U.S. patent application number 13/098234 was filed with the patent office on 2011-08-25 for apparatus for providing optical radiation.
Invention is credited to Michael Kevan Durkin, Fabio Ghiringhelli, Andrew Michael Gillooly, Louise Mary Brendan Hickey, Stephen Roy Norman, David Neil Payne, Andy Piper, Jayanta Kumar Sahu, Mikhail Nickolaos Zervas.
Application Number | 20110206074 13/098234 |
Document ID | / |
Family ID | 38694261 |
Filed Date | 2011-08-25 |
United States Patent
Application |
20110206074 |
Kind Code |
A1 |
Durkin; Michael Kevan ; et
al. |
August 25, 2011 |
Apparatus for providing optical radiation
Abstract
In one embodiment, a photo-darkening resistant optical fibre
includes a waveguide having a numerical aperture less than 0.15.
The waveguide includes a core having a refractive index n1 and a
pedestal having a refractive index n2, and wherein the fibre
includes a first cladding having a refractive index n3 surrounding
the pedestal, wherein n1 is greater than n2, n2 is greater than n3.
The core includes silica, a concentration of alumina of between
approximately 0.3 to 0.8 mole percent, a concentration of phosphate
of substantially 15 mole percent, a concentration of ytterbium
substantially in the range 20000 to 45000 ppm. The pedestal can
include silica, phosphate and germania. The core can have
substantially zero thulium dopant.
Inventors: |
Durkin; Michael Kevan;
(Southampton, GB) ; Norman; Stephen Roy;
(Ampfield, GB) ; Ghiringhelli; Fabio;
(Southampton, GB) ; Payne; David Neil;
(Southampton, GB) ; Hickey; Louise Mary Brendan;
(Windsor, GB) ; Sahu; Jayanta Kumar; (Southampton,
GB) ; Zervas; Mikhail Nickolaos; (Southampton,
GB) ; Piper; Andy; (Southampton, GB) ;
Gillooly; Andrew Michael; (Southampton, GB) |
Family ID: |
38694261 |
Appl. No.: |
13/098234 |
Filed: |
April 29, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11803036 |
May 11, 2007 |
7936796 |
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13098234 |
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60799456 |
May 11, 2006 |
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60812164 |
Jun 9, 2006 |
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60819439 |
Jul 7, 2006 |
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60834974 |
Jul 31, 2006 |
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Current U.S.
Class: |
372/25 |
Current CPC
Class: |
H01S 3/09415 20130101;
H01S 3/094076 20130101; H01S 3/094007 20130101; G02B 6/024
20130101; H01S 3/10015 20130101; H01S 3/10038 20130101; H01S 3/176
20130101; H01S 5/065 20130101; H01S 3/0064 20130101; H01S 3/06754
20130101; H01S 3/1691 20130101; H01S 3/1618 20130101; H01S 3/0014
20130101; H01S 3/06733 20130101; H01S 3/06716 20130101; H01S 3/1693
20130101; H01S 5/0656 20130101; H01S 2301/08 20130101; H01S 2301/03
20130101; H01S 3/06712 20130101 |
Class at
Publication: |
372/25 |
International
Class: |
H01S 3/10 20060101
H01S003/10 |
Claims
1. An apparatus for providing optical radiation, said apparatus
comprising a seed laser for emitting seeding radiation, at least
one optical amplifier, a reflector and a controller, wherein: the
seed laser comprises a semiconductor laser; the seeding radiation
comprises a plurality of seed laser pulses; the seed laser is
connected to the optical amplifier via the reflector; the seed
laser pulses are amplified by the optical amplifier to produce
optical radiation; the optical radiation comprises output pulses;
the output pulses are characterized by a peak power, a pulse
energy, and a pulse repetition frequency; the reflector is arranged
to reflect a proportion of the seeding radiation emitted by the
seed laser back into the seed laser; and the controller controls
the seed laser to vary the shape of the seed laser pulses to
maintain the peak power above 3 kW, the pulse energy in excess of
0.04 mJ, and the pulse repetition frequency at 1 Hz-500 kHz.
2. The apparatus according to claim 1, wherein the controller
varies the shape of the seed laser pulses to maintain the peak
power above 5 kW, the pulse energy in excess of 0.1 mJ, and the
pulse repetition frequency at 1 Hz-200 kHz.
3. The apparatus according to claim 1, wherein the controller
varies the shape of the seed laser pulses to maintain the peak
power at 18-26 kW, the pulse energy at 0.8 to 1 mJ, and the pulse
repetition frequency at 1 Hz-25 kHz.
4. The apparatus according to claim 1, wherein the apparatus
comprises two optical amplifiers.
5. The apparatus according to claim 1, wherein the reflector is
arranged to reflect less than 20% of the seeding radiation emitted
by the seed laser back into the seed laser.
6. The apparatus according to claim 5, wherein the reflector is
arranged to reflect between 1% and 10% of the seeding radiation
emitted by the seed laser back into the seed laser.
7. The apparatus according to claim 1, wherein the reflector is
characterized by a bandwidth which is greater than 1 nm.
8. The apparatus according to claim 1, wherein the reflector is
located at a distance less than 2 m from the seed laser.
9. The apparatus according to claim 8, wherein the distance is
between 0.5 m and 1.5 m.
10. The apparatus according to claim 8, wherein the distance is
between 5 mm and 50 cm.
11. The apparatus according to claim 1, wherein the optical
amplifier comprises a cladding pumped optical amplifier which
further comprises an optical fiber.
12. The apparatus according to claim 11, wherein the optical fiber
is a multimode waveguide at a signal wavelength.
13. The apparatus according to claim 11, wherein the optical fiber
is a single mode waveguide.
14. The apparatus according to claim 11, the optical fiber
comprises a waveguide having a numerical aperture less than
0.15.
15. The apparatus according to claim 11, wherein the optical fiber
is characterized by an increase in attenuation, which is no greater
than 5% in 2000 hours at a wavelength between 1000 nm and 1100 nm
when a 0.1 to 1 m length of the optical fiber is core pumped with
approximately 400 mW of light at a wavelength of 976 nm.
16. The apparatus according to claim 1, wherein the reflector is a
fiber Bragg grating.
17. The apparatus according to claim 16, wherein the fiber Bragg
grating is chirped.
18. The apparatus according to claim 1, further comprising a laser
delivery fiber, and a processing head.
19. The apparatus according to claim 18, wherein the controller
controls the seed laser and the optical amplifier such that the
output pulses have sufficient average power and peak power to
process a material over the range of pulse repetition
frequencies.
20. The apparatus according to claim 1, wherein the output pulses
are characterized by a pulse width, and the controller varies the
pulse width as the pulse repetition frequency is varied.
21. The apparatus according to claim 11, wherein the optical fiber
comprises a core having a refractive index, n1, and a pedestal
having a refractive index, n.sub.2, and wherein the optical fiber
comprises a first cladding made of glass having a refractive index,
n.sub.3, surrounding the pedestal, wherein n.sub.1 is greater than
n.sub.2; n.sub.2 is greater than n.sub.3; and the pedestal guides
optical radiation that escapes from the core, and the amplifier is
cladding pumped by coupling pump radiation from a pump into the
first cladding.
22. The apparatus according to claim 1, further comprising a pump
for pumping the optical amplifier, and wherein the controller
reduces power emitted by the pump.
23. The apparatus according to claim 22, wherein the power emitted
by the pump is reduced by modulation.
24. The apparatus according to claim 23, wherein the modulation
applied to the pump is synchronous with the seed laser pulses.
25. The apparatus according to claim 1, wherein the apparatus is
defined by a peak power obtained with a rectangular seed laser
pulse, and the controller reduces the peak power by controlling the
shape of the seed laser pulses.
26. The apparatus according to claim 1, wherein a portion of the
optical radiation is wavelength converted, and the controller
controls the seed laser to vary the shape of the seed laser pulses
to maintain the portion of the optical radiation that is wavelength
converted to less than 50%.
27. The apparatus according to claim 26, wherein the portion of the
pulse that is wavelength converted is less than 10%.
28. The apparatus according to claim 1, wherein the pulse
repetition frequency is 20 kHz.
29. An apparatus for providing optical radiation, the apparatus
comprising a seed laser for emitting seeding radiation, at least
one optical amplifier, and a controller, wherein: the seeding
radiation comprises a plurality of seed laser pulses; the seed
laser is connected to the optical amplifier; the seed laser pulses
are amplified by the optical amplifier to produce optical
radiation; the optical radiation comprises output pulses; the
output pulses are characterized by a peak power, a pulse energy,
and a pulse repetition frequency; and the controller controls the
seed laser to vary the shape of the seed laser pulses to maintain
the peak power above 3 kW, the pulse energy in excess of 0.04 mJ,
and the pulse repetition frequency at 1 Hz-500 kHz.
30. The apparatus according to claim 29, wherein a portion of the
optical radiation is wavelength converted, and the controller
controls the seed laser to vary the shape of the seed laser pulses
to maintain the portion of the optical radiation that is wavelength
converted to less than 50%.
