each of the outputs;
[0150] FIG. 15 illustrates an exemplary system for delivering
multiple output beams, each capable of modulation at high speed and
high power;
[0151] FIG. 16 illustrates generating supercontinuum or 4 wave
mixing with user-defined pulses on demand, according to an
embodiment;
[0152] FIG. 17 illustrates a system with combined dual output and
ability to adjust separation of pulses, according to an
embodiment;
[0153] FIG. 18 illustrates, by way of example, using pulse
separation in a modulator system to have overlapping or partially
overlapping pulses to optimise a nonlinear process,
pre-compensating for dispersion and temporal walk off.
[0154] FIG. 19 is an example schematic according to one system
design whereby different pulse durations can be delivered by a
laser MOPA with fast selection between the two pulses.
[0155] FIG. 20 illustrates an example in which unwanted pulses are
injected at an amplifier output.
[0156] FIG. 21 is a schematic of an exemplary dual wavelength
modulated system.
[0157] FIG. 22 is an exemplary timing diagram for an exemplary
2-wavelength modulated system.
[0158] FIG. 23 shows an apparatus for use with a sixteenth example
of the invention.
DESCRIPTION
Example 1
[0159] FIG. 1 illustrates a first example of an optical system 100
according to an embodiment of the invention. As shown optical
system 100 includes an optical apparatus 101 comprising an
acousto-optic modulator 104 and a polarization-altering component
in the form of phase plate 106. The optical apparatus 101 receives
linearly polarised light from an optical source in the form of an
oscillator 107. The oscillator 107 may for example be a modelocked
oscillator. The system 100 also includes a combiner 108, an
amplifier 110, and discrimination device in the form of
polarisation beam splitter (PBS) 112.
[0160] The operation of the AOM 104 is well known per se. Briefly,
on application of an acoustic field to the acousto-optic (AO)
crystal of the AOM, a standing wave is established within the
crystal and, under the correct designed phase-matching conditions,
a proportion of the propagating beam (determined by the diffraction
efficiency of the device which is typically 70-85%) is diffracted
and exits the crystal at a different angle to the non-diffracted
portion of the beam. By applying the acoustic field in short
pulses, it is possible to switch the diffracted beam on and off
very quickly, determined by the frequency of the acoustic field and
size of the beam. See FIGS. 2a and 2b. An RF amplifier can be
provided to control the size of the acoustic field within the AO
crystal. The angular separation of the diffracted and transmitted
beam means that one can separate the diffracted and undiffracted
beams. In this way, the AOM may be used to direct received light to
different paths by selectively controlling the relative amount of
light which is directed to different orders, responsive to an
electrical signal.
[0161] In the optical system 100, both the diffracted first order
114 and the transmitted zero order 116 of the AOM 104 are collected
and used in the system. The diffracted first order 114 propagates
with one state of polarisation, defined in the Figure as P-state of
polarisation.
[0162] The zero order light 116 is also collected and used, but the
state of polarisation of the zero order is rotated through 90
degrees by the phase plate 106, to become substantially orthogonal
to the diffracted order 114. In this way, optical apparatus 101
provides first and second signals 114, 116 with a polarisation
difference therebetween.
[0163] The two beams are received at combiner 108 and polarisation
combined into a single beam, which is injected into the amplifier
110. Between the AOM 104 and the combiner 108, it may also be
beneficial to have control of the optical delay and relative losses
between the two paths. The loss may be substantially balanced. The
optical paths may be deliberately imbalanced slightly to make sure
that pulses with substantially orthogonal states of polarisation do
not overlap in time. This may be exploited to minimise any unwanted
nonlinear effects within the amplifier or to pre-compensate for
temporal effects including polarisation dependent dispersion within
the amplifier or external optics.
[0164] As shown in FIG. 1, the polarising beam splitter 112 after
the amplifier 110 rejects the zero-order, rotated polarisation
pulses 106 and transmits the diffracted order pulses 114 to an
optical output.
[0165] With this configuration, one can select the gating signal to
the AOM 104 to achieve a desired pulse repetition rate or train of
pulsed at the laser output. However, since both the diffracted and
zero order 114, 116 are re-combined and injected into the amplifier
110, the seed to the amplifier 110 never changes and is effectively
at the fundamental pulse repetition rate of the laser oscillator
107.
[0166] As a result, even though the repetition rate may be changed
by adjusting the gating signal to the AOM 104, nonetheless the seed
to the amplifier 110 remains constant, and this avoids the build up
of excess noise which can otherwise occur if the amplifier is left
unseeded for too long. This approach also avoids the amplifier gain
becoming too high and unsustainable leading to catastrophic damage
of the amplifier by self-Q-switching, which can also occur if the
amplifier is left unseeded for too long.
[0167] Thus, according to embodiments of the invention a desired
repetition rate can be selected without the risk of excess noise or
catastrophic damage to the amplifier.
[0168] The optical system 100 can be considered to be a MOPA
system, since it comprises an amplifier 110, which amplifies light
generated by an oscillator 107,
[0169] FIG. 3 shows an exemplary timing diagram at different points
of the system 100. In this example, the oscillator 107 is a
modelocked oscillator at 20 MHz pulse repetition rate. Suppose that
the end user application wants to have a 4 MHz output--ie: pulse
picking of every 5.sup.th pulse.
[0170] This figure also takes into account that there is
approximately 15% non-diffracted light into the zero order.
However, this has no effect on the final output pulse train after
amplification and passing through the polarising beam splitter 112
after the amplifier 110, where the zero-order, rotated polarisation
pulses are rejected.
[0171] Using this approach, one can use a low-power modulator,
which is sufficiently fast to pulse pick at tens of MHz pulse
repetition rate, yet deliver on-demand modulation of a pulse train.
This system can, for example, function with a fundamental
repetition rate of 40 MHz and an amplified average power of 100
Watts, yet have single-shot to 40 MHz operation, burst mode and
very fast switching speed.
[0172] A second timing diagram (FIG. 4) shows the use of this
system to achieve pulses on demand.
[0173] Thus the invention, in one aspect, can advantageously
modulate at very high frequency and high average power.
[0174] Using this approach, the amplifier is seeded with pulses
having a rep rate higher than the pulse picked rep rate, and the
amplifier can be seeded at the fundamental pulse repetition rate
(or even higher, depending on the delay introduced between the
polarization and the diffraction efficiency), meaning that the
amplifier is free-running and is effectively immune to the
modulation sequence or pattern.
[0175] In many applications, one simply does not need extremely
high pulse repetition rates. In these circumstances, the repetition
rate of the oscillator can be first reduced prior to amplification,
such that the amplifier only amplifies pulses that are wanted.
[0176] This can be achieved by a first low-power modulator after
the oscillator and a second, slower, higher power modulator
following final stage amplification.
[0177] Many further variations of the first example are possible.
For example, although AOMs are referred to above for modulation,
alternatively an electro-optic modulator (EOM) could be used, or
another type of modulation apparatus having two outputs.
[0178] FIG. 5 describes a fiber-connectorised dual output version
of an EOM pulse picker based on a phase modulated Mach Zehnder
interferometer (MZI) 500. The EOM pulse picker is known per se, and
will be described only briefly here. Within the device, the input
beam (linearly polarised and delivered through a PM optical fiber)
can be split into two paths of a MZI 502, where one of these paths
passes through a Pockels Cell 504. Application of a voltage to this
Pockels cell 504 causes a phase change in light propagating in this
path of the MZI 502. The beams are then recombined, and two outputs
are delivered of as linear polarised light propagating within two
PM fibers.
