U.S. patent application number 17/361232 was filed with the patent office on 2022-03-03 for lifetime extending and performance improvements of optical fibers via loading.
This patent application is currently assigned to NKT PHOTONICS A/S. The applicant listed for this patent is NKT PHOTONICS A/S. Invention is credited to Carsten L. THOMSEN.
Application Number | 20220066285 17/361232 |
Document ID | / |
Family ID | |
Filed Date | 2022-03-03 |
United States Patent
Application |
20220066285 |
Kind Code |
A1 |
THOMSEN; Carsten L. |
March 3, 2022 |
LIFETIME EXTENDING AND PERFORMANCE IMPROVEMENTS OF OPTICAL FIBERS
VIA LOADING
Abstract
A method of making a microstructured optical fiber including
loading the core and cladding materials of the fiber with hydrogen
and deuterium at a loading temperature; annealing the fiber at a
selected temperature T.sub.anneal; pumping the fiber with
radiation; and reducing the temperature of the fiber and storing
the fiber at the reduced temperature before the step of pumping the
fiber; and wherein the method allows the hydrogen and the deuterium
to become bound to the core material and the cladding material.
Inventors: |
THOMSEN; Carsten L.; (Virum,
DK) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NKT PHOTONICS A/S |
Birkerod |
|
DK |
|
|
Assignee: |
NKT PHOTONICS A/S
Birkerod
DK
|
Appl. No.: |
17/361232 |
Filed: |
June 28, 2021 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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16658747 |
Oct 21, 2019 |
11048145 |
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17361232 |
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16256096 |
Jan 24, 2019 |
10474003 |
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16658747 |
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15963694 |
Apr 26, 2018 |
10281797 |
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16256096 |
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15683281 |
Aug 22, 2017 |
9971230 |
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15963694 |
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13003510 |
Jan 10, 2011 |
9746749 |
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PCT/DK2009/050158 |
Jul 3, 2009 |
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15683281 |
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International
Class: |
G02F 1/365 20060101
G02F001/365; G02B 6/02 20060101 G02B006/02; A61B 1/06 20060101
A61B001/06; F21V 8/00 20060101 F21V008/00; H01S 3/30 20060101
H01S003/30 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 11, 2008 |
DK |
PA 2008 00985 |
Claims
1. A method of making a microstructured optical fiber having a core
comprising a core material comprising hydrogen and/or deuterium
bound to the core material and a cladding comprising a cladding
material, where at least a part of the core comprises silica or
doped silica and the core having a Germanium content of less than
0.001 at %, where the microstructured optical fiber is a nonlinear
fiber in that the microstructured fiber can guide light for at
least a range of wavelengths .lamda..sub.min to .lamda..sub.max and
for a mode field diameter (MFD) of the fundamental mode over at
least a part of the range, the fraction (MFD)/.lamda. is less than
or equal to 5, the method comprising the steps of loading the core
material and the cladding material with hydrogen and deuterium at
loading temperature equal to or more than 120.degree. C.; annealing
the fiber at temperature T.sub.anneal of more than 80.degree. C.
for a period of more than 1 hour; pumping the fiber for a period of
more than 12 hours with radiation having an average power of 1 W or
more and a peak power density within the fiber equal to or higher
than 100 W/.mu.m.sup.2; reducing the temperature of the fiber and
storing the fiber at the reduced temperature for a period of time
before the step of pumping the fiber; and wherein the method allows
the hydrogen and the deuterium to become bound to the core material
and the cladding material.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation of U.S.
application Ser. No. 16/658,747, which was filed in the U.S. Patent
& Trademark Office on Oct. 21, 2019, and which will issue as
U.S. Pat. No. 11,048,145, which is a continuation of U.S.
application Ser. No. 16/256,096, which was filed in the U.S. Patent
& Trademark Office on Jan. 24, 2019, now U.S. Pat. No.
10,474,003, and which is a continuation of U.S. application Ser.
No. 15/963,694, which was filed in the U.S. Patent & Trademark
Office on Apr. 26, 2018, now U.S. Pat. No. 10,281,797, which is a
continuation of U.S. application Ser. No. 15/683,281, which was
filed in the U.S. Patent & Trademark Office on Aug. 22, 2017,
which is a continuation of U.S. application Ser. No. 13/003,510,
which was filed in the U.S. Patent & Trademark Office on Jan.
10, 2011, now U.S. Pat. No. 9,746,749, which is a national stage
application of PCT/DK2009/050158, which was filed on Jul. 3, 2009,
and which claims the priority of Danish Patent Application No. PA
2008 00985, which was filed on Jul. 11, 2008. The contents of U.S.
application Ser. Nos. 16/658,747; 16/256,096, 15/963,694;
15/683,281; 13/003,510; PCT/DK2009/050158; and Danish Patent
Application No. PA 2008 00985 are incorporated herein by
reference.
TECHNICAL FIELD
[0002] The invention relates specifically to an optical fiber
comprising a core and a cladding comprising a core material and a
cladding material, respectively.
BACKGROUND ART
[0003] Guiding of relatively high powers in an optical fiber may
have relevance for several commercial applications such as guiding
of surgical and/or therapeutic light, optical sensing, and
materials processing. Among such applications is transport of
optical energy and utilizing non-linear effects in the fiber, which
are commonly more pronounced with higher optical power inside the
fiber. The optical power may be continuous wave (CW), pulsed or a
mixture thereof. High optical power inside a fiber may be
particularly pronounced with pulsed light where a high peak power
may be obtainable even while having a relatively modest average
power.
[0004] One limitation of the average power/spectral density carried
by an optical fiber is the damage threshold of the fiber. The input
facet or the first few millimeters of fiber may be destroyed if the
optical power (CW, peak power or pulse energy) is above the bulk
glass or glass-air interface damage threshold. It has been observed
by the present inventors that even when the optical power is below
this threshold the optical fiber may still be observed to degrade
over time. This degradation is often observed as increased
absorption in the visible over time. For commercial applications a
long lifetime may be critical.
SUMMARY
[0005] An object of the present invention is to provide a fiber
where such degradation is either eliminated or reduced.
[0006] The objects of the invention are achieved by the invention
described in the accompanying claims and as described in the
following.
[0007] One object of the invention is achieved by an optical fiber
comprising a core and a cladding comprising a core material and a
cladding material, respectively, said optical fiber being obtained
by a method comprising loading said core material and optionally
said cladding material with hydrogen and/or deuterium under loading
conditions suitable to allow hydrogen and/or deuterium to bind
chemically to said material(s). In one embodiment said loading
condition comprises at least one of a) a raised temperature T, b) a
raised pressure P c) irradiation and/or d) subsequent irradiation.
Such an optical fiber may have particular suitable properties in
regard to long lifetime in applications where said fiber is
arranged to guide relatively high power, such as pulsed light with
relatively high peak power.
[0008] In one embodiment of the invention degradation of the fiber
in a supercontinuum light source for shorter wavelengths is reduced
by stripping of higher order modes. Accordingly, in one embodiment
the invention relates to a supercontinuum light source comprising a
fiber coupled to a pump source where said pump source and said
fiber are adapted to provide an optical output spanning over at
least one octave with at least 10 .mu.W/nm, and/or wherein said
pump and said fiber are adapted to provide a maximum modulation
instability gain .OMEGA..sub.max larger than 20, wherein at least
part of said fiber is arranged to strip higher order modes and/or
suppress coupling of light from the fundamental mode to higher
order modes. In one embodiment this stripping of higher order modes
is combined with loading of the fiber with deuterium and/or
hydrogen.
[0009] Loading by deuterium is sometimes applied in the art in
order to overcome absorptions in the so-called water-band, which
increases when the fiber is subjected to hydrogen-rich
environments, such as found for undersea communication cables. This
issue is not similar to the present problem. In one embodiment the
fiber is therefore applied in an environment where it is subjected
to a medium with a content of H.sub.2 and/or H.sup.+-ions of less
than 5 at %, such as less than 1 at %, such as less than 0.1 at %,
such as less than 0.01 at %, such as less than 0.001 at %.
[0010] Loading by deuterium is sometimes applied in the art in
order to increase the photosensitivity of the fiber in order to
enable changing its refractive index, e.g. to induce a Bragg
grating in the fiber by exposure through an external light source
perpendicular to the fiber. This issue is not similar to the
present problem. In one embodiment the fiber is annealed prior to
usage, where said anneal significantly reduces the fibers
photosensitivity. In one embodiment the fiber is a pure silica
fiber without dopants in which it is difficult to write Bragg
gratings even if it was H2 or D2 loaded without a subsequent
anneal. In one embodiment the photosensitivity of the fiber pre
and/or post anneal is less than 50% of that of a typical fiber used
for producing fiber Bragg gratings, such as less than 25%, such as
less than 10%, such as less than 5%, such as less 1%, such as less
than 0.1%, such as less than 0.001%. In one embodiment the
photosensitivity is substantially zero.
[0011] In one embodiment water-band absorption (such as the peak
absorption peak around 1400 nm) may be undesirable. In such an
embodiment it may be preferable to load the fiber with as little
hydrogen as possible so that in one embodiment the loaded fiber
comprises bound deuterium relative to bound hydrogen (and/or their
corresponding ions) of more than or equal to 1%, such as more than
or equal to 10%, such as more than or equal to 100%, such as more
than or equal to 10,000% by atom. However, hydrogen may be
preferable for applications where such absorption is either
insignificant or even preferable, particularly when it is noted
that hydrogen is commonly significantly cheaper than deuterium.