31. The apparatus according to claim 30, wherein the portion of the
optical radiation that is wavelength converted is less than
10%.
32. The apparatus according to claim 29, further comprising a pump
for pumping the optical amplifier, wherein a portion of the optical
radiation is wavelength converted, and the controller reduces the
power emitted by the pump to maintain the portion of the optical
radiation that is wavelength converted to less than 50%.
33. The apparatus according to claim 32, wherein the portion of the
optical radiation that is wavelength converted is less than
10%.
34. The apparatus according to claim 32, wherein the power emitted
by the pump is reduced by modulation, and wherein the modulation
applied to the pump is synchronous with the seed laser pulses.
35. The apparatus according to claim 29, wherein the pulse
repetition frequency is 20 kHz.
36. An apparatus for providing optical radiation, said apparatus
comprising a seed laser and at least one amplifier, wherein: the
seed laser is configured to provide seeding radiation; the seed
laser is connected to the amplifier; the amplifier is configured to
amplify the seeding radiation; wherein the amplifier comprises an
optical fiber, said optical fiber comprises a core having a
refractive index, n.sub.1, a pedestal having a refractive index,
n.sub.2, and a first cladding having a refractive index n.sub.3,
wherein: n.sub.1 is greater than n.sub.2; n.sub.2 is greater than
n.sub.3; and wherein the first cladding is made of glass and
surrounds the pedestal; the amplifier is cladding pumped by
coupling pump radiation into the first cladding; the pedestal is
adapted to guide optical radiation that escapes from the core; and
wherein, when in use, the amplifier emits a pulse having a peak
power greater than 1 kW when seeded by the seed laser.
37. The apparatus of claim 36, wherein: the core comprises silica,
a concentration of alumina in the range of 0.1 to 4 mole percent,
and a concentration of phosphate in the range of 2 to 20 mole
percent; and wherein, the pedestal comprises silica, phosphate and
germania.
38. The apparatus of claim 37, wherein the optical fiber is doped
with at least one rare earth dopant disposed in at least the core
or the pedestal.
39. The apparatus of claim 38, wherein the rare earth dopant is
ytterbium having a concentration in the range of about 2000 to
about 60000 ppm.
40. The apparatus of claim 39, wherein the concentration of
ytterbium is between approximately 15000 to approximately 50000
ppm.
41. The apparatus of claim 40, wherein the concentration of
ytterbium is between approximately 20000 to approximately 45000
ppm.
42. The apparatus of claim 37, wherein the concentration of
phosphate in the core is between approximately 12 to approximately
17 mole percent.
43. The apparatus of claim 42, wherein the concentration of
phosphate in the core is approximately 15 mole percent.
44. The apparatus of claim 37, wherein the concentration of alumina
is between approximately 0.20 to approximately 1 mole percent.
45. The apparatus of claim 44, wherein the concentration of alumina
is between approximately 0.3 and approximately 0.8 mole
percent.
46. The apparatus of claim 36, wherein the optical fiber is a
multimode waveguide at a signal wavelength.
47. The apparatus of claim 46, wherein the optical fiber is
configured to propagate single mode light without significant
distortion over a substantial length.
48. The apparatus of claim 36, wherein the optical fiber is a
single mode waveguide.
49. The apparatus of claim 36, further comprising at least one
stress producing region for inducing birefringence in the core.
50. The apparatus of claim 36, wherein the optical fiber comprises
a waveguide having a numerical aperture less than 0.15.
51. The apparatus of claim 36, wherein: the optical fiber is a
photo-darkening resistant optical fiber comprising a waveguide
having a numerical aperture less than 0.15; the core comprises
silica, a concentration of alumina of between approximately 0.3 and
approximately 0.8 mole percent, a concentration of phosphate of
substantially 15 mole percent, a concentration of ytterbium
substantially in the range of 20000 to 45000 ppm; and the pedestal
comprises silica, phosphate and germania.
52. The apparatus of claim 36, further comprising a laser delivery
fiber and a processing head.
53. The apparatus of claim 52, further comprising a controller
configured to control the seed laser and the amplifier such that
the optical radiation has sufficient average power and peak power
to process a material over a range of pulse repetition
frequencies.
54. The apparatus of claim 36, further comprising a controller,
wherein the optical radiation is characterized by pulses having a
pulse width, and wherein the controller varies the pulse width as
the pulse repetition frequency is varied.
55. The apparatus of claim 36, wherein the optical fiber is a
photodarkening resistant optical fiber characterized by an increase
in attenuation which is no greater than 5% in 2,000 hours at a
wavelength between 1000 nm and 1100 nm when a 0.1 to 1 m length of
the optical fiber is core pumped with approximately 400 mW of pump
light at 976 nm
56. The apparatus of claim 55, wherein the increase in attenuation
is less than 1% in 2,000 hours.
57. The apparatus of claim 36, further comprising a controller,
wherein the seeding radiation is characterized by a pulse shape,
the optical radiation is characterized by a pulse energy, a pulse
repetition frequency, and a pulse width, and wherein the controller
controls the seed laser to vary the pulse shape to maintain the
peak power above 3 kW, the pulse energy in excess of 0.04 mJ, and
the pulse repetition frequency at 1 Hz-500 kHz.
58. The apparatus of claim 57, further comprising a laser delivery
fiber and a processing head.
59. The apparatus of claim 36, wherein the amplifier is side
pumped.
60. The apparatus of claim 36, wherein the apparatus is configured
to emit a peak power greater than 5 kW with a pulse energy greater
than 0.04 mJ at pulse repetition frequencies from 1 Hz to 500 kHz,
and the apparatus further comprises at least two optical fiber
amplifiers which are connected to amplify the seeding
radiation.
61. The apparatus of claim 36, wherein the apparatus is configured
to emit a peak power greater than 18 kW with a pulse energy greater
than 0.8 mJ at pulse repetition frequencies from 1 Hz to 25 kHz,
and the apparatus further comprises at least two optical fiber
amplifiers which are connected to amplify the seeding
radiation.
62. The apparatus of claim 36, wherein the seed laser is a
superluminescent light emitting diode.
63. The apparatus of claim 36, wherein the seed laser is a
semiconductor laser.
64. The apparatus of claim 36, wherein the seed laser is defined by
a wavelength less than 1350 nm.
65. The apparatus of claim 36, wherein the seed laser is defined by
a wavelength less than 1100 nm.
66. A method for providing optical radiation comprising the steps
of: providing a seed laser; providing at least one amplifier which
comprises an optical fiber, wherein the optical fiber comprises a
core having a refractive index, n.sub.1, a pedestal having a
refractive index, n.sub.2, and a first cladding having a refractive
index, n.sub.3, wherein: n.sub.1 is greater than n.sub.2; n.sub.2
is greater than n.sub.3; the first cladding is made of glass and
surrounds the pedestal; and the pedestal is adapted to guide
optical radiation that escapes from the core; cladding pumping the
amplifier by coupling pump radiation into the first cladding;
controlling the seed laser to generate seeding radiation; and
amplifying the seeding radiation with the amplifier to generate a
pulse having peak power of greater than 1 kW.
67. The method of claim 66, further comprising the step of using
the pulse to mark a material.
Description
[0001] This application is a continuation application of U.S.
patent application Ser. No. 11/803,036 (filed May 11, 2007), which
claims priority under 35 USC .sctn.120 to U.S. Provisional patent
application Ser. Nos. 60/799,456 (filed May 11, 2006), 60/812,164
(filed Jun. 6, 2006), 60/819,439 (filed Jul. 7, 2006), and
60/834,974 (filed Jul. 31, 2006), all of which are hereby
incorporated herein in their entirety.
FIELD
[0002] This application relates to apparatus for providing optical
radiation. The apparatus can form the basis of an apparatus for
material processing.
BACKGROUND
[0003] Fibre pulsed lasers are increasingly being adopted as the
laser of choice in a number of industrial applications, such as
micromachining, drilling and marking. In peak-power-driven
applications, such as marking, it is essential to retain high peak
powers (in excess of 2.5 to 5 kW) at high repetition rates in order
to achieve faster character marking and increased throughput.
[0004] Conventional single-stage Q-switched lasers are very
efficient in storing energy. However, they are characterised by
variable average power and substantial peak-power drop as the
repetition rate is increased. In most cases, the peak power can
drop below the process (e.g. marking) threshold with an adverse
effect on speed and throughput. Master oscillator power amplifier
(MOPA) configurations, on the other hand, can offer more
controllability over the pulse characteristics and power
performance of the pulsed . laser and extend the operation space of
a marking unit to higher repetition rates offering increased
marking speed. There is a requirement for pulsed lasers which
maintain the peak power over a 5 kW level for repetition rates in
excess of 200 kHz. The average power should be in excess of 10 W,
the pulse energy lies in the 0.1-0.5 mJ range or higher, the pulse
duration to be variable between 10 ns and 200 ns, while the peak
power should remain substantially constant at about the 5 kW or 10
kW level for rep rates in the range 10 kHz to >200 kHz.
Additional requirements are good beam quality such as can be
provided by a low-moded or single-moded fibre laser.