[0179] The output intensity of the two outputs vary as a function
of the phase change directly generated by the applied voltage, with
the two outputs having inverse responses to the applied
voltage.
[0180] The benefits of EO devices is that, for low-voltage devices
which operate at modest optical powers (up to 1 Watt), very high
speed phase and amplitude modulation can be achieved. One of the
draw backs is that the amplitude response may not have be linear
with applied voltage (see FIG. 5). In addition, thermal effects
mean that the electronics need to be controlled very carefully to
attain a stable amplitude ratio and good extinction between the two
output arms.
[0181] With both AOM and EOM technologies the beam is typically
focussed quite tightly through the modulator crystal if one is
interested in fast switching speeds. For AOMs the speed of
modulation depends on the speed of sound waves within the AO
crystal and the distance that the sound wave must travel through
the crystal in order to interact with the optical field of the
laser beam--this is determined by the laser beam spot size and
therefore a small beam is required for the fastest rise time. For
EOM's, thermal effects limit the power density such that higher
power operation necessitates larger apertures of the Pockel's cells
and the required voltage becomes very large, resulting in
relatively slow switching rates due to capacitance effects.
[0182] Embodiments of the invention may, for example, be applicable
within the printing industry, e.g: in laser writing of printing
plates. For example, in producing printing plates, laser writing
may be used to modify the surface structure of the printing plate
locally where the laser beam interacts with the surface. Typical
resolutions (1200 to 2400 dots per inch), sizes of the printing
plates (often >1 m.sup.2) and the need to produce the plates
within a short time scale, means that the laser should be modulated
at very high speed, typically greater that 10 MHz and as high as
100 MHz. The printing application can benefit from full control of
the laser beam to select, at this speed, whether the laser beam
should be on or off. This is because the system can have no pulses
for long periods of time (for example to produce a large area of
printing area to be "inked") or can alternatively operate at the
maximum pulse repetition rate for long periods of time (to produce
a large print area where there is to be no ink). Each time a pulse
of light is delivered to the work piece (printing plate) a dot will
either be printed or not printed.
Example 2
[0183] A second system design, also based on the use of
polarisation and modulation, is shown in FIG. 6. The system of FIG.
6 is substantially the same as the system of FIG. 1, apart from the
following modifications. The same reference numerals are retained
for corresponding features.
[0184] This optical apparatus 101 of the system 600 of FIG. 6
includes two modulators 104a, 104b rather than one, with the zero
order output of the first modulator 104a used as the input to the
second modulator 104b.
[0185] The diffracted order output beam 602 from the first
modulator 104a propagates with one state of polarisation and the
diffracted order output beam 604 from the second modulator 104b
passes through a polarisation rotator (phase plate) 106 such that
the beam has an substantially orthogonal state of polarisation to
the first modulator diffracted beam 602.
[0186] This system can be operated in a number of ways. It can
operate as the same as the previously described system, can be
operated (e.g., by maintaining the fundamental repetition rate of
the oscillator throughout the system), enabling modulation from
single pulse to the fundamental frequency of the oscillator. In
this case, the non-diffracted light from the first modulator does
not pass through to the amplification stage, thereby making the
amplifier slightly more efficient.
[0187] A more useful aspect of this architecture is that one can
adjust the timing electronics 606 such that a maximum repetition
rate of the system is attained and set. For example, (FIG. 7 shows
timing diagrams for a pulsed MOPA) we choose a maximum pulse
repetition rate of 4 MHz for this system. It is not straight
forward to build a modelocked oscillator at 4 MHz, so the
oscillator is built at 20 Mhz in this example.
[0188] The system can be used simply as a direct pulse picker,
reducing the pulse repetition rate from 20 MHz to 4 MHz. Here the
first AOM 104a modulates at 4 MHz, providing the 4 MHz pulse train
to the diffracted order output beam. Modulator 2 104b in this
example is therefore left closed and all 4 MHz pulses are
transferred to the amplifier with the same state of polarisation.
Following amplification, all pulses transmit through the output
polarising beam splitter to give a 4 MHz amplified output.
[0189] A further demonstration of Example 2 is shown by the timing
diagrams of FIG. 8, showing the case where a system can provide
modulation from single shot up to a selected rate (e.g., 4 MHz,
typically defined by the application). This maximum repetition rate
can be defined anywhere up to the fundamental of the oscillator but
is preferably chosen to be as low as the application needs in order
that amplification can be efficient.
Example 3
[0190] The same approach can be used to achieve, not only pulse
selection but fast pulse amplitude control and hence the ability to
modify the pulse energy of each individual pulse following
amplification. FIG. 9 shows an example of how this can be achieved
with the aid of pulse timing diagrams.
[0191] One can achieve this performance using the two modulator
example of FIG. 6 and by having both amplitude and digital control
of each of the modulators. For AOMs amplitude control involves
adjustment of the diffraction efficiency of light into the
diffracted order. This is achieved by controlling the magnitude of
the RF amplifier power which controls the size of the acoustic
field within the AOM crystal. AOM amplitude control is well known
per se to those skilled in the art.
[0192] For EOM devices, amplitude control involves changing the
magnitude of the applied voltage to the crystal.
[0193] Referring to FIG. 6, maximum output pulse energy following
amplification and polarisation can be achieved by applying the
maximum level of RF power to the first AOM 104a and causing optimum
diffraction for a chosen pulse propagating through the first AOM
104a into the first diffracted order. Correspondingly, the second
AOM 104b may have a minimum applied RF power for the period of time
corresponding to this pulse, resulting in no output of light
through the diffracted 1' order of the second AOM 104b.
[0194] Correspondingly, to have minimum pulse energy for a given
pulse following amplification and polarisation, minimum RF power is
applied to the first AOM 104a, resulting in no pulse energy
propagating to the first order of the first AOM 104a, and maximum
RF power is applied to the second AOM 104a, diffracting the maximum
amount of pulse energy into its first order output.
[0195] In order to have any level of pulse energy in between
minimum and maximum, one can synchronise both AOM RF amplifiers to
provide different levels of RF power to each of the devices, such
that the sum of the two diffraction efficiencies results in a
constant total pulse energy when the two pulses are summed together
following re-combination (with substantially orthogonal states of
polarisation). This is depicted in FIG. 9 in the 4.sup.th timing
diagram.
[0196] Following transmission through the output polariser after
amplification, only one of the pulses of a given polarisation
passes through to the laser output, shown by the bottom timing
diagram in FIG. 9. The rejected pulses may be dumped within the
laser chassis, although these pulses may be utilised in some
applications.
Example 4
[0197] FIG. 10 shows a non-limiting example of how the system may
be implemented in an all-optical fiber approach, for example within
a modelocked fiber laser MOPA system. The optical system 1000 of
FIG. 10 is substantially the same as the system of FIG. 6, apart
from the following modifications. The same reference signs are used
for corresponding features.
[0198] In the example of FIG. 10, the modulators 104a, 104b
comprise AOMs and are fiber coupled. The second modulator 104b
comprises a standard single output, diffracted order AOM. The first
modulator is customised to have both the diffracted and the zero
order delivered through polarization-maintaining (PM) optical fiber
1002.