Hydrogen may also be preferable for applications where the
deuterium induced OD-absorption around 1870 nm is undesirable.
[0012] In one embodiment the lifetime of a fiber has been found to
increase with increased ambient temperature during loading (see
FIG. 7). This dependency indicates that providing energy during
loading benefits the lifetime. In one embodiment, the extended
lifetime depends on a chemical process occurring between the
material of the fiber and hydrogen/deuterium being loaded. In one
embodiment, the lifetime has been found to increase by introducing
energy subsequent to loading, such as by providing "subsequent
irradiation" which is the term used throughout the application for
the provision of energy to loaded material during and/or subsequent
to loading. In one embodiment subsequent irradiation is taken to
mean introducing energy at a rate higher than provided by the
general environment subsequent to loading, i.e. energy provided by
keeping the fiber at room temperature or energy provided by normal
lighting is not subsequent irradiation. In one embodiment this
introduction of energy stimulates the chemical process between
unbound Hydrogen/Deuterium and the material. In one embodiment it
is therefore preferable that subsequent stimulation comprises
localized contribution of energy. In one embodiment said chemical
process is binding of at least part of the loaded hydrogen and/or
deuterium to the material of the fiber. In one embodiment, the
fiber is being loaded by subjecting it to hydrogen and/or deuterium
under loading conditions suitably to allow hydrogen and/or
deuterium to bind chemically to said material(s). In one
embodiment, said loading condition comprising at least one of a)
raised temperature T, b) raised pressure P, c) irradiation and d)
subsequent irradiation. In one embodiment the fiber is a silica
fiber, so that in one embodiment said chemical process is binding
of at least part of the loaded Hydrogen and/or deuterium to the
silica matrix, such as forming an OH.sup.- or OD.sup.- bond. By
providing such loading conditions a fiber comprising an increased
amount of bound hydrogen and/or deuterium is obtainable so that in
one embodiment the loaded material comprises more than 0.1 atom
percent (at %) bound hydrogen and/or deuterium, such as more than 1
at %, such as more than 5 at %, such as more than 10 at %, such as
more than 20 atom percent, such as more than 50 at %.
[0013] In contrast to photosensitizing of fibers via loading the
temperature T is in one embodiment raised to allow for binding as
discussed above, so that T is more than or equal to 80.degree. C.,
such as more than or equal to 100.degree. C., such as more than or
equal to 120.degree. C., such as more than or equal to 140.degree.
C., such as more than or equal to 160.degree. C., such as more than
or equal to 180.degree. C., such as more than or equal to
200.degree. C., such as more than or equal to 220.degree. C., such
as more than or equal to 240.degree. C., such as more than or equal
to 260.degree. C., such as more than or equal to 280.degree. C.,
such as more than or equal to 300.degree. C., such as more than or
equal to 350.degree. C., such as more than or equal to 400.degree.
C., such as more than or equal to 450.degree. C., such as more than
or equal to 500.degree. C.
[0014] The fiber may and may not comprise a coating, such as a
polymer coating. In an embodiment where the fiber comprises a
polymer coating the loading temperature for loading deuterium
and/or hydrogen is kept below the melting point of the polymer,
such as below the softening temperature of the polymer. The
practical upper limit for increasing the loading temperature is
likely set by the coating of the fiber. In one embodiment high
temperature coating may extend the possible deuterium loading
temperature to above 250.degree. C. or even higher. One example of
a high temperature coating is DeSolite.RTM. DF-0005 or
DeSolite.RTM. DF-0009, manufactured by DSM Desotech Inc., which are
designed for high temperature fiber applications up to about
250.degree. C.
[0015] Alternatively, fibers without coating may be produced,
allowing even higher loading temperature e.g. up to and above
500.degree. C. and/or loading of the core (and optionally cladding)
material may be performed prior or during the process of forming
the fiber i.e. prior to coating. In one embodiment, the fiber is
stripped of its coating and subsequently recoated.
[0016] In one embodiment the chemical reaction time depends on the
temperature and/or pressure. In one embodiment the loading time
during which the fiber is loaded is at least sufficient to ensure
that thermal equilibrium has occurred. In one embodiment the
loading time is less than 6 days, such as less than or equal to 4
days, such as less than or equal to 1 days, such as less than or
equal to 1 day, such as less than or equal to 12 hours, such as
less than or equal to 6 hours, such as less than or equal to 3
hours, such as less than or equal to 2 hours, such as less than or
equal to 1 hours, such as less than or equal to 30 minutes, such as
less than or equal to 1 minute.
[0017] An increased pressure of the loading gas may in one
embodiment be utilized to provide a higher concentration of
indiffused hydrogen and/or deuterium in the fiber. Also, in one
embodiment the pressure P is more than or equal to 10 bars, such
more than or equal to 25 bar, such more than or equal to 50 bars,
such more than or equal to 75 bar, such more than or equal to 90
bar, such as more than or equal to 120 bar, such as more than or
equal to 160 bar, such as more than or equal to 200 bar, such as
more than or equal to 500 bar, such as more than or equal to 1000
bar, such as more than or equal to 2000 bar.
[0018] The above mentioned subsequent irradiation may in principle
be any irradiation suitable for providing energy to stimulate the
chemical process. In one embodiment said subsequent irradiation is
light, such as light introduced during use of the fiber. In one
embodiment said subsequent irradiation alters the glass structure
such as by creating and/or activating defects which interact with
hydrogen/deuterium. Further details of defects in glass may for
example be found in the paper "OPTICAL PROPERTIES AND STRUCTURE OF
DEFECTS IN SILICA GLASS" by D. L. Griscom (Journal of the Ceramic
Society of Japan. Vol. 99, no. 1154, pp. 923-942, 1991). In one
embodiment said irradiation is light guided by the fiber. In one
embodiment light otherwise applied in use of the fiber provides the
activation energy. In use of the fiber, the pump light, i.e. the
light by which the fiber is pumped, entering the fiber may in one
embodiment provide sufficient energy to allow unbound
hydrogen/deuterium to interact with the fiber and/or create new
defects with which remaining unbound hydrogen/deuterium may
interact. In embodiments where the fiber is subjected to signal
light and pump light or signal light alone, pump light may also
refer to signal light or pump and signal light in combination. In
the context of this application such subsequent irradiation of the
fiber may be referred to as photo activation. However, in principle
any types of irradiation of the fiber providing sufficient energy
may be applied. In one embodiment UV radiation is applied as
subsequent irradiation, since its higher photon energy enables
chemical reactions.
[0019] In one embodiment the fiber is cooled subsequent to loading
and/or annealing in order to reduce diffusion out of the fiber
prior to subsequent irradiation. Accordingly, in one embodiment the
fiber is stored at reduced temperature (cold storage) for a period
in the time from production, i.e. loading, to subsequent
irradiation. In one embodiment, reduced temperature is less than
-30.degree. C., such as less than -40.degree. C., such as less than
-50.degree. C., such as less than -60.degree. C., such as less than
-70.degree. C., such as less than -90.degree. C., such as less than
-100.degree. C. In one embodiment storing at reduced temperature
preserves or limits the loss of deuterium and/or Hydrogen over
time. In one embodiment storing at reduced temperature is applied
substantially until the fiber is put in operation. In one
embodiment cold storage is applied substantially until the fiber is
subjected to subsequent irradiation. In one embodiment the fiber is
stored at reduced temperature after loading either before and/or
after anneal. In one embodiment storing at reduced temperature is
applied at least part of the time between loading and/or anneal and
subsequent irradiation and/or operation. In one embodiment the
fiber is subjected to storing at reduced temperature during
operation.
[0020] In one embodiment the term subsequent in subsequent
irradiation, e.g. photo activation, refers to a maximum time
between loading and subsequent irradiation less than or equal to 2
months, such as less than or equal to 1 month, such as less than
three weeks, such as less than or equal to 14 days, such as less
than or equal to 7 days, such as less than or equal to 4 days, such
as less than or equal to 1 day, such as less than or equal to 12
hours, such as less than or equal to 6 hours, such as less than or
equal to 3 hours, such as less than or equal to 1 hour, such as
less than or equal to 1 minute. If the maximum time between loading
and subsequent irradiation is exceeded, this will in one embodiment
lead to that the lifetime extension effect of the loading is
decreased relative to subjecting the fiber to irradiation
immediately after loading. In one embodiment storing the fiber at
reduced temperature significantly reduces the effects which cause
this decrease in lifetime. In one embodiment the time where the
fiber has been stored at reduced temperature is therefore not
counted in the calculation of the maximum time as described above.
In other words, maximum time between loading and subsequent
irradiation refers in one embodiment to a maximum accumulated time,
not counting time wherein said fiber has been stored at reduced
temperature, between loading and subsequent irradiation of less
than 2 months, such as less than 1 month, such as less than 14
days, such as less than 7 days, such as less than 4 days, such as
less than 1 day, such as less than 12 hours, such as less than 6
hours, such as less than 3 hours, such as less than 1 hour, such as
less than 1 minute. In one embodiment the term stored refers to the
fiber not being used while in storage, whereas in one embodiment
stored refers to the fiber being in use while in storage.