[0005] At these intensity and peak-power levels, special care is
needed in the pulsed system to avoid the onset of optical
non-linearities and optical damage. In addition, under the
resulting high-gain, high inversion operating conditions the active
fibre is not subject to photodarkening effects as that will reduce
the efficiency and lifetime of the pulsed system.
[0006] A number of different pulsed fibre laser configurations have
been proposed and used in a stand-alone fashion or as part of a
master-oscillator power amplifier (MOPA) configuration. Q-switched
fibre lasers, in particular, are quite attractive because they can
produce high peak powers and several mJ pulse energies in a
relatively simple and stable configuration. One of the main
drawbacks of stand-alone Q-switched lasers, which are intended to
be used in an industrial application in a versatile manner in order
to increase the application space, is that all the parameters of
interest, such as the pulse repetition rate (PRR), energy, peak
power and pulse width, are interrelated and cannot be controlled
independently. In particular, the peak power reduces as the pulse
repetition frequency increases.
[0007] A number of these performance issues can be resolved and the
required high peak power performance can be extended in the high
PRR regime by using a multiple-amplification-stage MOPA
configuration.
[0008] In this case the pulsed seed can be either a low power
Q-switched laser or a directly modulated semiconductor laser. The
latter can be controlled directly and provides much more freedom in
defining the pulse shape and pulse repetition frequency, as well
as, it gives the possibility of changing them at will to better
fulfil the application needs. In addition, it is based on the
well-developed and extremely reliable semiconductor technology
developed over the years for the telecommunications industry.
Different parameters of the amplified pulse sequences are defined
accurately by controlling the gain distribution along the
amplification chain.
[0009] The local inversion in a fibre amplifier increases
considerably before the arrival of a pulse towards the output end
of the amplifier. Knowledge of the inversion distribution is very
important in defining the photodarkening rate in case where a fibre
prone to this performance-degrading effect is used. As the pulse
propagates, it depletes the inversion and increases its intensity.
The amplification process also results in significant pulse
reshaping and front-end sharpening. This is extremely important in
defining pulse width and peak-power and as a consequence defines
the onset of various non-linearities such as stimulated Raman
scattering (SRS) and stimulated Brillouin scattering (SBS). Above a
certain energy level, all pulses reshape (sharpen) considerably and
reduce their pulse width. This is due to the fact that the pulse
acquires enough energy to start saturating the amplifier. It is
known that under such conditions, energy is extracted primarily by
the leading edge of the pulse resulting in pulse reshaping and
distortion. Peak power increases nonlinearly with pulse energy and
inevitably exceeds the SRS threshold, which is typically around 5
kW to 10 kW, depending upon the fibre design and pulse shape.
[0010] Another important effect that limits the output power of
pulsed fibre lasers is the formation of giant pulses. These can
catastrophically damage the optical components in the system. The
effect is believed to be highly dependent upon the peak power and
the spectral properties of the laser and believed to arise from
stimulated Brillouin scattering (SBS). When the non-linear
threshold is reached, forward going pulses are reflected. Giant
pulses are observed, and these can catastrophically damage the
amplifiers (and other devices) in pulse laser systems.
Unfortunately, the effect is stochastic in nature, and by itself
very unpredictable. A single variation in the instantaneous
spectral properties of a seed laser (such as a laser diode) which
narrows the linewidth can result in an SBS event, and trigger giant
pulse formation and subsequent catastrophic damage. Such damage has
been observed in lasers months after they have been installed in
industrial processing equipment.
[0011] Fibre lasers are often pumped by laser diodes. These laser
diodes can be damaged by undesired optical radiation propagating
from the laser to the diodes. The effect is particularly severe in
pulsed lasers because laser diodes are damaged by peak power rather
than the energy of a pulse. Pulsed lasers have much higher peak
powers than continuous wave lasers. Therefore the requirement to
isolate the pumps from the laser is more stringent in a pulsed
laser than a continuous wave laser.
[0012] A very important issue related to the long-term behavior of
Yb3+ doped fiber lasers and amplifiers is the effect of
photodarkening. The effect shows as a gradual increase of the fibre
background loss with time, which reduces the output power and
overall efficiency of the optical system. It is believed to be
related to the optical activation of pre-existing fibre color
centres, with absorption bands mainly in the UV spectral region.
However, the tails of the absorption bands extend into the near-IR
adversely affecting the optical performance. Photodarkening results
in gradual degradation and is not known to result in catastrophic
sudden fibre failures. Photodarkening rate and final level is shown
to be dependent on the active fibre degree of inversion and, as a
result, different amplified systems will show different
degradation.
[0013] Many applications of optical fibres require the generation
and transmission of optical signals having intensities at which the
transparency of the optical fibre degrades with time. The effect is
known as photo-darkening, which is a light-induced change in the
absorption of glass. The increase in absorption is believed to be
due to the formation or activation of color centers that strongly
absorb light in the UV and visible part of the spectrum.
[0014] In the spectral domain, photodarkening shows as a sharp loss
increase below a wavelength of approximately 800 nm. The tail of
this strong absorption band extends well into the 1 micron to 1.5
micron region and affects adversely the losses at both the pump and
signal wavelengths. This has a severe limiting effect on the
performance and overall efficiency of fiber lasers and amplifiers
operating in this wavelength regime.
[0015] In the time domain, photodarkening shows as a gradual
pseudo-exponential decrease of the laser or amplifier output power
to an asymptotic value. The final power drop and related time scale
seems to depend on the fiber laser or amplifier operating
conditions, most notably the pump and average inversion levels, as
well as, the operating temperature. The output power drop could be
compensated by the provision of additional pump sources and/or the
increase of driving pump current. Both measures are highly
undesirable since the former results in increased unit cost while
the latter results in accelerated of the pump-unit ageing and
increased catastrophic failure probability.
[0016] Optical fibre lasers and amplifiers often include rare-earth
dopant which can lead to photo-darkening via multi-photon
processes. The effect is seen in at least Tm.sup.3+, Yb.sup.3+,
Ce.sup.3+, Pr.sup.3+, and Eu.sup.3+ doped silica glasses.
[0017] Photodarkening is problematic when optical fibres are used
in the industrial material processing. The effect can degrade the
transmission in fibres used to deliver laser radiation from lasers
(such as frequency-doubled, -tripled rod lasers, disk lasers, and
fibre lasers) to a work piece, It can also severely limit the
amount of optical power that can be generated in a fibre laser or
amplified by an optical amplifier.
[0018] Conventional methods to reduce photo-darkening in glass are
to use silica with high hydroxyl (OH) content, so-called "wet
silica". This can be loaded with deuterium and irradiated with
ultra-violet (UV) light. However these approaches are not well
suited for fibre lasers because the OH will increase the background
loss of the optical fibre.
[0019] There is a need for a pulsed laser that maintains its peak
power over a wide range of repetition frequencies and in which
non-linear effects are controlled.
[0020] There is a need for fibre lasers that are resistant to pump
damage.
[0021] There is a need for fibre lasers that are resistant to
catastrophic damage from giant pulse formation.
[0022] There is a need for a photo-darkening resistant optical
fibre. There is a related need for a fibre laser and amplifier that
is resistant to photo-darkening. By photo-darkening, it is meant
any light-induced decrease in transmission of glass, whether
temporary or permanent.
SUMMARY
[0023] According to a non-limiting embodiment an apparatus for
providing optical radiation includes a seed laser, at least one
amplifier, and a reflector, wherein the seed laser is a Fabry Perot
semiconductor laser. The seed laser is connected to the amplifier
via the reflector, and the reflector is arranged to reflect a
proportion of the seeding radiation emitted by the seed laser back
into the seed laser. The amplifier includes an optical fibre which
includes a core having a refractive index n1 and a pedestal having
a refractive index n2, and the optical fibre includes a first
cladding made of glass having a refractive index n3 surrounding the
pedestal. Further, n1 is greater than n2, and n2 is greater than
n3.
[0024] We have found that use of such a reflector has been found to
effectively eliminate the occurrence of giant pulses, believed to
arise as a result of stimulated Brillouin scattering.
[0025] Incorporation of the pedestal is advantageous because it can
reduce the cross-coupling of signal power to the first cladding.
This has been found to dramatically reduce pump diode failure in
cladding pumped fibre lasers and amplifiers.
[0026] The reflector can be a dispersive reflector.
[0027] The seed laser can be characterized by an effective optical
transit time. The reflector can be characterized by a bandwidth and
a round-trip reflective time-delay variation over the bandwidth.
The round-trip reflective time-delay variation can be greater than
the effective optical transit time.
[0028] The proportion of the seeding radiation emitted by the seed
laser reflected back into the seed laser can be less than 20%. The
proportion can be between 1% and 10%.
[0029] The reflector can be located a distance less than 5 m from
the seed laser. The distance can be less than 2 m. The distance can
be between 0.5 m and 1.5 m.
[0030] The reflector can be located a distance between 5 mm and 50
cm from the seed laser.
[0031] A peak power emitted from the apparatus can exceed 1 kW.