[0199] Instead of a phase plate 106, the optical apparatus 101
comprises a 90 degree splice 1004 between the
polarization-maintaining optical fiber 1002 at the output of the
second modulator and a second piece of PM fiber 1006. In this way,
the state of polarisation from the second AOM can be rotated
through 90 degrees, simply by splicing the polarisation maintaining
fiber at 90 degrees to a second piece of PM fiber.
[0200] The two orthogonally orientated fibers are combined using a
conventional 3 dB (50:50) PM optical coupler 1008. Alternatively a
fiber-coupled polarisation combiner can be used, having an
additional advantage of low transmission loss. The coupler provides
two outputs, one of which is used to seed a PM amplifier system
1010. The second is available either as a monitor or to seed a
second amplifier system, for example.
[0201] The optical apparatus 101 of system 1000 may also include a
delay line 1012 to balance the two paths but to leave the two pulse
sets of different polarisation with no overlap to eliminate XPM
effects.
[0202] These examples advantageously demonstrate a non-external
modulator being used to achieve fast modulation. In some
applications, an external modulator need not be used, and fast
modulation is achieved solely by internal modulation.
Example 5
[0203] FIG. 11 shows schematically (using an all-fiber
configuration of the source) an alternative approach to achieving
fast modulation using two modulators and exploiting polarisation
properties of the two modulator outputs. The system 1100 includes
an optical apparatus 1101 comprising a splitter in the form of
coupler 1102 which splits received light into different paths. The
optical apparatus 1101 further comprises a first fiber-coupled AOM
1104 located along one of said paths, and a second fiber-coupled
AOM 1106 located along the other path. The system also includes a
combiner in the form of a second coupler 1108 and an amplifier
1110. The system further comprises a discrimination device 1112.
The first AOM 1104 will also be referred to as AOM1, and the second
AOM 1106 will also be referred to as AOM2.
[0204] In this example, the diffracted order of each of the AOMs is
used to achieve the solution of having fast modulation whilst
maintaining a reasonably consistent or other desired seed level to
the power amplifier system. The zeroth orders or one or both need
not be used.
[0205] FIG. 11 omits the oscillator from the diagram. However,
working from the left side of the schematic, the output from a
laser oscillator (preferably but not necessarily a high repetition
rate pulsed master oscillator such as a modelocked ultrafast fiber
laser) is passed through the coupler 1102 which splits the laser
power into two output paths, preferably of similar output power.
For example, this coupler can be a 50:50 Polarisation Maintaining
fused optical fiber coupler and the two outputs of this coupler can
have nominally identical power and polarisation. Alternatively the
coupler can be a fiber-coupled polarisation combiner component.
[0206] Each of the outputs of the coupler can be spliced to the
input of a fiber-coupled acousto-optic modulator (AOM) 1104, 1106,
each of which has an output fiber coupled to its diffracted order,
thus delivering, with minimum insertion loss, diffracted light
through the device when activated.
[0207] As with previous examples, the system is arranged such that,
prior to re-combination in the second coupler 1108, the two outputs
have substantially orthogonal states of polarisation. This can be
achieved either within the AOM devices 1104, 1106 (by actively
exciting each of the AOM output fibers on different polarisation
axes) or by splicing either the input fiber or output fiber of one
of the AOMs at 90 degrees such as to ensure substantially
orthogonal polarisation states propagate through each of the AOM
arms prior to re-combination.
[0208] Each of the AOMs has a separate RF driver 1104a, 1106a,
which is timed to determine whether the AOM is on (diffracts the
light to the fiber-coupled output) or off (dumps the light within
the AOM package, or alternatively passes the un-diffracted light to
another port or beam dump (not shown).
[0209] Timing electronics for the system (not shown) can be used to
achieve various different functions of the invention as described
within examples of this invention record. For example, as a direct
pulse picker system, AOM1 and AOM2 would be driven in direct
anti-phase to one another, meaning that when AOM1 diffracts light
to its output, AOM2 is off and when AOM1 is off, AOM2 is driven to
be on, and diffracts light to its fiber-coupled output.
[0210] After recombination in the coupler 1108 prior to
amplification, the power is maintained at a fairly consistent
level, maintaining a consistent seed level to the amplifier. Within
the seed signal, there will be a varying state of polarisation of
the light with time, determined by the sequence of operation of the
two AOMs 1104, 1106.
[0211] Following amplification, a discrimination device in the form
of a polarizing device 1112 discriminates the signal into
substantially orthogonal states of polarisation, resulting in
time-varying signals at the two outputs of the polarizing device
(shown here as a polarising beam splitter).
Example 6
[0212] FIG. 12 shows another optical system 1200 according to a
sixth example. Optical system 1200 is substantially the same as the
system of example 4, described above with reference to FIG. 10,
apart from the following differences. The same reference numerals
are used for corresponding features.
[0213] In the sixth example, both outputs of the polarisation beam
splitter 112 are used.
[0214] Harmonic generation optics (not shown) may be used at one of
the outputs so that the optical system 1200 provides an output
signal at the fundamental wavelength and an output signal at a
harmonic wavelength (e.g: 2.sup.nd harmonic wavelength). These are
identified in FIG. 12 as output 1 and output 2.
[0215] The system of FIG. 12 can also be used to advantageously
non-mechanically switch the beam path of a laser beam.
[0216] The timing diagram of FIG. 13 shows how this system can
operate in one specific embodiment where it is desired have fast
switching between two outputs, in generating a fast, motion-free
switch of the beam path from output 1 to output 2 using the
combination of low-power modulators and polarisation switching and
re-combining.
[0217] To deliver fundamental wavelength output through output 1,
the first AOM 104a is used to deliver linear polarised light on the
slow axis of the amplifier 1010. To switch (within as little as 20
nanoseconds) from output 1 to the second harmonic laser output (2),
the first AOM 104a is switched off, resulting in all light
transmitted to the zero order and into the second AOM 104b.
[0218] This approach can advantageously allow fast switching, with
no mechanical moving parts, reliably and repeatably between two
outputs. And this can be attained a very high repetition rates.
Example 7
[0219] In a further example, the method of switching between output
beams described in Example 6 is combined with any of the previous
examples showing fast modulation of a MOPA system.
[0220] Using this combination, amplitude control of each of the two
modulators as described in example 3 can be used to independently
modulate the two outputs of a laser, for example delivering
different harmonics of the laser output.
[0221] This approach allows one to modulate the laser output 1 with
high speed and from single shot to high repetition rate but also
enables one to switch to a second output of the laser and achieve
the same fast modulation control. In this system one would
preferably but not always block the unwanted output using, for
example, an external laser shutter as is commonly installed in most
high power laser systems.
[0222] This system is particularly advantageous in a materials
processing system where one wishes to select between two different
outputs beams AND have full and fast control of the pulse energy of
either of the laser outputs. In this method the relative amplitudes
of each pulse at each of the two outputs can be adjusted without a
need to adjust the gain of the amplifier itself. This can be
particularly attractive if one wishes to have a switchable dual
colour laser system (Fundamental and SHG for example) AND one
wishes to be able to have fast modulation and energy control of
each of the outputs.
[0223] Alternatively, one can simply have two different beam paths
for the output of a laser and simply select between the two without
needing to have mechanical switching which results in spatial
deviation of the beam over time and between multiple switching
operations.