[0021] In one embodiment subsequent irradiation is performed by
irradiation of the fiber by applying irradiation transversely to
the optical axis. In one embodiment subsequent irradiation is
performed by applying irradiation to an end facet of the fiber,
such as by coupling light or other radiation into the fiber. In one
embodiment said irradiation is side coupled to the fiber. In one
embodiment the fiber is subjected to subsequent irradiation prior
to annealing the fiber. In one such embodiment the fiber ends of
the fiber may be annealed to allow splicing of the fiber. In one
embodiment said light is coupled into the fiber by splicing the
fiber to a light source of delivery fiber. In one embodiment light
is coupled into the fiber via free-space or butt-coupling.
[0022] In one embodiment subsequent irradiation, e.g. photo
activation, comprises pumping the fiber with light having a peak
power density within said fiber equal to or higher than 10
W/.mu.m.sup.2, such as equal to or higher than 50 W/.mu.m.sup.2,
such as equal to or higher than 100 W/.mu.m.sup.2, such as equal to
or higher than 500 W/.mu.m.sup.2, such as equal to or higher than 1
kW/.mu.m.sup.2, such as equal to or higher than 2.5 kW/.mu.m.sup.2,
such as equal to or higher than 5 kW/.mu.m.sup.2, such as equal to
or higher than 10 kW/.mu.m.sup.2. In one embodiment the subsequent
irradiation comprises pumping said fiber with irradiation having an
average power of 50 mW or more, such as 100 mW or more, such as 500
mW or more, such as 1 W or more, such as more than 10 W or more,
such as 15 W or more, such as 20 W or more, such as 50 W or more,
such as 100 W or more. In one embodiment the subsequent irradiation
has a duration of more than 1/1000 of a second, such as more then
1/100 of a second, such as more than 1/10 of a second, such as more
than 1 second, such as more than 1 minute, such as more than 1
hour, such as more than 12 hours, such as more than 24 hours, such
as more than 48 hours, such as more than 1 week.
[0023] In principle the materials may be loaded at any time in the
process of forming the fiber. However, consideration may have to be
taken to ensure that processes following the loading do not disrupt
the achieved extension of the lifetime of the final fiber. Such a
disruption may occur if a process following the loading comprises
conditions which allow the loaded hydrogen/deuterium to escape from
the fiber e.g. a sufficiently high temperature over a sufficiently
long time. Accordingly, in one embodiment the loading of the core
material, and optionally of said cladding material, being performed
prior to forming said fiber, during forming of said fiber and/or
after forming said fiber. Furthermore, as shown in FIG. 5 discussed
below, fibers may, at least partially, be regenerated so that in
one embodiment the fiber is loaded after use.
[0024] After deuterium and/or hydrogen loading, the fiber is
preferably annealed. The anneal enables splicing the fiber to other
fibers (plasma heating of hydrogen/deuterium, such as in fusion
splicing, may be explosive) and reduces any added photosensitivity
due to these molecules. However, as discussed below, an anneal may
also serve other functions such as providing increased lifetime of
the fiber and increased spectral stability of embodiments of
supercontinuum light sources according to the invention.
Furthermore, in one embodiment the fiber is annealed prior to
loading of the fiber, such as to relieve defects or stress in the
fiber material. In one embodiment the fiber is anneal as a part of
the drawing process. In one embodiment fiber is annealed between
pulling the fiber and coating of the fiber. In one embodiment the
temperature of this anneal is equal to or less than the temperature
applied to the preform when pulling the fiber, such as 90% or less
of that temperature, such as 80% or less of that temperature, such
as 70% or less of that temperature, such as 60% or less of that
temperature, such as 50% or less of that temperature, such as 40%
or less of that temperature, such as 30% or less of that
temperature, such as 20% or less that temperature, such as 10% or
less that temperature. In one embodiment the anneal is a flash
anneal providing a relatively brief high energy irradiation.
[0025] In one embodiment excessive anneal temperature above
approximate 1000.degree. C. may lead to out diffusion of the bound
hydrogen/deuterium and is therefore often undesirable. In one
embodiment the anneal is performed for more than 30 minutes, such
as more than 1 hours, such as more than 4 hours, such as more than
8 hours, such as more than 16 hours, such as more than 32 hours,
such as more 64 hours. In one embodiment the anneal is a flash
anneal of the fiber with or without coating. In one embodiment said
anneal is performed at a temperature T.sub.anneal of more than
40.degree. C., such as more than 50.degree. C., such as more than
60.degree. C., such as more than 70.degree. C., such as more than
80.degree. C., such as more than 90.degree. C., such as more than
100.degree. C., such as more than 110.degree. C., such as more than
120.degree. C., such as more than 130.degree. C., such as more than
140.degree. C., such as more than 150.degree. C., such as more than
160.degree. C., such as more than 200.degree. C., such as more than
250.degree. C., such as more than 300.degree. C., such as more than
350.degree. C., such as more than 450.degree. C., such as more than
550.degree. C. In one embodiment the anneal is performed in a
chamber comprising an atmosphere adapted to be less aggressive with
respect to the materials of the fiber, such as its coating,
relative to atmospheric air. Surrounding the fiber with such an
atmosphere during the anneal may provide a reduction in damage to
the fiber and/or its coating due to the raised temperature relative
to atmospheric air. In one embodiment said atmosphere of the
chamber comprises a mixture of gasses that have low tendency to
interact chemically with the optical fiber and/or its coating. In
one embodiment the anneal is performed in an atmosphere which is
essentially free of oxygen or other free radicals. In one
embodiment the anneal is performed in an atmosphere substantially
formed by Nitrogen, Argon, an inert gas, or a mixture thereof.
[0026] In one embodiment the anneal is performed in the same
chamber that is used for loading. In one embodiment the fiber is
subjected to subsequent irradiation during the anneal. In one
embodiment the fiber is exposed to a flash irradiation in the
chamber. In one embodiment the irradiation is coupled into the
fiber such as by irradiating the fiber end inside or outside the
chamber. In one embodiment a piece of transport fiber is spliced to
the fiber to allow the entire fiber to reside in the chamber. In
one embodiment the fiber end which the transport fiber is spliced
to is annealed sufficiently to allow splicing. In one embodiment
the irradiation is a general irradiation of at least part of the
fiber surface such as via a light source in the chamber and/or
through a window in the chamber.
[0027] In one embodiment the lifetime of the fiber is extended
relative to the lifetime of an otherwise identical fiber not
subjected to loading by deuterium and/or hydrogen by more than
50%,such as more than 100%, such as more than 200%, such as more
than 500%, such as more than 1000%, such as more than 10,000%. The
absolute lifetime of a fiber subjected to light suitable for
generating a supercontinuum may vary depending on the application
as well as on the particular material of the fiber core. In one
embodiment the lifetime is more than 100 operating hours, such as
more than 200 operating hours, such as more than 2000 operating
hours, such as more than 20000 operating hours, such as more than
50000 operating hours.
[0028] In one embodiment the lifetime of a fiber has been found to
scale inversely with the peak power of the guided light to the
fourth power. Accordingly, in such embodiment a reduction of the
peak power by a factor of 2 may extend the lifetime of the fibre by
a factor 16.
[0029] In one embodiment the lifetime is measured after an initial
burn out time of more than 1 minute, such as more than 1 hour, such
as more than 5 hours, such as more than 10 hours, such as more than
15 hours, such as more than 20 hours, such as more than 24 hours,
such as more than 2 days, such as more than 7 days. In one
embodiment, an initial burn out time was observed to impose a
decrease of 10% of the spectrum in the visible emitted by a
supercontinuum light source. In this context "burn out time" is
understood as an initial time of operation where after the
system/fiber/light source reaches a steady state operation. In this
context steady state is taken to mean that performance changes are
slow relative to the performance change during the burn out time,
such as at least 50% or lower, such as at least 100% slower, such
as at least 300% slower. In one such example a laser has an output
power which increases 2% and thereafter reduces about 10% during
the initial two days of operation after which it has a 2% reduction
of output power during the next 100 days of operation all else
maintained equal. In such an example "burn out time" may be defined
as the first two days of operation. In one embodiment lifetime
could be defined relative to the initial output power and in one
embodiment relative to the maximum output power and in one
embodiment relative to the output power after two days of
operation.
[0030] In one embodiment the lifetime of a fiber is defined as time
in which the fiber may be operated according to its operational
purpose. In one embodiment the lifetime of a fiber is defined as
the time after which the absorption has increased by more than or
equal to 1%, such as more than or equal to 5%, such as more than or
equal to 10%, such as more than or equal to 20%, such as more than
or equal to 30%, such as more than or equal to 40%, such as more
than or equal to 50%, such as more than or equal to 100%, such as
more than or equal to 200%, such as more than or equal to 500%,
such as more than or equal to 1000%. In one embodiment said fiber
forms part of a light source or another system emitting light. In
one such embodiment the lifetime of the fiber is defined as the
time after which the optical output power of the light source or
system is reduced by more than or equal to 1% all else equal, such
as more than or equal to 5%, such as more than or equal to 10%,
such as more than or equal to 20%, such as more than or equal to
30%, such as more than or equal to 40%, such as more than or equal
to 50%, such as more than or equal to 60%, such as more than or
equal to 70%, such as more than or equal to 80%, such as more than
or equal to 90%. In one embodiment the optical output of said light
source and/or system emitting light is pulsed, in which case the
optical output power may be taken to be the peak power, average
power and/or pulse power. In one embodiment the optical output
power relates to the contribution of pump energy to the light
source and/or system emitting light either via optical power (such
as via a pump laser) or electrically or by other means. In one
embodiment the lifetime is defined as more than or equal to a 1%
increase in pump energy that is required to maintain the same
optical output power, such as more than or equal to 5%, such as
more than or equal to 10%, such as more than or equal to 20%, such
as more than or equal to 30%, such as more than or equal to 40%,
such as more than or equal to 50%, such as more than or equal to
100%, such as more than or equal to 200%, such as more than or
equal to 500%, such as more than or equal to 1000%. In one
embodiment said absorption or reduction of optical power output
increase is within more than or equal to 1% of the range of
wavelengths for which the fiber/system/light source is operated,
such as more than or equal to 5%, such as more than or equal to
10%, such as more than or equal to 20%, such as more than or equal
to 30%, such as more than or equal to 40%, such as more than or
equal to 50%, such as more than or equal to 60%, such as more than
or equal to 70%, such as more than or equal to 80%, such as more
than or equal to 90%.