[0032] The core can include silica, a concentration of alumina in
the range 0.1 to 4 mole percent, a concentration of phosphate in
the range 2 to 20 mole percent. The pedestal can comprise silica,
phosphate and germania. The optical fibre is preferably (but not
necessarily) a photo-darkening resistant optical fibre. The
waveguide preferably (but not necessarily) has a numerical aperture
less than 0.15.
[0033] An advantage of the apparatus is that the optical fibre is
capable of transmitting optical radiation at high intensities. The
low numerical aperture means that the fibre can be configured to be
a so-called large mode area fibre, which in combination with the
high intensities, permits the output power and/or product lifetime
of fibre lasers and amplifiers to be increased. A further advantage
in the design of fibre lasers and amplifiers provided for herein is
that fewer pump diodes are required since the optical fibre
maintains its transmission quality over the product lifetime.
[0034] The core can include silica, a concentration of alumina in
the range 0.1 to 4 mole percent and a concentration of phosphate in
the range 2 to 20 mole percent. The alumina doped core in
combination with the germano-phosphorus doped pedestal permits low
numerical aperture fibres to be manufacturable with improved
repeatability as compared with the prior art. Numerical apertures
as low as 0.06 to 0.1, or preferably approximately 0.08 are readily
achievable.
[0035] The pedestal can include silica, phosphate and germania. The
concentration of phosphate and germania are selected to achieve the
desired numerical aperture. Incorporation of germania has the
advantage of increasing the fictive temperature of the pedestal and
thus assisting in the retention of both circularity and core to
pedestal concentricity and hence core to first cladding
concentricity during the optical fibre manufacturing process. Core
concentricity is important in the production of low-loss fusion
splices.
[0036] Preferably (but not necessarily) there is substantially zero
thulium dopant in the core. Preferably (but not necessarily) other
trace rare-earth dopants should also be avoided. The importance of
eliminating thulium dopant is important in the design of fibre
lasers and amplifiers as fibres containing thulium dopant have been
found to be especially susceptible to photo-darkening. It is
important therefore to use rare-earth dopants that have low trace
amounts of thulium. The thulium concentration should be less than
approximately 10 ppm, and preferably less than 1 ppm.
[0037] The optical fibre can be doped with at least one rare earth
dopant disposed in at least one of the core and the pedestal. The
rare earth dopant can be ytterbium having a concentration in the
range 2000 to 60000 ppm. The concentration of ytterbium can be
between approximately 15000 to 50000 ppm. The concentration of
ytterbium is preferably between approximately 20000 to 45000
ppm.
[0038] The concentration of phosphate in the core can be between
approximately 12 to 17 mole percent. The concentration of phosphate
in the core is preferably approximately 15 mole percent.
[0039] The concentration of alumina can be between approximately
0.20 to 1 mole percent. The concentration of alumina is preferably
between approximately 0.3 and 0.8 mole percent.
[0040] The waveguide can be a multimode waveguide at a signal
wavelength. The waveguide can be configured to propagate single
mode light without significant distortion over a substantial
length.
[0041] The waveguide can be a single mode waveguide.
[0042] The optical fibre can include at least one stress producing
region for inducing birefringence in the core.
[0043] The photo-darkening resistant optical fibre can include a
waveguide having a numerical aperture less than 0.15, wherein the
waveguide includes a core having a refractive index n1 and a
pedestal having a refractive index n2, and wherein the fibre
includes a first cladding having a refractive index n3 surrounding
the pedestal, wherein n1 is greater than n2, n2 is greater than n3.
The core includes silica, a concentration of alumina of between
approximately 0.3 to 0.8 mole percent, a concentration of phosphate
of substantially 15 mole percent, a concentration of ytterbium
substantially in the range 20000 to 45000 ppm. The pedestal can
include silica, phosphate and germania. Preferably (but not
necessarily) the core includes substantially zero thulium
dopant.
[0044] The waveguide can be a multimode waveguide at a signal
wavelength, and the waveguide can be configured to propagate single
mode light without significant distortion over a substantial
length. Alternatively, the waveguide can be a singlemode
waveguide.
[0045] The optical fibre can be used in an apparatus in the form of
a cladding pumped amplifying optical device comprising at least one
source of pump energy. This is particularly advantageous because
signal light scattering or leaking from the low numerical aperture
core is preferentially captured and guided by the pedestal, and
therefore is not routed back into the pumps. The apparatus
therefore removes one of the major failure mechanisms of cladding
pumped lasers, namely the catastrophic failure of pump diodes
caused by unwanted signal light.
[0046] The optical fibre can be used in an apparatus in the form of
an amplifier, a laser, a master-oscillator power amplifier, a
Q-switched laser or an ultra-fast laser comprising at least one
source of pump energy.
[0047] A further embodiment described and provided for herein is
for an apparatus in the form of a laser for material processing,
including at least one source of pump energy, an optical fibre, a
laser delivery fibre, and a processing head.
[0048] Yet another embodiment described and provided for herein is
a method of marking which method includes the step of providing a
reflector within a marking laser.
[0049] An additional embodiment described and provided for herein
is a material when processed using apparatus according to the
present disclosure. The material can be a semiconductor package
(plastic or ceramic), a key pad on a mobile phone, an iPOD, a
computer, a component, a package, or a commercial or industrial
product.
[0050] A further apparatus provided for herein can be in the form
of a laser for material processing, including at least one source
of pump energy, an optical fibre, a laser delivery fibre, and a
processing head.
[0051] Another embodiment provided for herein is to replace the
seed laser and the reflector with a surface emitting light emitting
diode (SLED). An SLED can be amplified such that its use also
dramatically reduces the onset of SBS.
BRIEF DESCRIPTION OF THE DRAWINGS
[0052] Embodiments of the present disclosure will now be described
solely by way of example and with reference to the accompanying
drawings in which:
[0053] FIG. 1 shows apparatus according to the present
invention;
[0054] FIG. 2 shows a refractive index profile of an optical
fibre;
[0055] FIG. 3 shows an optical fibre that includes stress producing
regions for inducing birefringence in the core;
[0056] FIG. 4 shows a cladding pumped amplifying optical
device;
[0057] FIG. 5 shows apparatus in the form of a laser for material
processing;
[0058] FIG. 6 shows the effect of photodarkening in four
continuous-wave fibre lasers;
[0059] FIG. 7 shows the effect of photodarkening in alumina doped
silica;
[0060] FIGS. 8 to 10 show results of accelerated ageing tests in
three different fibres;
[0061] FIG. 11 shows the elimination of photodarkening effects in a
master oscillator power amplifier MOPA that emits 12 W average
power with 20 kHz pulses having pulse energies of 0.6 mJ and pulse
widths of 35 ns;
[0062] FIG. 12 shows the elimination of photodarkening effects in a
fibre laser that is modulated at 10 kHz;
[0063] FIG. 13 shows the refractive index profile of a preferred
embodiment;
[0064] FIG. 14 shows an optical fibre that has an elliptical
pedestal;
[0065] FIGS. 15 and 16 show measurements of output power in 200 W
continuous wave fibre lasers;
[0066] FIG. 17 show a master oscillator power amplifier (MOPA);
[0067] FIG. 18 shows a MOPA in which the seed is a surface emitting
light emitting diode (SLED);
[0068] FIG. 19 shows a MOPA which includes a reflector;
[0069] FIG. 20 shows pulse shapes in normal operation;
[0070] FIG. 21 shows pulse shapes containing additional spikes;
[0071] FIG. 22 shows an example of a giant pulse;
[0072] FIG. 23 shows a typical spectrum from a laser diode;
[0073] FIG. 24 shows the spectrum of the laser diode with the
reflector in place;
[0074] FIG. 25 shows the design of a fibre Bragg grating used as
the reflector;
[0075] FIG. 26 shows the measured reflectivity of the fibre Bragg
grating;
[0076] FIG. 27 shows output pulses for different repetition
rates;
[0077] FIG. 28 shows the corresponding pulses emitted by the seed
laser;
[0078] FIGS. 29 and 30 show the pulses shown in FIGS. 27 and 28
respectively superimposed;
[0079] FIG. 31 shows the variation of peak power and pulse energy
with repetition frequency;
[0080] FIG. 32 shows the output power from the MOPA as the pump
power is increased;
[0081] FIG. 33 shows the optical spectrum of the pulse shown in
FIG. 32;
[0082] FIGS. 34 to 37 show the effect of varying the pulse
repetition frequency and pulse width on the shape of the output
pulse; and
[0083] FIG. 38 shows apparatus comprising a reflector and a
pedestal.
DETAILED DESCRIPTION
[0084] With reference to FIG. 38, there is provided an apparatus
380 for providing optical radiation 381 including a seed laser 382,
at least one amplifier 383, and a reflector 384, wherein the seed
laser 382 is a Fabry Perot semiconductor laser, the seed laser 382
is connected to the amplifier 383 via the reflector 384, the
amplifier 383 includes an optical fibre 1 (per FIG. 1) which
includes a pedestal 4 made from glass, and the reflector 384 is
arranged to reflect a proportion 388 of the seeding radiation 387
emitted by the seed laser 382 back into the seed laser 382.
[0085] The pedestal 4 protects the apparatus 380 from being damaged
by optical radiation 381 leaking from the optical fibre 1.