Further Embodiments
[0224] By way of example and not limitation, it is noted that using
the basic principles described within previous examples, there are
many additional applications and modifications of the systems that
can be implemented to achieve different end goals. These will be
described here by way of examples--
Example 8
Multiple Beam Delivery Systems
[0225] Many materials processing applications require multiple
beams to achieve parallel processing of different regions of a work
piece. Most commonly, each of these outputs requires individual
control of amplitude and, in the case of pulse systems, requires
its own modulation.
[0226] In large frame DPSS laser systems, capable of delivering
very high pulse energies, the laser beam is typically split into
multiple beams using partial reflectors and free-space beam
steering.
[0227] For fiber-based systems, due to compact size, it is possible
to have a single oscillator and multiple amplifiers, each of these
amplifiers being arranged in its own package to allow the
application of several different outputs in different
locations.
[0228] Such a system can be achieved at low repetition rate and
relatively low powers by having multiple amplifiers each with its
own external modulator. Such a multiple output system based is
shown in FIG. 14 where each of the four output amplifiers 1402,
1404, 1406, 1408 can have its own external modulator. The external
modulators can be capable of relatively low speed (up to a few MHz)
modulation.
[0229] In building this system with a series of external
modulators, the size of each of the laser output enclosures becomes
quite large, each requiring alignment of a beam through the
modulator. Moreover, these systems are limited to modulate at
relatively low speeds.
[0230] An alternative approach is to generate multiple seeds, each
modulated at low power using the techniques described previously,
and injecting each of these seeds into one or more amplifiers with
output polarisation discrimination and/or beam deflection.
[0231] FIG. 15 shows a schematic of such a system showing two
remote heads, by way of example.
Example 9
Nonlinear Spectral Generation with User Defined Modulation
[0232] Embodiments of the invention are applicable not only to high
pulse energy laser systems for materials processing, but also for
scientific lasers including lasers generating harmonics, super
continuum and other nonlinear optical phenomena such as 4 wave
mixing.
[0233] The schematic of FIG. 16 shows an example system whereby
supercontinuum pulses, generated within photonic crystal fiber, can
be delivered at repetition rates from single shot up to tens of
MHz. Whilst variable repetition rate supercontinuum sources are
commercially available (from Fianium SC400-PP for example), they
typically work only at continuous repetition rate and typically do
not provide an ability to reduce to below 100 KHz where amplifier
noise becomes excessive and it becomes difficult to achieve
sufficient peak power from the amplifier due to this noise
limitation.
[0234] By applying the techniques described within this invention
and ensuring that the amplifier is continuously seeded at a fairly
constant level, one can switch the supercontinuum off completely
for long periods of time without affecting the amplifier. This can
be achieved with very fast rise times, not achievable using
external modulation, a chopper or shutter, and can therefore modify
the amplitude and spectral bandwidth of each individual pulse,
allowing single shot, burst mode operation and high repetition rate
quasi-cw from a single laser system. As is evident to the skilled
worker from a reading of the present disclosure, beam
discrimination, though shown upstream of the nonlinear fiber in
FIG. 16, can alternatively be provided downstream of the nonlinear
fiber.
Example 10
Dual Pumping of Nonlinear Materials
[0235] Optical systems according to embodiments of the invention
may be used to synchronously pump a nonlinear material using two
different outputs. An example is the generation of both Fundamental
and Second Harmonic in two different beam paths and combining
these, injecting them into a nonlinear element.
[0236] FIG. 17 illustrates an example in which both outputs of the
optical system 1200 of FIG. 12 are injected into a nonlinear
element 1700. Nonlinear element 1700 may comprise for example a
photonic crystal fiber or a nonlinear crystal.
[0237] Exemplary configurations include--
-i- Dual wavelength pumping of a nonlinear photonic crystal fiber
at 1064 nm and 532 nm to produce improved supercontinuum
generation. In this process the Photonic Crystal Fiber can be
designed to have a complex dispersion profile with two zero
dispersion wavelengths corresponding to the 1064 nm and 532 nm pump
sources. -ii- Third Harmonic generation by synchronous pumping of a
nonlinear crystal using in-phase photons at the fundamental and
second harmonic wavelengths.
[0238] In both of these examples, effects including dispersion and
temporal walk off within non-linear crystals and fibers may reduce
the overlap of pulses throughout the nonlinear process, which may
adversely affect efficiency of the nonlinear process.
[0239] However, according to embodiments, a delay element 1702 may
be provided to adjust the relative delay between corresponding
pulses from outputs 1 and 2, to pre-compensate for dispersion and
walk-off and to optimise the conversion process.
[0240] According to embodiments, pulses of substantially orthogonal
polarisation may be produced from outputs 1 and 2 of the optical
system 1200 respectively.
[0241] See FIG. 18 for an example of using the pulse separation in
the modulator system to have overlapping or partially overlapping
pulses to optimise the non-linear process, pre-compensating for
dispersion and temporal walk off. This approach can produce
interleaved pulses with a definable separation of the pulses
determined by the required application.
Example 11
[0242] In a variation of Example 10, rather than combining the two
outputs of the laser system, these outputs can each drive a
different nonlinear process. Exemplary nonlinear processes include
frequency doubling, trebling, quadrupling, 4 wave mixing and
supercontinuum generation. This variation can be applicable for
applications in which multiple outputs of lasers are required, with
each output having a different wavelength or wavelength range.
[0243] In embodiments, two different outputs are delivered from the
laser system and, using the modulation capability, one can either
have both of these outputs working simultaneously or have fast
selection between the two.
[0244] Options can include (non-limited) having--
[0245] Two different nonlinear PCF's which each produce a different
4 WM signal and idler wavelength
[0246] Two different nonlinear PCF's which each produce different
levels of spectral broadening to cover specific ranges of the
spectrum covering the visible and/or Near IR range
[0247] A combination of nonlinear PCF at one output and a second,
third or fourth harmonic generator from the second output.
Example 12
[0248] FIG. 19 shows another optical system 1900 according to a
twelfth example. Optical system 1900 is substantially the same as
the system of example 6, but with the following modifications. The
same reference numerals are used for corresponding features.
[0249] The system 1900 is different to the system 1200 of example 6
in that the optical apparatus 101 includes an additional optical
circulator 1902 and chirped fiber Bragg grating (CFBG) 1904 to
impart a chirp on the pulse and produce a pulse with a longer or
shorter temporal duration. This system therefore combines two
pulses within the amplifier 1010 of different time duration but
each with their own, substantially orthogonal state of
polarisation. Following amplification, the polariser separates the
two pulses into two different outputs.
[0250] In this way, the pulse duration of the pulses within the
modulation system is modified such that the two output pulses with
substantially orthogonal states of polarisation have different
pulse durations.
[0251] The output pulses can be combined using polarisation
combination, used to generate their own nonlinear effects or used
independently within different applications.
[0252] This embodiment may be applicable in a material ablation
process, where one wishes to use different pulse durations for
different stages of the process, or on different areas of the work
piece. This approach and apparatus is a method of achieving this
requirement, and also allows this to be achieved both with
simultaneous delivery of the pulsed outputs or by switching between
the two outputs--something that can be achieved quickly regardless
of the output power.
[0253] In this system, nonlinear limitations of the amplifier might
clamp the maximum peak power delivered by the amplifier, for
example stimulated Raman scattering (SRS) within optical fiber
amplifiers. In this case it would be beneficial to be able to
change the pulse energy of the system when operating with the
different pulse durations. For a given amplifier output power,
pulses with longer wavelengths can be provided with a higher
maximum pulse energy than those of shorter pulses, due to SRS
limited peak power within the amplifier.