[0031] The amount of bound deuterium and/or hydrogen may in one
embodiment be determined by spectroscopy. In one embodiment, such
determination comprises determining the height of the absorption
peak around 1380 nm of the OH.sup.- bond and/or the absorption peak
at 1870 nm of the OD.sup.- bond. As the absorption cross sections
of these bonds are known the measured peak height may in one
embodiment be used to determine the concentration of such bonds. In
one embodiment said peak(s) may be applied to determine an
overrepresentation of hydrogen and/or deuterium relative to an
unloaded fiber. In one embodiment, the amount of bound deuterium
and/or hydrogen may be determined by spectroscopy of the UV
spectrum particularly around 240 nm. As glass is highly absorbent
in this region only a fairly short fiber may often be investigated
at a time. Further details of such measurements may be found in M.
J. Yuen, "Ultraviolet absorption studies of the germanium silicate
glasses", Appl. Opt., Vol. 21, 1, 1982, pp. 136-140.
[0032] In one embodiment, an over representation of bound deuterium
relative to an unloaded fiber, consistent with loading of the fiber
according to invention, may be determined at least partially by
secondary ion mass spectroscopy (SIMS). In one such embodiment the
amount of OD.sup.- is determined. In one embodiment the amount of
SiD.sup.- is determined as the measurement chamber may comprise
gaseous oxygen whereas there is commonly little or no gaseous
silicium in the measurement chamber. In one embodiment the
determination is performed relative to an unloaded fiber as
variations in isotope concentration etc. may be cancelled.
[0033] In one embodiment determination of whether a fiber has been
loaded according to the invention is performed by observing changes
in the lifetime of the fiber pre and post an anneal designed to
reduce bound deuterium and/or hydrogen. In one embodiment the fiber
is annealed at a temperature where the stability of the components
of the fiber, such as the coating, is ensured while the duration of
the anneal may be determined by this temperature and the knowledge
of the activation energy for breaking the bond in question. A drop
in lifetime due to the specially designed anneal is consistent with
a loading of the fiber according to the invention. Such a drop in
lifetime may be a change in absorption spectrum or a change in the
emission spectrum for a supercontinuum light source (such as
discussed below).
[0034] In one embodiment the optical fiber is a non-linear optical
fiber. In one embodiment a non-linear fiber is taken to mean a
fiber which guides light for at least a range of wavelengths
.lamda..sub.min to .lamda..sub.max and has a non-linear parameter
.gamma., wherein for at least part of said range the product
.gamma..lamda. is more than or equal to 410.sup.-9 W.sup.-1, such
as more than or equal to 510.sup.-9 W.sup.-1, such as more than or
equal to 610.sup.-9 W.sup.-1, such as more than or equal to
710.sup.-9 W.sup.-1, such as more than or equal to 810.sup.-9
W.sup.-1, such as more than or equal to 1010.sup.-9 W.sup.-1, such
as more than or equal to 2010.sup.-9 W.sup.-1, such as more than or
equal to 4010.sup.-9 W.sup.-1. The non-linear parameter .gamma. is
defined as
.gamma. = 2 .times. .pi. .times. .times. n 2 .lamda. .times.
.times. A eff , ##EQU00001##
[0035] where here n.sub.2 is the nonlinear refractive index of the
fiber material and A.sub.eff is the effective mode area of the
fiber. As an example, n.sub.2 is commonly approximately
2.610.sup.-20 m.sup.2/W for silica glass.
[0036] In one embodiment a non-linear fiber is taken to mean a
fiber having a non-linear parameter .gamma. when guiding a
wavelength of 1550 nm wherein .gamma. is more than or equal to
310.sup.-3 (Wm).sup.-1, such as more than or equal to 510.sup.-3
(Wm).sup.-1, such as more than or equal to 1010.sup.-3 (Wm).sup.-1,
such as more than or equal to 1510.sup.-3 (Wm).sup.-1, such as more
than or equal to 2010.sup.-3 (Wm).sup.-1, such as more than or
equal to 3010.sup.-3 (Wm).sup.-1, such as more than or equal to
4010.sup.-3(Wm).sup.-1, such as more than or equal to 5010.sup.-3
(Wm).sup.-1.
[0037] In one embodiment a non-linear fiber is taken to mean a
fiber having a non-linear parameter .gamma. when guiding a
wavelength of 1064 nm wherein .gamma. is more than or equal to
510.sup.-3 (Wm).sup.-1, such as more than or equal to 1010.sup.-3
(Wm).sup.-1, such as more than or equal to 1510.sup.-3 (Wm).sup.-1,
such as more than or equal to 2010.sup.-3 (Wm).sup.-1, such as more
than or equal to 3010.sup.-3 (Wm).sup.-1, such as more than or
equal to 4010.sup.-3 (Wm).sup.-1, or such as more than or equal to
5010.sup.-3 (Wm).sup.-1.
[0038] In one embodiment a non-linear fiber is taken to mean a
fiber where said fiber guide light for at least a range of
wavelengths .lamda..sub.min to .lamda..sub.max and a mode field
diameter (MFD) of the fundamental mode least part of said range the
fraction MFD/.lamda. is less than or equal to 5, such as less than
or equal to 4, such as less than or equal to 3, such as less than
or equal to 2, such as less than or equal to 1.
[0039] In the above embodiments the range of wavelengths
.lamda..sub.min to .lamda..sub.max may be selected from the group
of 350 nm to 2000 nm, 980 nm to 1550 nm, 1100 to 1550 nm, 1300 nm
to 1450 nm, 1500 nm to 4000 nm, 1500 nm to 6000 nm, 1500 nm to
11000 nm. In one embodiment .lamda..sub.min is more than or equal
to 300 nm, such as more than or equal to 350 nm, such as more than
or equal to 450 nm, such as more than or equal to 600 nm, such as
more than or equal to 750 nm, such as more than or equal to 900 nm,
such as more than or equal to 1050 nm, such as more than or equal
to 1200 nm, such as more than or equal to 1350 nm, such as more
than or equal to 1500 nm, such as more than or equal to 2000 nm,
such as more than or equal to 3000 nm. In one embodiment
.lamda..sub.max is less than or equal to 11000 nm, such as less
than or equal to 10000 nm, such as less than or equal to 8000 nm,
such as less than or equal to 5000 nm, such as less than or equal
to 2500 nm, such as less than or equal to 2000 nm, such as less
than or equal to 1500 nm, such as less than or equal to 1000 nm,
such as less than or equal to 800 nm, such as less than or equal to
600 nm, such as less than or equal to 500 nm. In one embodiment the
range of wavelengths .lamda..sub.min to .lamda..sub.max is selected
so as to limit the consideration to the range of wavelengths
wherein the fiber is single mode.
[0040] In one embodiment a non-linear fiber is taken to mean a
fiber having an MFD when guiding a wavelength of 1550 nm wherein
said MFD is less than or equal to 10 .mu.m, such as less than or
equal to 8 .mu.m, such as less than or equal to 6 .mu.m, such as
less than or equal to 5 .mu.m, such as less than or equal to 4
.mu.m, such as less than or equal to 3 .mu.m, such as less than or
equal to 2 .mu.m, such as less than or equal to 1 .mu.m.
[0041] In one embodiment a non-linear fiber is taken to mean a
fiber having an MFD when guiding a wavelength of 1064 nm wherein
said MFD is less than or equal to 6 .mu.m, such as less than or
equal to 5 .mu.m, such as less than or equal to 4 .mu.m, such as
less than or equal to 3 .mu.m, such as less than or equal to 2
.mu.m, such as less than or equal to 1 .mu.m.
[0042] In a preferred embodiment the fiber is a silica fiber,
wherein at least a part of the core being of silica or doped
silica, preferably at least the entire core being of silica or
doped silica, such as the entire core and part or all of the
cladding being of silica or doped silica.
[0043] In one embodiment the fiber is at least partly made of glass
from the group of soft glasses, such as e.g. fluoride classes (such
as e.g. ZBLAN, Fluorozirconate, Fluoroindate, Fluoroaluminate or
Fluorogallate) or a chalcogenide (such as sulphide, selenide, and
telluride and the IRT-SU product family offered by Corative) This
embodiment may be particularly suitable for applications where the
wavelengths range comprises wavelengths larger than 2 .mu.m. In one
embodiment it might be preferable to load with hydrogen instead of
deuterium to avoid the OD absorption peak around 1870 nm that may
otherwise arise.