[0086] The reflector 384 protects the apparatus 380 from damage.
The reflector 384 is believed to prevent the seed laser 382 from
emitting the seeding radiation 387 in a single longitudinal mode
(or a few longitudinal modes), thus suppressing the onset of
stimulated Brillouin scattering.
[0087] Preferably (but not necessarily) the reflector 384 is a
dispersive reflector.
[0088] Preferably the seed laser 382 is characterized by an
effective optical transit time 386, the reflector 384 is
characterized by a bandwidth 3810 and a round-trip reflective
time-delay variation 389 over the bandwidth 3810, and the
round-trip reflective time-delay variation 389 is greater than the
effective optical transit time 386.
[0089] The proportion 388 may be less than 20%. The proportion is
preferably 1% to 10%.
[0090] The reflector 384 is located a distance 3811 from the seed
laser 382. A 1 m distance corresponds to approximately 10 ns round
trip delay in an optical fibre. The build up time for stimulated
Brillouin scattering in optical fibres is approximately 20 ns to 40
ns depending upon the material composition. It is important that
the reflector 384 is positioned such that the feedback has time to
influence the emission from the seed laser 382. The distance 3811
should be less than 5 m. The distance 3811 should ideally be less
than 2 m. The distance is preferably between 0.5 m and 1.5 m.
Alternatively, the reflector 384 may be packaged with the seed
laser 382, and the distance 3811 may be between 5 mm and 50 cm.
[0091] The invention has most utility for apparatus 380 configured
as a pulsed fibre lasers which may have a peak powers greater than
1 kW within the first 200 ns of switch on. The above distances 3811
may be selected to ensure that the feedback from the reflector 384
occurs while the apparatus 380 is emitting power above 1 kW.
[0092] By effective optical transit time 386, it is meant the time
taken for light to propagate in a forwards direction through the
seed laser 382.
[0093] The optical fibre 1 includes a waveguide 2 such as shown in
FIG. 1. The waveguide 2 includes a core 3 having a refractive index
n1 and a pedestal 4 having a refractive index n2, and a first
cladding 5 made of glass and having a refractive index n3.
Preferably n1>n2>n3. The optical fibre 1 is preferably coated
with a coating (not shown) that has a refractive index less than
n3. The fibre 1 can then be cladding pumped by coupling pump
radiation into at least the first cladding 5. The coating is
preferably a polymer. It has been found that cladding pumped fibre
lasers are less resistant to optical radiation damaging the pump
diodes when pedestal fibres such as shown in FIG. 1 are used within
the amplifiers. Preferably (but not necessarily) the optical fibre
1 is side pumped.
[0094] The optical fibre 1 is preferably (but not necessarily) a
photo-darkening resistant optical fibre. By "resistant", we mean
that the increase in attenuation of the optical fibre 1 during
operation should be no greater than 10%. Preferably the increase in
attenuation should be no greater than 5% in 2,000 hours.
Preferably, the increase should be less than 1% in 2,000 hours.
Measurement data on photo-darkening is described in detail
below.
[0095] The optical fibre 1 preferably has a numerical aperture less
than 0.15.
[0096] In the following and in the claims, there is frequent
reference to materials such as silica; phosphate, alumina, and
germania. By silica, it is meant pure silica, doped silica, and
heavily doped silica glasses, which glasses are sometimes referred
to as silicate or silicic glasses. By phosphate, it is meant oxides
of phosphorus such as phosphorus pentoxide (P.sub.2O.sub.5). By
alumina, it is meant oxides of aluminum (referred to as "aluminium"
in Europe) such as Al.sub.2O.sub.3. By germania, it is meant the
oxides of germania, and in particular (but not exclusively)
GeO.sub.2. By ytterbium, it is meant ytterbium incorporated into
the glass as an oxide, and the concentration in ppm is the
concentration of the ions of ytterbium, and in particular
Yb.sup.3+. Similarly, by reference to other rare-earth metals (such
as thulium, erbium etc), it is meant the rare-earth metal
incorporated into the glass as an oxide, and the concentration in
ppm is the concentration of the ions of rare-earth metal.
[0097] FIG. 2 shows a refractive index profile 20 of the fibre 1.
The refractive indices of the core 3, pedestal 4, and first
cladding 5 are n1, n2 and n3 respectively. It is preferred that n1
is greater than n2 and n2 is greater than n3.
[0098] One desirable embodiment has a core 3 that comprises silica
glass with a concentration of alumina of between approximately 0.3
to 0.8 mole percent, and a concentration of phosphate of
substantially 15 mole percent. It is preferred (but not required)
that there is substantially zero thulium dopant. It is preferred
(but not required) that the pedestal 4 includes silica, phosphate
and germania. The first cladding 5 can be pure silica, fluorine
doped silica, or other cladding materials (including polymers) used
in the manufacture of optical fibres. In the event that the first
cladding 5 is a glass, the fibre 1 is preferably (but not
necessarily) coated with a polymer which can have an index lower
than the refractive index of the first cladding 5 in order to guide
pump light in amplifiers and lasers. For application as the gain
medium in fibre lasers and amplifiers, at least one of the core 3
and pedestal 4 is doped with rare-earth dopant. The rare earth
dopant can be ytterbium whose concentration is preferably
substantially in the range 20000 to 45000 ppm.
[0099] An advantage of using a photo-darkening resistant fibre is
that the optical fibre 1 is capable of transmitting optical
radiation under high pumping and inversion conditions as well as
high intensities. The low numerical aperture means that the fibre 1
can be configured to be a so-called large mode area fibre, which in
combination with the high pumping and inversion conditions, permits
the output power and/or product lifetime of fibre lasers and
amplifiers to be increased. The design of large mode area fibres is
described in U.S. Pat. No. 6,614,975, which is hereby incorporated
herein by reference. A further advantage in the design of fibre
lasers and amplifiers is that fewer pump diodes are required since
the optical fibre maintains its transmission quality over the
product lifetime.
[0100] Although the above figures represent a preferred embodiment,
the core 3 can comprise silica, with a concentration of alumina in
the range 0.1 to 4 mole percent and a concentration of phosphate in
the range 2 to 20 mole percent. The alumina doped core in
combination with the germano-phosphorus doped pedestal permits low
numerical aperture fibres to be manufacturable with improved
repeatability as compared with the prior art. Numerical apertures
as low as 0.06 to 0.1, or preferably approximately 0.08 are readily
achievable.
[0101] The pedestal 4 can include silica, phosphate and germania.
The concentration of phosphate and germania are selected to achieve
the desired numerical aperture. Incorporation of germania has the
advantage of increasing the fictive temperature of the pedestal 4
and thus assisting in the retention of both circularity and core 3
to pedestal 4 concentricity, and hence core 3 to first cladding 5
concentricity, during the optical fibre manufacturing process. Core
concentricity is relevant in the production of low-loss fusion
splices.
[0102] Preferably (but not necessarily) there is substantially zero
thulium dopant in the core 3. The importance of eliminating thulium
dopant is significant in the design of fibre lasers and amplifiers
as fibres containing thulium dopant have been found to be
especially susceptible to photo-darkening. It is important
therefore to use rare-earth dopants that have low trace amounts of
thulium. The thulium concentration should be less than
approximately 10 ppm, and preferably less than 1 ppm.
[0103] The optical fibre 1 can be doped with at least one rare
earth dopant disposed in at least one of the core 3 and the
pedestal 4. The rare earth dopant can be ytterbium having a
concentration in the range 2000 to 60000 ppm. The concentration of
ytterbium can be between approximately 15000 to 50000 ppm. The
concentration of ytterbium is preferably between approximately
20000 to 45000 ppm.
[0104] The concentration of phosphate in the core 3 can be between
approximately 12 to 17 mole percent. The concentration of phosphate
in the core 3 is preferably approximately 15 mole percent.
[0105] The concentration of alumina can be between approximately
0.20 to 1 mole percent. The concentration of alumina is preferably
between approximately 0.3 to 0.8 mole percent.
[0106] The optical fibre 1 can be manufactured using chemical vapor
deposition and solution doping. Techniques are described in U.S.
Pat. Nos. 4,787,927, 4,815,079, 4,826,288, 5,047,076, and
5,151,117, which are all hereby incorporated herein by
reference.
[0107] The waveguide 2 can be a multimode waveguide at a signal
wavelength. The waveguide 2 can be configured to propagate single
mode light without significant distortion and/or mode coupling over
a substantial length. Such designs are important for the design of
high power fibre lasers and amplifiers, particularly for
application in spectroscopy, industrial materials processing, laser
surgery, and aerospace applications. Examples of core designs,
techniques to propagate single mode light, and use of bend losses
to remove (at least partially) unwanted higher order modes are
described in U.S. Pat. Nos. 5,818,630, 6,496,301, 6,614,975, and
6,954,575, all of which are hereby incorporated herein by
reference.
[0108] The waveguide 2 can be a single mode waveguide.