[0254] One potential issue of using polarisation modulation as
described within these various examples is that the polarisation
extinction can be a factor if one needs to have high extinction
between pulses at the MOPA output.
[0255] For example, if the amplifier has a polarisation extinction
ratio (PER) of only 99%, in each of the timing diagrams shown by
way of example, there will be 1% leakage of pulses through the
output polariser where pulses were not required.
[0256] This problem rises since the two signals of orthogonal state
of polarisation propagate in the same direction through the
amplifier. This issue can be overcome by the variation of FIG. 11
shown in FIG. 20. As shown, in the example of FIG. 20 the unwanted
pulses are injected at the amplifier output.
Example 13
[0257] In all previous examples, we describe the use of different
states of polarisation as the method of discriminating the two
modulated beams after amplification. Use of polarisation as the
means for signal discrimination will typically mean that that the
system use a polarisation maintaining architecture and/or is
sensitive to polarisation mode coupling within components and
amplifiers such that it is advisable to have high quality optical
components and fibers. Also, in between amplifier stages (in a
cascaded amplifier design) one will typically avoid polarising
optics. Components are best polarisation independent, with minimal
polarisation dependent loss.
[0258] Other optical parameters may be used to discriminate between
modulated signals. Here we describe systems relying on use of
different wavelengths to generate signals with different modulation
patterns which can be discriminated at any point by using
wavelength filtering, for example using a dichroic mirror or
wavelength dependent combiners, separators, couplers or
switches.
[0259] When using polarisation discrimination according to
embodiments of the invention, a single oscillator can
advantageously be used and the output of this oscillator can be
split into the two or more different signal paths, With wavelength
discrimination, it is more difficult to have a single oscillator
producing more than a single wavelength and therefore in using
wavelength discrimination, the use of more than a single oscillator
is often preferred. These oscillators can operate as CW, pulsed,
modulated, Q-switched, modelocked or a combination of the different
laser operating regimes.
[0260] By way of example, here we consider an application where one
wishes to have a CW MOPA laser system and modulate the output of
this MOPA as required by the application. The CW MOPA laser in this
example is in the form of a fiber laser and is most likely to be a
high power fiber laser for use in industrial manufacturing.
[0261] FIG. 21 shows a laser MOPA 2100 whereby two CW laser
oscillators 2102, 2104 are provided to seed a high power amplifier
2106 comprising multiple gain stages. The two seed oscillators are
made to operate at two different wavelengths selected within the
gain bandwidth of the high power amplifier. Preferably the
wavelengths are chosen to operate at wavelengths where the
amplifier provides similar optical gain, but it might be
advantageous to have the seeder wavelengths operate at very
different gains. Furthermore, the two wavelengths may be closely
spaced by only 1 or a few nanometers, for example. In various
practices of the invention, the wavelengths are spaced by at least
5 nm, at least 10 n, at least 15 nm, or at least 25 nm. The two
wavelengths can be have a difference of between 1 nm and 5 nm, or
between 1 nm and 10 nm, or of between 2 nm and 5 nm or of between 2
nm and 10 nm. The two wavelengths can be spread across the gain
bandwidth or even operating at two different short and long
wavelength extremes of the amplifier gain bandwidth.
[0262] The outputs of the two oscillators 2102, 2104 pass through
two different modulators 2108, 2110 (in this example shown as
AOMs), whereby the two seeds are modulated as required.
[0263] Following modulation, the two seeds are combined using a
dichroic filter 2112 into a single seed signal which is injected
into the high power amplifier system 2106.
[0264] FIG. 22 shows an example timing diagram showing how such a
system works. The end user would have complete control of the
desired output modulated signal at a user wavelength (1064 nm or
wavelength 2). According to this modulation requirement, the second
AOM 2110 is modulated to pass through the desired 1064 nm output.
Simultaneously, the first AOM 2108 is modulated in anti-phase to
the first AOM 2108, such that the combination of 1064 nm and 1060
nm modulated signals at the input to the high power amplifier, is a
constant seed power.
[0265] The amplifier is operated continuously at high power.
However, after passing through a discrimination device in the form
of an output dichroic splitter 2114, only the 1064 nm output is
passed to the output of the laser for use by the end user.
[0266] This system configuration makes it possible to achieve any
modulated output, at very high speeds, whilst ensuring that the
amplifier is seeded at an acceptable level.
[0267] Clearly the same performance can be achieved using a single
laser oscillator configured to provide two different wavelengths
rather than having two separate oscillators. In such a
configuration, the oscillator output would be wavelength separated,
for example using a dichroic splitter, prior to modulation and
subsequent re-combination and amplification.
[0268] The system in this example would also work if one had two
very different oscillator designs and used wavelength (or
polarisation) as the optical discrimination means. For example, the
first oscillator 2102 might operate at a first wavelength and be
continuous wave whereas the second oscillator 2104 may be a
nanosecond or modelocked oscillator at a second wavelength (note as
in previous examples, wavelength or polarisation can be used but
are not limiting examples of parameters).
[0269] This particular approach would allow, for example, very fast
switching of two different laser outputs, one being CW and the
other being pulsed.
Example 14
[0270] Use of wavelength discrimination is not limited to CW
operation, it can also be used for ultrafast or Q-switched MOPA
systems. Here we use an example of a modelocked laser system.
[0271] Example 14 comprises two laser oscillators operating at two
different wavelengths. The configuration of this example (in the
form of a fiber-MOPA) will be described with reference to FIG. 21
as an example of how this system
[0272] At least one of the oscillators is modelocked or Q-switched
and operates with the desired pulse duration and wavelength of the
laser system. The other oscillator can also be modelocked or
Q-switched (preferably time-synchronised to the first oscillator)
but can equally be a CW laser. However, the second oscillator
operates at a different wavelength to the first oscillator. Both
wavelengths are chosen to fit within the gain bandwidth of the
amplifier system (typically a Yb-based system) and preferably at
wavelengths where the gain is relatively flat. However, the
important point within this system is that the laser amplifier
operates well at both of the wavelengths used.
[0273] The wavelengths are also preferably chosen such that they
are sufficiently separated that nonlinear spectral broadening
during amplification do not result in spectral overlap between the
two different oscillator pulses and preferably such that dichroics
can be used to efficiently combine or separate the two
wavelengths.
[0274] Each of the oscillators is inputted into a separate
modulator 2108, 2110 (in this example we show AOMs). The outputs of
the two AOMs are combined using a dichroic filter combiner 2112
into a single seed signal, injected into a high power amplifier
system. Following amplification, the wavelength signals are
separated in a discrimination device comprising a dichroic splitter
2114, or an alternative means such as a Volume Bragg Grating for
example.
[0275] The output modulation signals previously described by way of
the example of using polarisation modulation can also be attained
through wavelength discrimination and modulation.
Example 15
[0276] One specific embodiment of the invention is an ultrafast
laser system which delivers high power, high pulse energy output
with a user-defined modulation pattern and at 1030 nm
wavelength.
[0277] The MOPA itself is based around an Ytterbium doped fiber
system, where the oscillator and amplifier stages all have gain
within the Yb-fiber gain bandwidth.