[0044] In one embodiment the invention relates to an optical system
comprising an optical fiber according to the invention and a
feeding unit wherein said feeding unit is adapted to feed said
fiber with light with a peak power density within said fiber equal
to or higher than 10 W/.mu.m.sup.2, such as equal to or higher than
50 W/.mu.m.sup.2, such as equal to or higher than 100
W/.mu.m.sup.2, such as equal to or higher than 500 W/.mu.m.sup.2,
such as equal to or higher than 1000 W/.mu.m.sup.2, such as equal
to or higher than 2500 W/.mu.m.sup.2, such as equal to or higher
than 5,000 W/.mu.m.sup.2, such as equal to or higher than 10,000
W/.mu.m.sup.2. Application where such high peak power density in
the fiber are in the present document referred to as high power
applications. It should be noted, that while reference is made to
peak power which commonly is a feature of pulsed light, peak power
may in this context refer to CW light as well.
[0045] In one embodiment said feeding unit comprises a pump light
source and in one embodiment the feeding unit also comprises one or
more amplifiers. In principle the feeding unit may be any optical
system feeding light to the fiber.
[0046] As the fiber according to the invention has no or reduced
degradation due to exposure to high peak power such a system may
have an extended lifetime of operation depending on the
significance of the fiber performance and life-time relative to
that of other components in the system.
[0047] In one aspect the invention relates to a method of producing
an optical fiber comprising a core and a cladding comprising a core
material and a cladding material, respectively, said fiber having
extended lifetime in high power applications, the method comprising
[0048] a. loading said core material and optionally said cladding
material with hydrogen and/or deuterium [0049] b. optionally
annealing said for a time t.sub.anneal at a temperature
T.sub.anneal.
[0050] Such a method may be advantageously applied to produce a
fiber according to the invention and any features described in
relation to features of the fiber may apply mutatis mutandis to the
method of producing the fiber.
[0051] In one embodiment the said loading is performed by
subjecting the core material and/or the cladding material to
hydrogen and/or deuterium under loading conditions suitable to
allow hydrogen and/or deuterium to bind chemically to said
material(s). In one embodiment the said loading is performed by
subjecting the core material and/or the cladding material to
hydrogen and/or deuterium under loading conditions comprising at
least one of a) a raised temperature T, b) a raised pressure P c)
irradiation and/or d) subsequent irradiation.
[0052] In an embodiment the invention relates to an apparatus
comprising an optical fiber according to the invention, an optical
system according to the invention, a light source according to the
invention and/or a fiber produced according to the invention. In
one embodiment the apparatus constitutes a system for performing
one from the group of laser precision spectroscopy, various forms
of fluorescent microscopy, guiding of surgical and/or therapeutic
light, astronomy (guide star generation), confocal microscopy
endoscopy, optical coherence tomography (OCT) and combinations
thereof.
[0053] As mentioned above, it has been found that it is often
possible to regenerate a degenerated fiber, thus providing a fiber
with extended lifetime compared to an identical fiber not subjected
loading. Accordingly, in one embodiment the invention relates to a
method of regenerating a fiber comprising a core and a cladding
comprising a core material and a cladding material, respectively,
said fiber having increased absorption in the visible due to
subjection to light in a high power application the method
comprising loading the fiber with hydrogen and/or deuterium. In one
embodiment loading of said fiber is carried out under loading
conditions allowing hydrogen and/or deuterium to bind to the core
material and/or a cladding material. In one embodiment loading of
said fiber is carried out under loading conditions comprising at
least one of a) a raised temperature T, b) a raised pressure P c)
irradiation and/or d) subsequent irradiation. Furthermore, any
features described in relation to loading of the fiber prior to use
discussed above may in one embodiment apply mutatis mutandis to the
method of producing the fiber.
[0054] It should be emphasized that the term "comprises/comprising"
when used in this specification is taken to specify the presence of
stated features, integers, steps or components but does not
preclude the presence or addition of one or more other stated
features, integers, steps, components or groups thereof.
BRIEF DESCRIPTION OF DRAWINGS
[0055] The invention will be explained more fully below in
connection with preferred embodiments and with reference to the
drawings in which:
[0056] FIG. 1 shows a part of a typical supercontinuum spectra in
initial operation of a prior art microstructured optical fiber (A)
and after 35 hours of operation (B) all else equal. The reduction
in the visible spectrum testifies to the degradation of the
fiber;,
[0057] FIG. 2 shows measured attenuation for a prior art
microstructured nonlinear fiber operated for 35 hours as a function
of the position of the fiber;
[0058] FIG. 3 shows 633 nm absorption as function of position in
the microstructured nonlinear fiber, where position means length
from the entrance of the pump light;
[0059] FIG. 4 shows the initial visible supercontinuum spectra (A),
after 35 hours of operation where the visible dip is observed (B)
and again after heating the fiber to 250.degree. C. (C);
[0060] FIG. 5 shows supercontinuum spectra after 35 hours of
operation (A) where a visible dip is observed and again after
heating the fiber to 250.degree. C. (B) and after the fiber has
been deuterium loaded and annealed (C);
[0061] FIG. 6 shows measured visible power as function of time for
microstructured nonlinear fibers deuterium loaded at 160 C (A), at
80 C (B) and not deuterium loaded (C);
[0062] FIG. 7 shows extracted lifetime as function of three
different deuterium loading temperatures (A) and an exponential fit
to the measurements (B);
[0063] FIG. 8 shows measured spectra for a deuterium loaded
microstructured nonlinear fiber after 0 hours (A), 188 hours (B),
260 hours (C), 305 hours (D) and 450 hours (E) of operation;
[0064] FIG. 9 shows measured visible power as function of time for
microstructured nonlinear fibers with less glass impurities
compared to the previous figures;
[0065] FIG. 10 shows results from a transmission experiment
involving a microstructed fibre, showing the transmission just post
loading (C), and 22 hours after loading where the ends of the fiber
has been sealed (B). For comparison, a transmission curve is shown
for a similarly loaded microstructured with unsealed fiber ends 2.5
hours post loading (A);
[0066] FIG. 11 shows one example of spectra obtained from a
supercontinuum light source comprising a non-linear microstructured
fibre according to the invention. The spectra are an initial
spectrum (A) and a spectrum after a 160 hour operation (B);
[0067] FIG. 12 shows the coupling efficiency spectra from a
supercontinuum light source according to the invention to a single
mode fiber. The spectra are an initial spectrum (A), and spectra
after a 273 hours (B) and 535 hours of operation (C). The
microstructured fiber has been annealed for 4 hours at 80 C at a
pressure of a standard atmosphere;
[0068] FIG. 13 shows M.sup.2 measurements vs. wavelength for a
supercontinuum light source according to the invention. The
microstructured fiber has been annealed for 4 hours at 160 C in an
atmosphere of nitrogen. The spectra are an initial spectrum (A) and
a spectrum after 3500 hours of operation where the fiber has
degraded in the visible region (B);
[0069] FIG. 14 shows spectra obtained from a supercontinuum source
according to the invention after 3200 hours of operation. The
microstructured fiber has been annealed for 4 hours at 160 C in an
atmosphere of nitrogen. The spectra A is for a supercontinuum
source with an uncoiled loaded nonlinear fiber according to
invention, whereas in B, the nonlinear fiber is coiled with a
radius of 25 mm; and
[0070] FIG. 15 is a schematic cross-sectional view of an optical
fiber according to an embodiment of the invention. The drawing is
not made to scale, and is not intended to illustrate the relative
diameters and thicknesses of the core material and cladding.
[0071] Further scope of applicability of the present invention will
become apparent from the detailed description given hereinafter.
However, it should be understood that the detailed description and
specific examples, while indicating preferred embodiments of the
invention, are given by way of illustration only, and should not be
taken to limit the inventions as set forth by the appended set of
claims.
DETAILS OF THE INVENTION
[0072] In the following some examples will comprise discussion of
the invention based on measured data. The conclusions drawn from
these should not necessarily be considered limited to the specifics
of the underlying experiments.
[0073] Super Continuum Generation in Microstructured Fibers
[0074] In one embodiment, of the invention the optical fiber is a
microstructured optical fiber. Microstructured optical fibers are a
relatively new technical field where the properties of the
waveguide may be designed with a relatively large degree of
freedom. Such fibers are commonly made of pure silica comprising a
pattern, often made of holes or doped glass, extending in the
longitudinal direction of the fiber. The freedom of design makes
such fibers interesting for application requiring specific
non-linear properties of the fiber. One such application is
supercontinuum generation wherein a fiber based source is cable of
generating a wide spectral output. Supercontinuum (SC) generation
in microstructured fibers has been studied for several years as a
source of broadband light (termed white light or supercontinuum).
While new applications of such sources are continuously discovered,
several have already been identified, such as various forms of
fluorescent microscopy, laser precision spectroscopy, and optical
coherence tomography (OCT). High brightness emission in the visible
part of the spectrum is especially important for confocal
fluorescent microscopy. SC-generation with relatively high power in
the visible has been targeted in the experiments presented here.
Most research has so far been based on seeding the microstructured
fiber with femtosecond (fs)-lasers but SC-generation using
nanosecond- and picosecond (ps)-lasers has also been
demonstrated.