[0109] FIG. 3 shows an optical fibre 30 which includes stress
producing region 31 for inducing birefringence in the core 3. The
core 3 can be circular or elliptical. Alternatively, or
additionally, birefringence can be induced by making at least one
of the pedestal 4 and core 3 elliptical. FIG. 14 shows an optical
fibre 140 having an elliptical pedestal 4 for inducing
birefringence. The optical fibre 140 also includes an inner
cladding 141 designed to have a higher viscosity than that of the
pedestal 4. This can be achieved, for example, by including boron
dopant in the pedestal 4 in order to depress its refractive index.
Techniques to manufacture such birefringent fibres are described in
U.S. Pat. Nos. 4,274,854 and 4,426,129, which are hereby
incorporated herein by reference. It should be noted that it can be
advantageous in such fibres to reduce or eliminate the dopant
concentration of germania in the pedestal in order to allow the
pedestal to be reshaped into an elliptical jacket. Alternatively,
or additionally an additional cladding (not shown) doped with
oxides of boron can be included in the fiber 1 in order to form the
elliptical jacket as taught by the above patents. The optical
fibres 30 and 140 can also be single polarization optical fibres
which can be single mode or multimode, as described in U.S. Pat.
No. 6,496,301 and co-pending and commonly-owned U.S. patent
application Ser. No. 10/528,895, which patent and patent
application are hereby incorporated herein by reference.
[0110] FIG. 4 shows an apparatus in the form of a cladding pumped
amplifying optical device 40 comprising pump 44 and an optical
fibre 41. The optical fibre 41 can be the optical fibre 1, 30 or
140. The cladding pumped optical device 40 utilizes a composite
fibre 42 comprising the optical fibre 41 and pump fibres 43 within
a common coating 46. The composite fibre 42 is described in U.S.
Pat. No. 6,826,335, which is hereby incorporated herein by
reference. Other cladding pumped fibres and arrangements are also
possible, such as those described in U.S. Pat. Nos. 4,815,079,
5,854,865, 5,864,644, and 6,731,837, all of which patents are
hereby incorporated herein by reference. The embodiments described
herein are particularly advantageous because signal light
scattering or leaking from the low numerical aperture core 3 is
preferentially captured and guided by the pedestal 4, and therefore
is not routed back into the pumps 44. The apparatus therefore
removes one of the major failure mechanisms of cladding pumped
lasers, namely the catastrophic failure of pump diodes caused by
unwanted signal light. The cladding pumped optical device 40 can be
an amplifier, a laser, a master-oscillator power amplifier, a
Q-switched laser or an ultra-fast laser including at least one
source of pump energy. By ultra-fast laser, it is meant a laser,
including for example a laser in the form of a mode locked laser
and/or a master-oscillator power amplifier, that emits pulses
having pulse widths less than 1 ns, and more preferably less than
10 ps. Designs and applications of ultra-fast lasers are disclosed
in U.S. Pat. Nos. 6,885,683, 6,275,512, 5,627,848, 5,696,782, and
5,400,350, all of which patents are hereby incorporated by
reference herein.
[0111] FIG. 5 shows an apparatus in the form of a laser 50 for
material processing. The laser 50 includes the amplifying optical
device 40, a laser delivery fibre 51, and a processing head 52. The
processing head 52 can be a scanner, a galvanometer, or a focusing
lens. The laser 50 is particularly useful for marking,
microwelding, printing, micromachining and cutting of metals,
plastics and other materials.
[0112] FIG. 6 shows the effect of photodarkening in four
continuous-wave fibre lasers that each include a core 3 doped with
ytterbium, alumina, germania, and oxides of boron. The figure shows
a measurement of output power 61 with time 66 for four different
fiber lasers. The cores 3 of each of these fiber lasers were doped
with a standard commercially-available source of ytterbium dopant.
Line 62 shows a measurement from a fiber laser that is doped with
standard commercially-available ytterbium dopant that has a high
thulium content. A rapid decrease in output power 61 is observed.
Line 63 shows a measurement taken at 30C from a fiber laser that
outputs a continuous wave output power of 60 W. The fiber core 1
was doped with a standard commercially available ytterbium. Again,
there is a rapid decrease in output power 61 observable. Line 64
shows a measurement taken at 70C from a fiber laser that outputs a
continuous wave output power of 110 W. Finally, line 65 shows a
measurement taken at 30C from a fiber laser that outputs a
continuous wave output power of 60 W. The ytterbium dopant is high
purity, with a specified impurity level of less than 1 part in
10.sup.6. The ytterbium dopant is high purity, with a specified
impurity level of less than 1 part in 10.sup.6 (1 ppm). Comparing
lines 63 and 65, the use of high purity ytterbium has reduced the
effect of photodarkening when measured at the same levels of output
power and temperature. Comparing lines 64 to 65, increasing
temperature from 30C to 70C has caused an increase in
photodarkening. There is clearly a significant improvement in the
resistance to photodarkening achieved by utilizing Yb-dopant with
very low levels of thulium and other impurities. It is believed
that the remaining photodarkening effect is due to the other
refractive-index-controlling core co-dopants, namely alumina,
germania and oxides of boron.
[0113] Kitabayashi et al., in their paper entitled "Population
inversion factor dependence of photodarkening of Yb-doped fibers
and its suppression by highly aluminum doping" published in the
Proceedings of Optical Fiber Communications 2006 Conference,
disclose that photodarkening in Yd-doped fibers can be reduced, but
not entirely eliminated, by incorporating aluminum doping.
[0114] Surprisingly, the inventors have discovered that further
improvements can be achieved by removing germania from the core 3
of the fibre 1 and by including phosphate dopant. FIG. 7 shows the
improvement obtained by including phosphate dopant in a core 1 that
had been doped with alumina. The core 1 contains no germania and no
boron doping. The core 1 was doped with high purity oxides of
ytterbium (total impurity content specified to be better than 1
ppm) in order to keep contamination due to thulium and other
rare-earths to a minimum. Curve 67 shows the result with alumina
dopant, and curve 68 shows the result with alumina and phosphate
dopant. It is shown that, in accordance with the Kitabayashi et al.
results, alumina on its own cannot entirely suppress the
photodarkening effects. However, we have discovered that the
inclusion of phosphate co-dopants in addition to alumina results in
a dramatic improvement and dramatically reduces photodarkening
effects in ytterbium-doped fibers. Importantly, use of alumina
dopant in the core also enables low numerical aperture waveguides
to be produced with higher reproducibility than with phosphate
doping alone.
[0115] Photodarkening of optical fibres can also be characterized
by monitoring the increase in absorption (around the 1 micron
region of interest) of the optical fibre, subjected to an
accelerated-ageing test. The ageing is accelerated by pumping the
ytterbium doped core to achieve the maximum possible inversion. The
measurements shown with reference to FIGS. 8 to 10 were obtained by
core-pumping approximately 0.1 to 1 m lengths of the fibres with
approximately 400 mW of pump light at 976 nm, and coupling white
light from a tungsten filament into the core. The spectral
absorption was then measured by a cut-back measurement. The
measurement method follows the method published in a paper,
"Photodarkening in ytterbium-doped silica fibers" by L. L. Koponen
et al, Proceedings of SPIE Volume 5990. Measurements of loss 81
versus wavelength 83 were taken before and after the accelerated
ageing. FIG. 8 shows the measurement of loss 81 before 84 and after
85 accelerated ageing in a fibre laser whose core was doped with
high-purity Yb, germania and boron. FIG. 9 shows the same
measurement using a commercially available photodarkening-resistant
Yb-doped fibre. FIG. 10 shows the same measurement on a fibre laser
containing the fibre 1 shown in FIG. 1. The ytterbium absorption
peak from around 850 to 1050 nm is evident in each of FIGS. 8-10.
For this reason, the attenuation curves 84 and 85 have been
extrapolated by curves 86 and 87 (in FIGS. 8 and 9) and by curve
101 in FIG. 10 (since there is no discernable difference between
curves 84 and 85). Photodarkening is evident in FIG. 8 by the
increase in loss 81 which increases as the wavelength 83 reduces. A
region of interest 88 is shown by the hatched area in FIGS. 8 and 9
which shows the increase in loss 81 between 1000 nm and 1100 nm.
The fibre corresponding to FIG. 9 clearly shows a much smaller
increase in loss than the fibre corresponding to FIG. 8. However,
as evident from FIG. 10, there is no photodarkening evident in this
wavelength band as evidenced by the highlighted area 102. Fibre
designs according to this aspect of the present disclosure have
therefore eliminated photodarkening.
[0116] FIGS. 11 and 12 show the output power 110 versus time 111
from fibre lasers utilizing the fibre of FIG. 1, with dopants
similar to those used in the accelerated ageing test of FIG. 10.
The upper measurement line in each figure is the output power 110,
whereas the lower line is the ambient temperature 112 at which the
measurements were taken. The fibre laser used to obtain the results
shown in FIG. 11 was a master oscillator power amplifier MOPA (not
shown), emitting 12 W average power with 20 kHz pulses having pulse
energies of 0.6 mJ and pulse widths of 35 ns. The fibre laser used
to obtain the results shown in FIG. 12 was a continuous wave laser
which was modulated at 10 kHz by turning the laser diodes used as
pump sources off and on repeatedly at 10 kHz and with a 70% duty
factor. No degradation of the output power 110 with time 111 is
observable in either FIG. 11 or FIG. 12 over two to three months of
continuous operation for both the high-peak power pulsed MOPA, and
the continuous wave laser.