[0278] In ultrafast amplifiers, the length of the fiber amplifier
is typically made to be as short as possible in order to minimise
optical nonlinear effects due to propagation of highly intense
pulses within the fiber. In such Yb-doped amplifiers, typically the
peak gain of the system operates in the region around 1030 nm but
extends from 1020 nm to beyond 1080 nm.
[0279] In this example, referring to FIG. 21, the first oscillator
2102 can be a modelocked fiber oscillator operating at 1030 nm and,
for example, 20 MHz pulse repetition rate. The second oscillator
2104 can comprise a cw laser or a modelocked oscillator operating
at a different wavelength within the Yb-fiber gain bandwidth. In
this embodiment, the second oscillator 2104 is a modelocked fiber
oscillator operating at a pulse repetition rate of 80 MHz. The
oscillator having the wavelength to be discriminated out from the
desired wavelength (1030 nm) so as to be separated from the 1030 nm
has a higher repetition rate.
[0280] The objective of this system is to be able to provide 1030
nm pulses on demand, and this is achieved by modulating the first
AOM 2108 accordingly.
[0281] In its simplest form, AOM1 2108 and AOM2 2110 are operated
in anti-phase such that, when a 1030 nm pulse is required, AOM1
2108 is open and AOM2 2110 is closed but when zero 1030 nm laser
output is required, AOM1 2108 is closed and AOM2 2110 is opened. In
this case, since the second oscillator 2104 operates at 80 MHz and
the first oscillator 2102 at 20 MHz, synchronization is not
performed.
[0282] In actual fact, since the amplifier has wavelength dependent
gain, optical components within the amplifier will have wavelength
dependent loss and the oscillators will most likely deliver
different powers, it is important to operate AOM2 2110 and AOM1
2108 with different transmission (achieved with driving the two
AOMs with different RF power levels). In this case, it will be
necessary to have different seed levels into the amplifier
depending on the mix of 1030 nm light and 1064 nm light
signals.
[0283] The outputs from the two AOMs are combined using a
wavelength dependent combiner (shown in FIG. 21 as a dichroic
combiner), and this combined, time-wavelength modulated signal
propagates to the high power amplifier.
[0284] Following amplification, the beam passes through a
wavelength discrimination device 2114 (e.g: a dichroic mirror)
which passes the 1030 nm output to the laser output aperture for
use by the end user and delivers the unwanted 1064 nm light to be
either "dumped" or used in another application if required.
[0285] In specific embodiments of the invention, the main goal is
to aim to have a relatively constant seed power at the input to the
amplifier such that the amplifier can operate at high power and/or
high gain regardless of the desired modulation duty cycle of the
oscillator signal of interest.
[0286] Use of wavelength discrimination has certain benefits over
polarisation, since there is little interdependence of the two
signals as a result of polarisation extinction ratio of the
amplifier system. This approach can work in a polarisation
maintaining or non-polarisation maintaining MOPA architecture.
Example 16
[0287] Example 16 can use signal discrimination to achieve high
speed nonlinearly produced pulses on demand, from single shot to
several tens of MHz repetition rates. The signal discrimination can
be wavelength based and/or based on differences in an optical
response of the optical system to different signals. With reference
to FIG. 23, the system 2300 can include a source apparatus 2310
that can include a first source 2312A and a second source 2312B.
The modulation apparatus 2313 may comprise a first modulator 2314A
to modulate the first source and a second modulator 2314B to
modulate the second source 2312B. The modulators 2314A and 2314B
can, for example, comprise AOMs.
[0288] The combiner 2318 can combine signals from the modulators
2314A and 2314B, and the amplifier 2320 downstream of the combiner
is followed by a nonlinear element 2324. A discrimination device in
the form of a separator 2324, downstream of the nonlinear optical
element 2322, separates out a desired signal 2334, which proceed to
an output, from an undesired signal 2332. In some practices, the
"undesired" signal is simply provided with the desired signal to an
output. In this practice the separator need not be used.
[0289] The first source 2312A can comprise a modelocked oscillator
operating at 40 MHz, producing pulses of approximately 10
picoseconds temporal duration and at a wavelength of 1064 nm and
the second source 2312B can comprise a continuous wave oscillator
operating at a wavelength of 1030 nm.
[0290] Preferably the nonlinear process of the nonlinear element
comprises supercontinuum generation within a photonic crystal fiber
or fibers. Accordingly, the non-linear element 2322 shown in FIG.
23 can comprise a photonic crystal fiber configured for producing a
supercontinuum.
[0291] As one of ordinary skill will appreciate, based on
consideration of this disclosure, the system 2300 of FIG. 23 can,
in the practices of the invention under discussion in this Example
16, operate in a similar manner to that of Example 15.
[0292] The first and second modulators 2314A and 2314B,
respectively, can be driven substantially out of phase (e.g., when
one modulator transmits a substantial optical signal the other does
not and vice versa). This can be done, in one practice of the
disclosure, such that the input signal to the amplifier following
the combiner 2318 of FIG. 23, is maintained at a selected power
level, which limits energy storage (e.g., at relatively constant
level), which is preferably selected such that the amplifier can be
continuously pumped at a high power level without risk of
self-Q-switching.
[0293] In certain practices of the invention disclosed herein, the
overlap of one of the signals relative to the other of the signals
within the nonlinear element (such as by viewing propagation of the
signals along the nonlinear element at a moment frozen time) can be
0%, no greater than 5%, no greater than 10%, no greater than 15%,
no greater than 20%, no greater than 25%, or no greater than
35%.
[0294] It is also within the scope of the invention as disclosed
herein that source 2 simply provide a substantially unmodulated
signal to the combiner 2318. For example source 2 can provide a cw
signal that is combined and amplified and provided to the nonlinear
element 2322, such as without passing through a modulator. This
example also provides an opportunity for a discussion of "overlap".
In this instance the cw signal would overlap a pulsed signal from
modulator 1 completely (e.g., 100%) but the modulated signal from
modulator 1 would typically overlap the cw signal by a much smaller
percentage because of the modulation, such as with the bounds noted
above for the overlap.
[0295] The system 2300 can produce supercontinuum (spectrally
broadened) light when high power pulses (pulses from the first
source 2312A, amplified in the amplifier 2320, are injected into
the nonlinear fiber of the nonlinear optical element 2322. If
modulator 1 is permanently open, the amplifier is seeded with 40
MHz pulses. The system 2300 is preferably configured such that the
amplifier 2320 is pumped at a sufficient level (provides enough
gain) to pump the nonlinear element 2322 so as to generate
spectrally broadened (supercontinuum) output from the nonlinear
element 2322.
[0296] When modulator 1 is open, the amplifier 2320 is seeded with
pulses from source 1, at 40 MHz. In this example, the amplifier
2320 amplifies the pulses to an average power of up to 20 Watts, a
pulse energy of 500 nanoJoules, and a peak power of 50 kilowatts.
After the nonlinear fiber of the nonlinear element 2322, there is a
broadband supercontinuum output, which can have, for example, an
average power of approximately 10 Watts, at 40 MHz (250 nanoJoules)
with spectrum spanning from the blue (450 nm) to the near infra-red
(beyond 2000 nm).
[0297] The amplifier 2320 can be configured to be pumped at
relatively constant power and deliver a relatively constant output
power of approximately 20 Watts.
[0298] At any point in time, modulator 1 2314A can driven so as to
modulate, preferably driven synchronised with the repetition rate
of source 1, to change the number of 1064 nm pulses passing through
the modulator 1 2314A into the amplifier 2320. The modulator can
"pulse pick" to provide any desired selection of pulses.