[0075] As microstructured fibers often guides by holes extending in
the cladding such fibers often consist entirely of un-doped silica
(i.e. both core and cladding are made of silica) in opposition to
e.g. standard single mode communication fibers where the core is
commonly doped with germanium in order to change the refractive
index. Accordingly, in one embodiment the core of the fiber
comprises a Germanium content of less than or equal to 20 at %,
such as less than 5 at %, such as less than 3 at %, such as less
than 2 at %, such as less than 0.1 at %, such as less than 0.01 at
%, such as less than 0.001 at %.
[0076] In one embodiment, submitting the fiber to subsequent
irradiation may significantly improve the lifetime of the fiber. In
one embodiment experimental results have shown a reduction of
optical power of a supercontinuum light source of 30% after 40
hours of operation when the fiber has not been loaded, 30% after 80
hours when the fiber has been loaded and the ends subsequently
sealed and the fiber stored at room temperature for over 1 month
and only 4% after 200 hours when the fiber was loaded and photo
activated within 1 week of load with the ends sealed. It may be
noted that in one embodiment experiments have shown that the above
degradation scales with applied peak power to the fourth power.
Accordingly, the above periods may in one embodiment be extended by
e.g. a factor of 16 by reducing the applied optical peak power with
50%. In one embodiment the latter result was found to drop
approximately 10% over the first 24 hours where after the source
showed this stability.
[0077] In one embodiment, the application of irradiation of the
fiber subsequent to loading utilizes gaseous hydrogen/deuterium
residing in the microstructures formed by holes of a
microstructured fiber. In one embodiment the fiber is annealed to
improve the life time of the fiber and/or to allow splicing of the
end, such as discussed above. However in one embodiment it may be
preferable to allow gaseous hydrogen/deuterium to reside in the
holes of the microstructure just prior to or during subsequent
irradiation, such as photo activation. In such embodiments it may
be preferable to seal one or both ends of the fiber. Said sealing
is performed prior to storage at reduced temperature and/or prior
to loading. In one embodiment sealing is performed by supplying
sufficient thermal power, such as by an arch or fusion splicer. In
one embodiment sealing is performed by applying a resin, such as
epoxy to the fiber ends. In one embodiment said resin may be UV
cured. As shown in FIG. 10, the initial absorption shown by the dip
in transmission around 1710 nm (101) is substantially unchanged for
a fiber just after loading (B) and 22 hours post loading with
sealed ends (C). On the other hand the same peak is shown to be
substantially gone after just 2.5 hours when the ends are left
unsealed (A). In some embodiments of the invention the application
of such microstructured fibers is selected from the group of
supercontinuum generation, optical power transport, optical
amplification and forming part of a laser cavity. It is preferred
that the irradiation is performed subsequent to loading; however,
in one embodiment, it is performed during loading.
[0078] In one embodiment sealing of the fiber ends may be
preferable to minimize the entrance of impurities into the fiber
via the microstructures. In one embodiment such impurities
comprises water, which may condensate on the fiber post having the
fiber stored at reduced temperature. In one such embodiment said
water may otherwise travel to the inside of the fiber via capillary
forces and/or diffusion.
[0079] As will be apparent to the skilled person, the
considerations in regard to gaseous hydrogen/deuterium in the
microstructures and the sealing of these may in one embodiment be
relevant for applications of microstructured fibers other than
super continuum generation, such as power delivery.
[0080] In one embodiment the invention relates to a supercontinuum
light source comprising a pulsed pump light source and an optical
fiber according to the invention wherein said pump source is
adapted to provide light with a peak power density within said
fiber equal to or higher than 10 W/.mu.m.sup.2, such as equal to or
higher than 50 W/.mu.m.sup.2, such as equal to or higher than 100
W/.mu.m.sup.2, such as equal to or higher than 0.5 kW/.mu.m.sup.2,
such as equal to or higher than 1 kW/.mu.m.sup.2, such as equal to
or higher than 2.5 kW/.mu.m.sup.2, such as equal to or higher than
5 kW/.mu.m.sup.2, such as equal to or higher than 10
kW/.mu.m.sup.2, and/or wherein said pump and fiber is adapted to
provide an output spanning over at least one octave with at least
10 .mu.W/nm and/or wherein said pump and said fiber is adapted to
provide a maximum modulation instability gain .OMEGA..sub.max such
as larger than 20, such as larger than 40.
[0081] Here the modulation instability gain .OMEGA..sub.max is
given by
.OMEGA. max = .+-. 2 .times. .gamma. .times. .times. P peak .beta.
2 , ##EQU00002##
[0082] where .beta..sub.2 is the group velocity at the pump
wavelength, P.sub.peak is the peak power of the pump and .gamma. is
the pump wavelength.
[0083] In one example more than one octave span has been achieved
with the microstructured nonlinear fiber SC-5.0-1040 from the
Danish company Crystal Fiber A/S. Using this fiber with a peak
power of 200 W pumped at 1064 nm provided .OMEGA..sub.max=22 (A
peak power of 200 W is e.g. obtained through a 50 MHz, 100 mW input
signal with 10 ps pulses).
[0084] In one embodiment the pump light source comprises a laser
which may be pulsed or continuous wave. The laser may in principle
be any suitable laser to provide the desired wavelength(s), power
and/or temporal performance (i.e. pulse length, repetition rate
etc.). In one embodiment said laser is a fiber laser, such as a
mode locked fiber laser. In one embodiment the pump light source
further comprises one or more amplifiers arranged to amplify the
output of said laser. In one such embodiment the laser light source
is formed by a so-called MOPA configuration.
[0085] The phrase spanning over at least one octave with at least a
specific power value (per nm wavelength) is in this context of the
present invention taken to mean that the optical spectrum of the
output of the light source spans at least an octave defining the
outer limits of said spectrum by said specific power value. The
spectrum may have holes; however, it is assumed that more than 25%
of the spanned spectrum has at least the specific power value. In
an embodiment at least 30% of the spanned spectrum has at least the
specific power value, such as at least 40%, such as at least 60%,
such as at least 80%, such as at least 99%, such as at least
99.9%.
[0086] In one embodiment, the output spans over at least one octave
with at least 50 .mu.W/nm, such as more than or equal to 500
.mu.W/nm, such as more than or equal to 1 mW/nm, such as more than
or equal to 5 mW/nm, such as more than or equal to 10 mW/nm.
Depending on the chosen power limit one embodiment may also span
over more than or equal to 0.5 octave, such as more than or equal
to 1.5 octave, such as more than or equal to 2 octaves.
[0087] In one embodiment the spectral degradation of a
supercontinuum light source light comprising a non-linear
microstructured fiber is less than 5% over more than 50 hours, such
as over more than 100 hours, such as over more than 500 hours, such
as over more than 1000 hours. In one embodiment the light system
with which the light source is made to interact is recalibrated at
least every 1000 hours, such as at least every 500 hours, such as
at least every 100 hours, such as at least every 50 hours. In one
embodiment anneal of the fiber is preferred in order to improve the
spectral stability.
[0088] In one embodiment the non-linear fiber is polarization
maintaining (PM) as this in one embodiment may provide a similar
spectrum with a 50% reduction of the necessary peak power. In one
embodiment the degradation scales with the applied peak to the
fourth power so that a significant extension of the lifetime of the
fiber and thereby the light source may be available. In one
embodiment this requires a good polarization extinction ratio of
the feed system pumping the non-linear fiber, such as more than 10
dB, such as more than 13 dB, such as more than 15 dB, such as more
than 17 dB, such as more than 20 dB.
[0089] In one embodiment the non-linear fiber and the feed system,
i.e. feeding unit, are coupled using bulk optics. However, as a
bulk optic coupling system may provide many degrees of freedom and
is often prone to mechanical and thermal instability splicing of
the two components may be preferred. In one embodiment the feed
system is spliced to the non-linear fiber. In one embodiment the
feed system comprises an optical amplifier providing the output of
the feed system into the non-linear fiber. In one embodiment there
is a significant mismatch in core size between the feed system
(e.g. a diameter of 11 .mu.m) and the non-linear fiber (e.g. a
diameter of 3 .mu.m). In one embodiment this mismatch is reduced by
allowing the core of the non-linear fiber to expand during
splicing.
[0090] In the following measured data were obtained for a
supercontinuum light source comprising a pump source and a
non-linear microstructured silica fiber. The fiber was pumped at
1064 nm with 8 ps pulses at a repetition rate of 80 MHz providing a
15 W input average power (23 kW peak power). The fiber had a mode
field diameter of 3.5 .mu.m, and air-filling fraction of about 50%
and was approximately 7 meters in length. The pump light source was
formed by a master-oscillator power amplifier (MOPA) design
comprising a mode-locked laser, a preamplifier and a power
amplifier followed by two pre-amplifiers.
[0091] The length of the fiber is preferably kept short to keep the
consumption of fiber to a minimum while still providing sufficient
length to allow the non-linear processes underlying a
supercontinuum to provide a desirable spectrum. This length
commonly depends on the shape of the pulses as shorter fiber is
commonly sufficient for shorter pulses. In one embodiment the
non-linear fiber have a length of 1 cm or longer, such 10 cm or
longer, such 1 m or longer, such 5 m or longer, such as 8 m or
longer, such as 10 m or longer.
[0092] In one embodiment the non-linear microstructured fiber is
500 m or less, 100 m or less, 50 m or less, such as 30 m or less,
such as 10 m or less.