[0117] FIG. 13 shows the refractive index profile 130 for a
preferred embodiment. The profile 130 differs from the idealized
profile of FIG. 2 in that it contains a central depression 130 in
the core 3 owing to evaporation of dopants (particularly phosphate)
in the collapse. The refractive index difference 131 between core 3
and pedestal 4 is given by n.sub.1-n.sub.2, which is approximately
0.0032. This corresponds to a numerical aperture of approximately
0.096. Similarly, the refractive index difference 132 between
pedestal 4 and first cladding 5 is approximately 0.0097, which
corresponds to a numerical aperture of approximately 0.17. The core
3 was doped with alumina with a dopant concentration of
approximately 0.70 mole percent, phosphate with a dopant
concentration of approximately 15 mole percent, and ytterbium with
a dopant concentration of approximately 25000 ppm. The ytterbium
dopant was provided by the oxide of ytterbium with an impurity
content less than 1 ppm, and thus substantially no thulium dopant
was included in the core 3. The pedestal 4 was doped with
phosphorus and germania at levels sufficient to provide the desired
core 3 to pedestal 4 refractive index 131 of approximately 0.0032.
It should be noted that the pedestal refractive index difference
132 can be manufactured with high precision because the phosphate
in the pedestal 4 will not evaporate during the collapse of the
preform by virtue of the core 3 being deposited. Similarly, the
core refractive index difference 131 can also be manufactured with
high precision because unlike phosphate, the alumina dopant is
relatively immune to evaporation during the collapse of the
preform. The preferred embodiment thus provides a solution to the
photo-darkening problem that can be manufactured with high
precision and without significant evaporation of dopant from the
core 3 during manufacture. The ytterbium dopant concentration of
25000 ppm can be varied to within approximately 20000 ppm and 40000
ppm. Higher values allow increased pump absorption within a fibre
laser or amplifier. This has the additional advantage of reducing
the required length of active fibre being used. The diameter 133 of
the core 3 can be varied by oversleaving the preform during
manufacture, or by other techniques known in the industry, in order
to produce cores with diameters in the range 5 .mu.m to 50 .mu.m,
or even higher (such as 100 .mu.m if used for very high power
lasers and amplifiers). The core 3 can be single mode, or
multimode, and the refractive index difference 131 can be varied
such that the equivalent numerical aperture is in the range
approximately 0.06 to 0.15, and preferably in the range 0.08 to
0.15 by adjusting the dopant concentrations with the core 3 and/or
pedestal 4. The diameter 134 of the pedestal 4 can be in the range
approximately 1.5 to 5 times the diameter 133 of the core 4,
preferably in the range 1.5 to 4 times the diameter 133 of the core
4, and more preferably between approximately 2.0 and 4 times the
diameter 133 of the core 4. The design shown in FIG. 13 can be used
as a basis for the core 3 and pedestal 4 designs of the fibres 1,
30, 41, or 140 described herein.
[0118] The use of an active fibre with a central dip (as shown in
FIG. 13) in the final stage of amplification of the MOPA pulsed
laser was found to be greatly beneficial in a MOPA configuration
comprising a semiconductor seed laser followed by a low power
pre-amplifier stage.
[0119] This was unexpected since the non-gaussian mode profile
caused by the presence of the central dip has a poor matching to
the fibre pigtails with respect to the modes in a flat-top
refractive index. The increased signal splice losses were thought
to result in reduced amplifier efficiency, defined as the ratio
between the pump power used in the amplifier and the output signal
power. Conversely, improved amplifier efficiency was obtained by
introducing the dip in the active fibre core. High efficiency
provides a significant advantage since fewer pump diodes are
required to reach a desired laser output power.
[0120] An input signal power of 300 mW was used in the
characterization of different active fibres. This is typical for
the considered MOPA configuration with a 5-10 mW average output
power from the semiconductor seed laser and .about.25 dB of
amplification in the pre-amplifier stage.
[0121] The measured efficiency of fibres with a flat-top index
profile was 50-55% when multiple transversal modes were excited in
the active fibre (Multi-Mode efficiency), and as low as 25-35% when
only the lower order modes were excited (referred to as Single Mode
efficiency for brevity). The latter is more relevant in fibre
lasers since good output beam quality is targeted.
[0122] Fibres with a central dip, instead, showed MM efficiency of
65-70% and SM efficiency as high as 50-55%. These results were
obtained for a variety of fibres presenting a different dip in the
centre of the core, with a depression index change ranging from
-0.003 to -0.010.
[0123] This can be explained by considering the different energy
extraction efficiency of the different designs, defined as the
ratio of the energy extracted from the laser medium to the maximum
energy available in the medium. Energy extraction efficiency can be
increased by increasing the input saturation of the active medium,
i.e., by using a larger signal input power or improving the overlap
of the fibre modes with the active medium. The relatively low power
that can be obtained from a semiconductor seed+1 pre-amplification
stage means that good overlap must be targeted.
[0124] In a flat-top fibre, the lower order modes are concentrated
around the centre of the core, preventing an efficient use of the
energy stored in a large amount of Yb ions near the core edge.
Indeed, it is reasonable to assume that the Yb dopant profile
roughly follows the refractive index profile of the core due to the
high molar refractivity of Yb ions.
[0125] Conversely, the lower order mode profiles match the Yb
distribution profile in a fibre with central dip. All sections of
the dopant are effectively used by the input signal, resulting in
better average level of saturation and better extraction
efficiency. A significant advantage is theoretically found when a
depression as small as -0.003 in refractive index is introduced,
with negligible changes if the depression is further increased.
[0126] Additional benefits are a relaxed manufacturing tolerance in
the fibre fabrication (since the dip is related to dopant
evaporation in the preform collapse process, which is difficult to
control) and that the central dip lowers the peak intensity of the
fundamental at a given output energy, which reduces non-linear
effects.
[0127] A possible drawback is given by the increased pulse
reshaping in the amplifier. Better extraction efficiency results in
worse pulse deformation in the amplifier, and increased peak power
at the pulse leading end for the same total pulse energy. This is
detrimental for both non-linear effects and pulse shape control.
Optimal balance between efficiency and reshaping is obtained with a
small dip (index change .about.0.003), at the expense of tighter
manufacturing requirements.
[0128] FIG. 15 shows the output power 110 versus time 111 for a
continuous wave 200 W laser. The 200 W laser includes a 20 W fibre
laser whose output is amplified by a 200 W optical fibre amplifier.
The 200 W fibre amplifier used a fibre with a refractive index
profile similar to that shown in FIG. 13. The core 3 was doped with
alumina, phosphate and ytterbium. The output power 110 reduced by
approximately 2 W (0.7%) in 70 hours. This small reduction can have
been caused by reasons other than photodarkening. By contrast, FIG.
16 shows an equivalent result in which a more conventional step
index fibre with a depressed inner cladding was used in the 200 W
optical amplifier. The core was doped with alumina, boron and
ytterbium. The output power 110 is seen to degrade by approximately
25 W (10%) in 35 hours. The improvement obtained by using alumina
and phosphate dopant in the core is clearly demonstrated.
[0129] Experimental results have been presented which demonstrate
that photodarkening occurs at mW power levels through to hundreds
of Watts. Key drivers are the dopant composition, and the amount of
inversion in the fibre which should be reduced as much as
practical. Reducing inversion can be achieved by reducing the small
signal gain at every stage throughout the optical amplifying
system. Photodarkening appears to be problematical for optical
gains greater than 10 dB, and especially problematical for optical
gains greater than 20 dB.
[0130] The solutions described here are believed to be applicable
for power levels from fractions of Watts through to many thousands
of Watts, for continuous wave lasers, and pulsed lasers having
pulse widths from tens of femtoseconds through to hundreds of
milliseconds, for both single mode and multi-mode outputs, and for
both randomly polarized and polarized outputs. Although the work
described has focused on ytterbium doping, the inventors contend
that the results are also applicable to fibre lasers and amplifiers
based on stimulated Raman scattering, and those containing
Neodymium, Ytterbium, Erbium, Neodymium, Praseodymium, Thulium,
Samarium, Holmium and Dysprosium, Erbium codoped with Ytterbium, or
Neodymium codoped with Ytterbium. The solutions are important for
fibre lasers and amplifiers operating at wavelengths less than
around 1350 nm, and especially important for fibre lasers and
amplifiers operating at wavelengths less than 1100 nm. The results
are also applicable for the design of beam delivery fibres (such as
fibre 51 in FIG. 5), especially when the output from a fibre laser
has a wavelength less than 800 nm. This can be achieved through
frequency doubling or tripling, for example using non-linear
crystals, waveguides, or fibres, or materials such as chirped
periodically poled lithium niobate. The solutions can be used in
conjunction with conventional methods to reduce photo-darkening
such as using silica with high hydroxyl (OH) content, so-called
"wet silica". This can be loaded with deuterium and irradiated with
ultra-violet (UV) light.