[0299] As noted above modulator 2 2314B is typically configured to
operate out of phase with modulator 1 2314A such that, when
modulator 1 is closed, reducing the number of pulses (and hence the
1064 nm average seed power) entering the amplifier, modulator 2
23148 is open, allowing 1030 nm cw light to seed the amplifier
2320.
[0300] The power of source 2 2312B and the operation of modulator 2
2314B can be arranged such that the amplifier seed (e.g., the
average input seed power from the 1030 and 1064 nm light) over time
is predictable and optimised to enable consistent gain of the
pulses within the amplifier, to a pulse energy of 500 nanojoules.
However, more generally it is just desired to have some seed to the
amplifier in between the pulses of the 1064 nm light so as to
reduce the risk of damaging or otherwise undesired behaviour (e.g.,
self q-switching).
[0301] The amplified output of the amplifier 2320 becomes a series
of 500 nJoule, 50 KW peak power pulses at 1064 nm and a high power
1030 nm CW output (albeit modulated in an "on" or "off" fashion if
modulator 2312B is included).
[0302] When injected into the nonlinear fiber comprised by the
nonlinear element 2322 the pulses can, for example, generate 250
nanoJoules of pulsed supercontinuum energy, spanning from 450 nm to
beyond 2 um, with a high power background cw wavelength at 1030 nm.
Other bandwidths are possible as well, as one of ordinary skill in
the art understands.
[0303] If modulator 1 2314A is closed completely, then the
nonlinear fiber output will be purely CW 1030 nm light at high
power.
[0304] The system 2300 can optionally include a discrimination
device such as for example in the form of a separator 2324, which
can be located downstream of the non-linear element 2322, as shown
in FIG. 23, for separating out an undesired part 2332 of the signal
traversing the nonlinear element 2322 from a desired portion that
can be output as the output signal 2334.
[0305] In one practice of the invention, such as where source 1 and
source 2 providing signals having different wavelengths, the
separator 2324 can comprise a reflective dichroic (which can be
narrowband at 1030 nm) that is placed in the supercontinuum output
beam filter out the cw 1030 nm light. The separator could
alternatively be placed before the nonlinear element, such that the
1030 nm light does not reach the nonlinear element. The separator
can include a simple blocking filter that includes a stopband for
the 1030 nm light.
[0306] Although systems that rely on either the wavelength or state
of polarisation to differentiate between signals are described
above, other optical parameters, or combinations of other optical
parameters, could alternatively be exploited in order to achieve
the objective of embodiments of this invention.
[0307] For example, the separator may comprise a saturable absorber
or other device, such as a nonlinear device, that can distinguish
signals based on optical power, as the spectrally broadened light
will typically have higher peak power and can be passed to the
output whereas the other signal does not have such peak power or
does not substantially contribute to the spectrally broadening
process and is of much lower power and can be absorbed or otherwise
treated differently by the saturable absorber or nonlinear device
of the separator 2324.
[0308] However, placing the separator 2324 downstream of the
nonlinear element 2322 can be particularly advantageous in the
practice of the disclosure wherein the non-linear element comprises
a microstructured fiber or PCF or other optical fiber as it allows
the output fiber of the amplifier 2320 to be spliced directly to
the microstructured fiber comprised by the nonlinear element 2322.
A practical supercontinuum fiber laser typically includes a length
of nonlinear fiber within a housing that can also include one or
more (such as all) of a source, such as one of the sources 2312A
and 2312B, one or more modulators, such as one or more of the
modulators 2314A and 2314B, one or more combiners (e.g., combiner
2318), and one or more amplifiers (e.g., fiber amplifier 2320). As
noted above, the housing can also include a nonlinear element, such
as a length of microstructured that receives pump energy and
provides spectral broadening responsive to receiving the pump
energy, and a length of delivery fiber extending out of the
housing. The length of delivery fiber extends outside of the
housing and is often terminated by a beam delivery module, which
can comprise a casing or other housing in which the beam delivery
fiber terminates. The beam delivery module can also include a
collimating lens or output window for providing environmental
protection for the end of the delivery fiber.
[0309] In one embodiment of the disclosure, the beam delivery
module can also comprise the separator 2324, which, for example,
can comprises a filter or dichroic that attenuates or blocks an
undesired wavelength or wavelength band, such as by directing such
wavelengths out of the beam delivery module. This can differ from
merely filtering out unused pump in that the signal attenuated or
blocked (attenuated means attenuated relative to becoming part of
output signal, which can include redirecting as well as attenuating
without redirecting) often has not contributed substantially to the
spectral broadening or other nonlinear processes or process of
interest in producing the output light. The signal being attenuated
could, in many practices of the disclosure, been attenuated
upstream of the nonlinear element rather than downstream without a
substantially different effect on the output signal.
[0310] Part or substantially all of the length of the delivery
fiber between a housing and the beam delivery module can be
configured to behave nonlinearly so as to participate in the
nonlinear process of interest, such as by, for example, provide
spectral broadening responsive to propagation of signal
therealong.
[0311] Note that in an interesting variation of Example 16 source 1
2312A and source 2 2312B can comprise substantially the same center
(i.e., fundamental) wavelength. One of the sources can be a cw
source that is modulated by one of the modulators and the other
source can be a pulse source, such as, for example, a picosecond
pulse source, that is modulated by the other of the modulators to
"pulse pick" selected pulses. This approach has benefits in that,
as compared to using sources having different center wavelengths,
the can be less of a need to balance any wavelength dependent gain
in the amplifier 2320 or, as another example, less of a need to
operate the amplifier 2320 such that it efficiently amplifies
different wavelengths.
[0312] In a preferred practice the amplifier 2320 can more readily
be seeded at substantially the same power level, regardless of the
particular pulse picking of the pulsed laser source (e.g., source
1), by adjusting the powers of the sources (and/or of any
attenuation of the modulators 1 and 2) and driving modulators 1 and
2 such that they modulate substantially out of phase, as noted
above. Again, however, the more general goal is to avoid
undesirable consequences or effects, and this does not necessarily
require substantially constant average input power to the amplifier
2320.
[0313] In an exemplary configuration, the signal from source 2
2312B is such that the signal after amplification does not cause
any substantial non linear effect when the amplified signal of
source 2 is provided to the nonlinear element (e.g: photonic
crystal fiber). In particular, the amplified second signal does not
generate a supercontinuum. On the other hand, after amplification,
the pulses from source 1 cause supercontinuum generation in the
nonlinear element. Due to the spectral broadening of the signal
from source 1, a spectral difference is introduced between the
signals from source 1 and source 2, which is sufficient for the
signals to be separated downstream by a wavelength-dependent
discriminator 2324.
[0314] In a specific example, source 1 2312A may comprise a 1064
nm, 20 MHz pulsed laser producing 5 ps pulses at an average energy
of about 5 mw and source 2 2312B may comprise a 1064 nm
substantially cw source (or high rep rate or long pulse source). In
this example, the cw signal from source 2 2312B is such that the
peak power after amplification is not sufficient to cause any
substantial nonlinear effect when the amplified signal of source 2
is provided to the nonlinear element 2322. In particular, the power
is insufficient to generate a supercontinuum. However, in this
example, the amplifier outputs the signal from source 1 as pulses
having 400 nj of energy at an average power of 8 Watts, which is
sufficient to cause supercontinuum generation in the nonlinear
element, thereby introducing a spectral difference between the
signals from source 1 and source 2, which is sufficient for the
signals to be separated by a suitable discriminator 2324.