[0093] FIG. 1 shows typical supercontinuum spectra in initial
operation of a prior art microstructured optical fiber (A) and
after 35 hours of operation (B) all else equal. The reduction in
the visible portion of the spectrum extending from about 450 nm to
about 750 nm testifies to the degradation of the fiber. The
phenomenon is investigated further by the measurements shown in
FIG. 2 showing attenuation for the prior art microstructured
nonlinear fiber operated for 35 hours as a function of the position
of the fiber from the entrance of the pump light. A is measured
through the first 3 m of the nonlinear fiber (NL-fiber), B is
through 3-4 m, C through 4-5 m and D through 5-7 m. The curves are
obtained by subtracting a spectrum obtained with a 7 m long
reference non-linear fiber which has not been operated with high
power for a longer duration of time. Very large absorption is
observed in the visible part of the spectrum due to the degradation
of the fiber. The dip at 0.9 .mu.m and 1.4 .mu.m likely stems from
the single-mode cut-off for the microstructured nonlinear fiber and
differences in O--H peak absorption for the microstructured
nonlinear fiber and the reference fiber, respectively. If the
degradation is caused by interaction with the relatively high peak
powered pump pulse, then the degradation is expected to be larger
closer to the pump laser where the peak power is maximal. As the
pump pulses travel through the fiber their average power decreases
due to attenuation. Furthermore, non-linear effects will tend to
broaden the pulse to reduce the peak power of the pulses along the
fiber. Therefore less degradation is expected along the fiber
further from the injection of the pump pulses. This tendency is
seen in this example as the absorption drops as the fiber sections
are taken from parts which were operated further and further from
the pump. This trend is also found in FIG. 3 showing that
measurements of the absorption at 633 nm as a function of distance
from the entrance of the pump light fit well to an exponential.
[0094] FIG. 4 shows the supercontinuum spectra in the beginning of
the experiment (A), after 35 hours were the visible dip is observed
(B) and again after heating the fiber to 250.degree. C. (C).
[0095] The heating seems to partly regenerate the fiber. The
inventors hypothesize that the regeneration of the fiber may be an
indicator of the pump light altering the structure of at least a
part of the glass. Allowing the glass to reach a higher temperature
may allow the glass to resettle causing it to at least partly
regenerate.
[0096] FIG. 5 shows supercontinuum spectra after 35 hours (A) where
a visible dip is observed and again after heating the fiber to
250.degree. C. (B) and after the fiber has been deuterium loaded
and subsequently annealed (C). The deuterium loading clearly
regenerated the fiber and the spectrum resembles the initial
spectrum (see FIG. 4) without any visible dip in the spectrum.
[0097] FIG. 6 shows measured visible power of a supercontinuum
source as function of time for 3 pieces of identical
microstructured nonlinear fibers deuterium loaded at 160 C (A), at
80 C (B) and not deuterium loaded (C). The lifetime of the
deuterium loaded fibers is increased by at least 2 orders of
magnitude compared to unloaded fibers. All fibers are loaded at 100
bar pressure with 100% deuterium.
[0098] FIG. 7 shows lifetime extracted from FIG. 6 as a function of
deuterium loading temperature (A) and an exponential fit to the
measurements (B). In this example the lifetime was defined as the
time where the visible power has decreased 30%. Depending on the
application the lifetime may be defined as where the visible power
has decreased by more than 40%, such as more than 50%, such as more
than 70%, such as more than 80%, such as more than 90%. Visible
light may in the context be defined as an integral of light in the
range 0.4 to 0.7 .mu.m. Alternatively, one or more wavelength
values may be specified such as the power at 650 nm and/or at 633
nm. As discussed above these results may indicate that the lifetime
of the fiber increases exponentially with loading temperature, at
least for the temperatures applied here and that loading at
increased temperature may be advantageous as long as practical
factors such as the temperature tolerance of the coating is
considered.
[0099] FIG. 8 shows measured spectra for a deuterium loaded
microstructured nonlinear fiber after 0 hours (A), 188 hours (B),
260 hours (C), 305 hours (D), and 450 hours (E) . The prominent
broad dip for the non-loaded microstructured fiber in the visible
spectrum from 0.4 to 0.7 .mu.m is no longer observed. In addition
to increasing the lifetime of the microstructured nonlinear fiber
the deuterium loading has also shown in this embodiment to
significantly alter the spectral changes of the fiber under
operation compared to unloaded fibers. Relative to an unloaded
fiber the degradation is no longer observed as a dip in the visible
spectrum, but as a broadening of the long wavelength peak around
475 nm and a slow overall decrease of visible power.
[0100] FIG. 9 shows measured visible power as function of time for
microstructured nonlinear fibers with less glass impurities for a
Deuterium loaded (A) and unloaded (B) fiber. Again the lifetime of
the deuterium loaded fiber (A) is significantly increased compared
to the unloaded fiber (B). The increase in output power for the
deuterium loaded fiber after 750 hours is due to an increase in
pump power. Compared to FIG. 5 the lifetime is significantly
extended indicating that the lifetime may also depend on the glass
impurity level.
[0101] FIG. 11 shows one example of spectra obtained from a
supercontinuum light source comprising a non-linear microstructured
fibre according to the invention. The shown spectra are an initial
spectrum (A) and a spectrum after a 160 hours of operation (B). It
has been observed that in many of the embodiments of the invention
the spectrum shows a reduction in a peak at short wavelengths,
commonly around 480 nm, whereas a peak rises from the remaining
spectrum around 550 nm in the present embodiment. This peak around
550 nm has not been observed for supercontinuum light sources
comprising an unloaded non-linear microstructured fiber as evident
from the other figures.
[0102] In one embodiment of a supercontinuum light source according
to the invention it has been observed that the increase in output
power around 550 nm shown in FIG. 11 occurs simultaneously with a
decrease in spatial mode quality of the output light at wavelengths
lower than about 550 nm, i.e. the light is increasingly multimoded.
This decreased beam quality may be identified in many ways, e.g. by
measuring the coupling efficiency to a single-mode fiber or by
measuring the M-square value. FIG. 12 shows the coupling efficiency
from a super continuum source according to the invention to a
single mode fiber as a function of wavelength for 0, 273 and 535
hours of operation. The measurement uncertainty is a few percent
and thus the coupling efficiency above 550 nm is within the
measurement uncertainty unchanged with time. However, below 550 nm
the coupling efficiency drops with time.
[0103] FIG. 13 shows the M.sup.2 spectra for a supercontinuum
source according to the invention. The spectra are an initial
spectrum (A) and a spectrum after 3500 hours of operation where the
fiber has degraded in the visible region (B). Notice that the time
until degration occurs is much longer than in FIG. 12. The
difference between FIGS. 13 and 12 is that in FIG. 13 the nonlinear
fiber has been annealed for 4 hours at 160 C in an atmosphere of
nitrogen prior to use, whereas in FIG. 12 the nonlinear fiber has
been annealed for 4 hours at 80 C in a standard atmosphere prior to
use.
[0104] In one embodiment the time of operation until the described
change around 550 nm occurs depends on the anneal conditions of the
fiber used to generate the super continuum, such as a
microstructured or standard nonlinear fiber. In one embodiment
increased temperature of the anneal extends this time of operation
before such changes around 550 nm are observed. In one embodiment
the time during which the fiber is annealed has a similar effect.
In one embodiment this correlation indicates that too much residual
hydrogen or deuterium in the fiber may affect said time of
operation. In one embodiment the fiber is annealed after subsequent
irradiation. In one embodiment this has the effect of allowing
residual hydrogen and/or deuterium to provide a benefit during
subsequent irradiation and subsequently to that removing at least
part of the residual hydrogen/deuterium. In one embodiment the
fiber is annealed prior and post subsequent irradiation.
[0105] As the light below about 550 nm becomes more multimoded more
optical energy may be coupled to these higher order modes. In one
embodiment the fiber is coiled to strip these higher order modes
for wavelengths of less than about 550 nm and/or to prevent
coupling to such higher order modes. In one embodiment the fiber
comprises a chirally coupled core to strip these higher order modes
for wavelengths of less than about 550 nm and/or to prevent
coupling to such higher order modes. FIG. 14 shows the output
spectrum of a supercontinuum light source according to the
invention after 3200 hours of operation. The microstructure fiber
has been annealed for 4 hours at 160 C in an atmosphere of nitrogen
prior to use. The spectra A is for a normal supercontinuum source,
whereas in B the nonlinear fiber is coiled with a radius of 20 mm.
It is observed that the coiling decreases the output power at the
550 nm peak and furthermore increases the short wavelength peak
around 480 nm as well as restores the M.sup.2 value.
[0106] The inventors have surprisingly found that in one embodiment
a relatively narrow coil is required but also that such a coil may
in one embodiment function even when the fiber is a micro
structured fiber, which would otherwise be considered sensitive to
such mechanical stress. In one embodiment at least part of the
fiber is coiled with a minimum diameter R where R is less than or
equal to 40 mm, such as less than or equal to 30 mm such as less
than or equal to 25 mm, such as less than or equal to 20 mm, such
as less than or equal to 15 mm, such as less than or equal to 10
mm, such as less than or equal to 5 mm. In a coil the radius of
each winding may vary e.g. depending on the winding method and
whether the winding is the inner most winding or not. In this case,
the minimum radius R refers to the radius of the winding with the
smallest radius.
[0107] In one embodiment the position of the coil relative to where
the pump light is injected affects the efficiency of the coil to
prevent the drop in average optical output power). Closer to the
injection of the pump light the peak power of the pump pulse are
higher and the formation of the different wavelengths of the output
spectrum of the super continuum source may in one embodiment be at
least partly related to the position along the fiber. Accordingly,
in one embodiment the fiber has an input end coupled to said pump
light source and an output end, wherein said coiling is performed
less than 50 cm from the input end, such as less than 40 cm, such
as less than 30 cm, such as less than 20 cm, such as less than 10
cm, such as less than 5 cm. To use the stripping effect provided by
the coil at a position where specific wavelengths of the output
spectrum are generated, the coil may, in one embodiment, cover at
least a distance which is more than 5 cm from the input end, such
as more than 10 cm from the input end, such as more than 20 cm from
the input end, such as more than 30 cm from the input end, such as
more than 40 cm from the input end, such as more than 50 cm from
the input end, such as more than 70 cm from the input end, such as
more than 80 cm from the input end. In one embodiment little of the
fiber is required to be coiled to reduce the said degradation at
wavelengths below 550 nm. In one embodiment more of the fiber is
required to be coiled, such as to prevent the degradation occurring
in an uncoiled section of the fiber. In one embodiment more than or
equal to 10% of said fiber is coiled, such as more than or equal to
20% of said fiber is coiled, such as more than or equal to 30% of
said fiber is coiled, such as more than or equal to 40% of said
fiber is coiled, such as more than or equal to 50% of said fiber is
coiled, such as more than or equal to 60% of said fiber is coiled
such as more than or equal to 70% of said fiber is coiled, such as
more than or equal to 80% of said fiber is coiled, such as more
than or equal to 90% of said fiber is coiled, such as 100% of the
fiber is coiled. In one embodiment one winding is sufficient to
prevent the degradation discussed above and/or to strip higher
order modes and/or suppress coupling of light from the fundamental
mode to higher order modes. However, in one embodiment two or more
windings may be required to provide sufficient effect. Accordingly,
in one embodiment said coil comprises 1 or more windings, such as 2
or more windings, such as 10 or more windings, such as 25 or more
windings, such as 50 or more windings, such as 100 or more
windings. In one embodiment the number of required winding
decreases with the winding radius. In one embodiment it may be
preferable to wind a long section, such as all, of the fiber with a
lesser larger radius rather than submit the fiber to the mechanical
stress imposed by a smaller radius.
[0108] As will be obvious to the skilled person, the detailed
effects of increasing and decreasing peaks as well as the region of
wavelengths for which the beam quality drops discussed here are
exemplary relating to the embodiments. It is clear that one or more
of the discussed effects may depend on the overall design of the
supercontinuum light source such as pump properties (e.g.
wavelength, peak power, pulse energy etc.) and/or fiber (e.g.
material, core size, dopants, mode field diameter etc.). Therefore,
it should be recognized that increased anneal temperature and/or
stripping of higher order modes, such as described above, may have
application in any of the embodiments of the invention including
the optical fiber as such and making of the same.
[0109] In one embodiment lifetime extension is provided by bound
deuterium/hydrogen relative to total number of impurities and/or
defects in the core and in some application also in the cladding
material. Accordingly, in one embodiment the core of the fiber
being a solid core (preferably silica) wherein the fraction of
bound hydrogen and/or deuterium relative to the total number of
impurities and/or defects is more than or equal to 10%, such as
more than or equal to 20%, such as more than or equal to 30%, such
as more than or equal to 40%, such as more than or equal to 50%,
such as more than or equal to 60%, such as more than or equal to
70%, such as more than or equal to 80%, such as more than or equal
to 90%, such as more than or equal to 99%, such as more than or
equal 99.9%. In this context all compounds in the glass apart from
SiO.sub.2 are considered impurities.
[0110] The invention is defined by the features of the independent
claim(s). Preferred embodiments are defined in the dependent
claims. Any reference numerals in the claims are intended to be
non-limiting for their scope.
[0111] Some preferred embodiments have been shown in the foregoing,
but it should be stressed that the invention is not limited to
these, but may be embodied in other ways within the subject-matter
defined in the following claims.
[0112] Super Continuum Generation in Standard Fibers
[0113] In the present context the term standard fiber refers to
fibers guiding light in a solid material substantially free of
microstructures. Guiding is obtained by total internal reflection
at the interface between a core of the fiber and a cladding, so
that the refractive index of the core is higher than the refractive
index of the cladding. The fiber may comprise multiple cores and/or
claddings such as in double clad fibers or panda fibers.
[0114] Similarly to microstructured fibers, supercontinuum (SC)
generation may be possible in standard fibers provided that
suitable nonlinear and dispersion properties are provided.
[0115] In one embodiment SC generation is obtained from standard
fiber such as a conventional single-mode fiber similar to what was
demonstrated by Watt et al., Generation of supercontinuum radiation
in conventional single-mode fibre and its application to broadband
spectroscopy, Appl. Phys. B 90, 47-53 (2008). Here a single mode
fiber (SMF28, Corning) is pumped at 1064 nm with 5 ps pulses. In
one embodiment an unloaded SMF28 fiber will experience similar
degradation to that observed in a microstructured fiber, and this
degradation may be prevented, reduced and/or repaired by way of the
invention. In one embodiment, increased load time relative to that
of a similar microstructured fiber will be beneficial as a standard
fiber lacks holes and therefore comprises more material in and/or
around the core.
[0116] In one embodiment, guiding in the standard fiber is obtained
by doping the cladding in order to reduce the refractive index,
such as by fluoride. In one embodiment guiding of the standard
fiber occurs in undoped silica.
[0117] In one embodiment SC is obtained, at least in part, from
tapering of the fiber, similarly to what was demonstrated by Lu et
al., Generation of broadband continuum with high spectral coherence
in tapered single-mode optical fibers, Opt. Expr., Vol. 12, No. 2
(2004). Here 100 fs pulses were pumped into the fiber at
wavelengths ranging from 780 nm to 920 nm. In one embodiment, a
tapered standard fiber will experience similar degradation to that
observed in a microstructured fiber and that this degradation may
be prevented, reduced and/or repaired by way of the invention.
[0118] Optical Power Transport
[0119] Optical fibers (standard and microstructured) may be applied
to transport optical energy (CW and/or pulsed) in applications such
as guiding of surgical and/or therapeutic light, optical sensing,
materials processing and measuring technology. Accordingly, in one
embodiment an optical fiber is arranged to receive light from a
feeding unit and transport said light substantially unchanged. In
this context substantially unchanged is taken to mean that the
output of the fiber may be substantially calculated by multiplying
with a linear transfer function. Substantially calculated is taken
to mean that the calculation is accurate within less than 20%
deviation, such as less than 10% deviation, such as less than 5%
deviation, such as less than 1% deviation. In one embodiment such
an optical fiber will experience similar degradation to that
observed in a microstructured fiber described above, and that this
degradation may be prevented, reduced and/or repaired by way of the
invention.
[0120] Active Fibers
[0121] In one embodiment the fiber according to the invention is an
active fiber suitable for providing optical amplification when
pumped with excitation light also referred to as pump light. In one
embodiment said active fiber forms part of an optical amplifier and
in one embodiment said active fiber forms part of a laser, such as
forms part of a laser cavity.
[0122] In one embodiment a fiber according to the invention forms
part of a laser cavity. Even though a laser may operate at a
wavelength far from where deterioration is commonly observed (e.g.
in the visible range) a tail of absorption may still be present at
the operating wavelength. In a laser light may pass the fiber a
high number of times before being coupled out. For this, and other
reasons, even a small deterioration may affect the performance of
the laser. In one embodiment, the gain medium of the laser
compensate at least partly for a small change in absorption due to
deterioration. In this event, this may result in stable power
output relative to the deterioration; however, with an increase in
noise, such as an increase in Relative Intensity Noise (RIN).
[0123] In one embodiment the fiber forms part of a laser with an
operating wavelength higher than 600 nm, such as higher than or
equal to 800 nm, such higher than or equal to 1000 nm, such higher
than or equal to 1064 nm, such higher than or equal to 1150 nm,
such higher than or equal to 1300 nm, such higher than or equal to
1550 nm. For such a fiber an increase in absorption in the visible
may occur as a function of operating time, in the absence of
loading according to the invention, similar to that occurring in
microstructures fiber in supercontinuum generation. As argued
above, such absorption may influence the performance of the laser.
By way of the invention such problems may be prevented, reduced
and/or repaired so that a fiber according to the invention is
applied for part of or the entire optical path of the laser. In one
embodiment loading is performed in a manner similar to that of the
microstructured fiber for supercontinuum or the fiber for
transport.
[0124] In one embodiment the fiber forms part of an optical
amplifier. In optical amplifiers high optical densities may arise,
so that one will observe that deterioration causing increased
absorption occurs. This may be prevented, reduced and/or repaired
by way of the invention so that a fiber according to the invention
is applied for part of or the entire optical path of the amplifier.
For lasers as well as amplifiers it is within the scope of the
invention that the fiber according to invention is active and/or
passive fiber.
[0125] FIG. 15 is a schematic cross-sectional view of an optical
fiber 100 according to an embodiment of the invention. The drawing
is not made to scale, and is not intended to illustrate the
relative diameters and thicknesses of the core material 110 and the
cladding 112. The optical fiber may be a microstructured fiber and
include a pattern (not shown in FIG. 15) of holes or doped glass,
extending in the longitudinal direction (i.e., into the page) of
the optical fiber 100.
* * * * *