[0315] However, more generally speaking, there may be any useable
difference between the response of system 2300 such that any
undesired signal can be separated and treated differently. It is
possible the source 2 could provide a signal that after
amplification does trigger a nonlinear effect. There just needs to
be some optical difference between signals provided by the sources
1 and 2 at some point during their traverse of the system 2300 or
on the effect they have on the system that allows selective removal
from the final output of undesired signal components.
[0316] The separator can remove the 1064 nm light of the amplified
signal of source 2 by comprising a notch filter or a narrowband
dichroic. Of course the spectrally broadened output signal 2334
will also have 1064 nm removed as well, though usually this is not
a problem in many applications. In applications where only visible
(or near infrared) wavelengths are needed, then the separator 2324
can include an appropriate bandpass filter with a cutoff that also
rejects the 1064 nm light.
[0317] Note that although two sources and two modulators are shown
in FIG. 23, one of ordinary skill in the art, cognizant of the
disclosure herein, understands that in the broad sense the system
2300 of FIG. 23 can be considered to comprise a laser source
apparatus 2340 that includes at least one laser source. For
example, where the signals combined by the combiner comprise
substantially the same wavelength, the laser source apparatus
could, in certain practices of the disclosure, comprise a laser
source and an optical splitter (with perhaps a attenuator for
attenuating one of the split signals relative to the other) for
providing first and second optical signals to modulators 1 and 2
respectively. Similarly, the system 2300 can be considered to
include a modulation apparatus 2313 that can include at least one
modulator. For example, the modulation apparatus 2313 can comprise
an AOM having two outputs, such as is disclosed above, which case
one of the modulators can comprise the AOM with two outputs and the
other modulator is not required.
[0318] The techniques described herein are not limited to the
generation of a supercontinuum but can be used with other
non-linear effects that are, for example, predominantly driven by
four wave mixing or frequency doubling.
[0319] This configuration enables supercontinuum pulses on demand,
whilst enabling an all-fiber (spliced) supercontinuum source,
whereby the amplifier output fiber can be spliced directly to the
nonlinear fiber, without the need for any intermediary bulk
components, which cause attenuation and potential failure points
due to free-space launching of high power light into the optical
fiber core.
[0320] Many of the examples above consider a pulsed laser system
and pulse picking modulation as the form of modulation of the
pulsed laser output. However, according to embodiment the similar
configurations can be used to modulate CW sources or even a
combination of CW and pulsed lasers to provide a multi-functional
hybrid laser system.
[0321] The invention can for example, in one or more of its various
aspects: [0322] Enable fast modulation at very high power, which in
many instances cannot be achieved with commercially available
optical modulator technology; [0323] Provide a means for ensuring
that an optical amplifier can be seeded at a constant level and
operate at a constant gain yet achieve fast modulation with low
duty cycle without issues relating to noise generation due to
fluctuating gain requirements; [0324] Enable fast modulation lasers
without the need for external free-space optical modulators which
typically affect beam quality and beam pointing stability; [0325]
Provide a solution for switching between 2 beam paths in a laser,
for example in switching between fundamental and second harmonic.
This can be achieved with no moving parts and with MHz switching
speed; and [0326] Provide a means for producing several novel
apparatus capable of exploiting multiple output beams with
different parameters and allowing either combination of these
outputs with variable temporal delay and/or fast selection between
the two outputs without any need for moving parts; [0327] Make use
of nonlinear effects as described herein.
[0328] The invention has several and varied aspects, including,
alone or in combination, providing methods and apparatus for (i)
modulating with high speed (>200 nanoseconds), high power (e.g.,
>1 Watt, >10 Watts, >50 Watts) laser systems, such as MOPA
systems; modulating high repetition rate (>1 MHz, >5 MHz,
>50 MHz, high power (e.g., as noted above, >1 Watt, >10
Watts, >50 Watts) pulsed lasers systems, such as a MOPA system;
and (iii) switching a laser beam between two different steered
paths, with a reduced need for, or even elimination of,
mechanically or electrically moving mirrors, which often have the
drawbacks of hysteresis and/or lack of beam position
consistency.
[0329] The present disclosure is directed to each individual
feature, system, material and/or method described herein. In
addition, any combination of two or more such features, systems,
materials and/or methods, if such features, systems, materials
and/or methods are not mutually inconsistent, is included within
the scope of the present invention. For example, any
arrangement/method disclosed for providing two signals may be used
with any disclosed method/arrangement for downstream use of the
signals, e.g: non-linear generation. Moreover, in any of the
foregoing embodiments, signal discrimination can be provided before
or after nonlinear generation, or forgone. Moreover, methods of the
invention are deemed to include the functional steps shown for
operation of apparatus. Those skilled in the art would readily
appreciate that all parameters, dimensions, materials and
configurations described herein are meant to be exemplary and that
in certain practices of the invention actual parameters,
dimensions, materials and configurations can depend on specific
applications for which the teaching of the present disclosure is
used. Accordingly, one of ordinary skill understands that the
invention may be practiced otherwise than as specifically described
and remain within the scope of the appended claims and equivalents
thereto.
[0330] In the claims as well as in the specification above all
transitional phrases such as "comprising," "including," "carrying,"
"having," "containing," "involving" and the like are understood to
be open-ended. Only the transitional phrases "consisting of" and
"consisting essentially of" shall be closed or semi-closed
transitional phrases, respectively, as set forth in the U.S. Patent
Office Manual of Patent Examining Procedure .sctn.2111.03.
[0331] The phrase "A or B" as in "one of A or B" is generally meant
to express the inclusive "or" function, meaning that all three of
the possibilities of A, B or both A and B are included, unless the
context clearly indicates that the exclusive "or" is appropriate
(i.e., A and B are mutually exclusive and cannot be present at the
same time). "At least one of A, B or C" (as well as "at least one
of A, B and C") reads on any combination of one or more of A, B and
C, including, for example the following: A; B; C; A & B; A
& C; B & C; A & B; as well as on A, B & C.
[0332] It is generally well accepted in patent law that "a" means
"at least one" or "one or more."
[0333] Nevertheless, there are occasionally holdings to the
contrary. For clarity, as used herein "a" and the like mean "at
least one" or "one or more." The phrase "at least one" may at times
be explicitly used to emphasize this point. Use of the phrase "at
least one" in one claim recitation is not to be taken to mean that
the absence of such a term in another recitation (e.g., simply
using "a") is somehow more limiting. Furthermore, later reference
to the term "at least one" as in "said at least one" should not be
taken to introduce additional limitations absent express recitation
of such limitations. For example, recitation that an apparatus
includes "at least one widget" and subsequent recitation that "said
at least one widget is coloured red" does not mean that the claim
requires all widgets of an apparatus that has more than one widget
to be red. The claim shall read on an apparatus having one or more
widgets provided simply that at least one of the widgets is
coloured red. Similarly, the recitation that "each of a plurality"
of widgets is coloured red shall also not mean that all widgets of
an apparatus that has more than two red widgets must be red;
plurality means two or more and the limitation reads on two or more
widgets being red, regardless of whether a third is included that
is not red, absent more limiting explicit language (e.g., a
recitation to the effect that each and every widget of a plurality
of widgets is red).
* * * * *