[0131] FIG. 17 shows apparatus 171 for providing optical radiation
1713 including a light source 172, at least one amplifier 173, and
a controller 174. The light source 172 emits optical pulses 175
which are amplified by the amplifier 173 to produce output pulses
1719. The output pulses 1719 are characterized by a pulse
repetition frequency 176, a pulse duration 177, a peak power 179,
an average power 178, and a pulse energy 1710. The pulse energy
1710 is shown in FIG. 17 as the shaded area underneath the pulse
1719. The apparatus 171 has an optional fiber optic cable 1711
between the amplifier 173 and an optional processing head 1712
which directs the optical radiation 1713 to a material 1714. The
controller 174 controls at least one of the light source 172 and
the amplifier 173 such that the apparatus 171 can process the
material 1714. In FIG. 17, the processing is depicted as a mark
1715. The light source 172 can include the seed laser 382 of FIG.
38. The light source 172 can also include the reflector 384 of FIG.
38. The amplifier 173 can further include the amplifier 383 of FIG.
38.
[0132] Processing can also encompass marking, printing, cutting,
drilling, welding, microwelding, brazing, annealing, as well as
other materials processing applications. Processing can also
encompass biological processes such as tissue (such as skin)
treatment, dentistry, and surgery.
[0133] The processing head 1712 can include a scanning head or
galvanometer for scanning the optical radiation 1713. Alternatively
or additionally, the processing head 1712 can include at least one
lens for collimating and/or focusing the optical radiation 1713.
Preferably (but not necessarily) the processing head 1712 can
include a high power optical isolator to prevent destabilization of
the amplifier 173 by back-reflections originating from the material
1714.
[0134] The disclosure provided herein can also be viewed as the
material 1714 including a mark 1715 made using the apparatus 171.
The material 1714 can be an article such as a semiconductor package
(plastic or ceramic), a key pad on a mobile phone, iPODs, or a
computer, a component, a package, or a commercial or industrial
product.
[0135] Pre-amplifiers, power amplifiers, and optical arrangements
that can be used in the apparatus of FIG. 17, but without the
reflector 196, are described in U.S. Pat. No. 6,433,306, which is
hereby incorporated herein by reference. As that patent describes,
non-linear effects begin to appear as the peak power 179 of the
apparatus 171 is increased. A solution proposed to avoid stimulated
Brillouin scattering is to use a laser diode as the light source 2
which laser diode has multiple wavelengths so as to increase the
SBS threshold by the number of multimodes present so that the
stages of amplification are relatively free of SBS. The inventors
have found this approach to be generally suitable for peak powers 9
less than around 1 kW to 5 kW. However, as seen below, MOPAs based
on this solution are prone to random pulsing which can damage the
laser. The effect is believed to be due to SBS. Solutions such as
increasing the mode area of the fibres increases the SBS threshold,
but it has been proven difficult to increase it above around 5 kW
reliably. U.S. Pat. No. 6,433,306 does not disclose or suggest the
use of the reflector 196 to increase the SBS threshold or to avoid
random pulsing.
[0136] As an example of the problems posed by SBS, a laser diode
having greater than 50 modes was used in an apparatus emitting 12
kW peak power. When modulated at 20 kHz for extensive periods (such
as 1 hour to several months), a transient pulse was observed at the
output which caused internal damage to the waveguides with the
amplifier 173. The effect is believed to occur owing to random
effects within the semiconductor laser resulting in occasional line
width narrowing and consequential triggering of SBS. However it is
not proven that the effect is due to SBS although this appears to
be quite likely. Nevertheless, transient pulse damage has been
found to be very difficult to quantify and remove from such
devices, particularly as it occurs so infrequently.
[0137] Surprisingly, the inventors have managed to solve this
problem using the apparatus 180 of FIG. 18. The semiconductor laser
has been replaced by a superluminescent diode (SLED) 181, an
isolator 186 and a preamplifier 182. The preamplifier 182 is pumped
by a pump 189. The output of the preamplifier 182 is coupled to an
amplifier 183 via optical isolator 186, which is then coupled to a
power amplifier 1810 via another optical isolator 186. The
amplifier 183 and the power amplifier 1810 are pumped by pumps 184
and 1811 respectively. The pumps 189, 184 and 1811 are preferably
(but not necessarily) semiconductor lasers.
[0138] Replacing the semiconductor laser with an SLED 181 is a
counter-intuitive approach. This is because the SLED 181 has very
much less power than semiconductor lasers. Consequently the
preamplifier 182 is not saturated and generates ASE between pulses
1719 which saturates the gain of the amplifier 183 and the power
amplifier 1810. ASE is generated if the pump 189 is on all the
time. This is because it pumps the preamplifier 182, creating
inversion and subsequent generation of ASE. The effect of ASE can
be reduced by modulating the pump 189 such that it is turned off
between pulses, and turned on prior to the pulses arriving from the
SLED 181. The exact timing depends upon the design of the
preamplifier 182 and in particular the power available from the
pump 189, the higher the pump power, the less time that is required
to have the pump 189 turned on. Alternatively or additionally, an
optical switch 1812 can be inserted between the preamplifier 182
and the power amplifier 1810. Preferably (but not necessarily) the
optical switch 1812 is inserted between the preamplifier 182 and
the amplifier 183. Alternatively or additionally a filter (not
shown) can be used to filter the ASE but this is not believed to be
as effective as either modulating the pumps or using the optical
switch 1812. The optical switch 1812 can be an acousto-optic
modulator, a waveguide switch, a Kerr cell or a pockels cell.
[0139] The SLED 181 and the pumps 189, 184 and 1811 are controlled
by the controller 25, which can also control the optical switch
1812 if fitted. These devices can be controlled to be in
synchronism and to reduce non-linear and damage effects.
[0140] The preamplifier 182, amplifier 183, and power amplifier
1810 can be core-pumped or cladding pumped. Preferably (but not
necessarily), the preamplifier 182 is a core-pumped preamplifier.
This is because a core-pumped preamplifier is shorter device which
is more efficient. Preferably (but not necessarily) the amplifier
183 and power amplifier 1810 are cladding pumped. Such an
arrangement provides for efficient devices that can be produced
with low cost.
[0141] The optical fibres contained within the preamplifier 182,
amplifier 183 and power amplifier 1810 can each be solid core
fibres or so-called holey fibres. They are preferably doped with
rare-earth dopant such as ytterbium, erbium, neodymium, holmium,
thulium or praseodymium. Preferably (but not necessarily) the
optical fibres are resistant to photodarkening. Such fibres are
described with reference to FIGS. 1 to 16.
[0142] Preferably (but not necessarily) the SLED 181 has a
bandwidth greater than 10 nm, and preferably between 20 nm and 40
nm, or larger. The higher the bandwidth, the higher the SBS
threshold, and the more reliable the apparatus of the present
disclosure.
[0143] Other broadband sources can be used as the light source 172
in FIG. 17 to replace the SLED 181, such as other forms of LEDs
(such as edge emitting LEDs) and superluminescent fibre sources
(such as ASE sources filtered by gratings).
[0144] The inventors have also discovered that SBS events can be
prevented by using the apparatus shown in FIG. 19. The seed laser
192 is preferably the seed laser 382. The reflector 196 is
preferably the reflector 384. The amplifier 193 is preferably the
amplifier 383. In an experiment, the apparatus 190 included a
depolarizer 191, isolators 186, a pre-amplifier 192 pumped by a
pump 199, and the power amplifier 193 pumped by a pump 194. The
controller 174 controlled the light source 172, and the pumps 199
and 194.
[0145] The seed laser 192 was a single transverse-mode Fabry-Perot
semiconductor laser that emitted approximately 50 to 100
longitudinal modes at a centre wavelength of approximately 1060 nm.
The reflector 196 was a fibre Bragg grating having a bandwidth of
approximately 2 nm and which was chirped. Its reflectivity was
around 4%. The depolarizer 191 comprised two lengths of
polarization maintaining fibre spliced at 45 degrees to each other.
Use of the depolarizer 191 is preferable in many materials
processing applications (but not all) to reduce variations in
processing conditions caused by polarization fluctuations. It is
therefore important that the seed laser 192 has sufficient optical
bandwidth to enable its emission to be depolarized. The isolators
186 were commercial off-the-shelf isolators chosen to prevent
amplified spontaneous emission from being redirected back into the
seed laser 192. The preamplifier 192 and power amplifier 193 were
ytterbium-doped cladding pumped fibre amplifiers. The pumps 194,
199 included many single-emitter semiconductor laser diodes that
were combined together. A tap coupler 197 was provided in order to
monitor backward traveling optical radiation from the power
amplifier 193. The tap coupler 197 and back reflection detector 198
were included for diagnostic purposes, and need not form part of a
finished apparatus.
[0146] In normal operation, the seed laser 192 is pulsed to provide
10 ns to 250 ns pulses 175 at frequencies between 1 kHz and 500
kHz. At 25 kHz, the peak power from the seed laser is approximately
300 mW, the peak power from the preamplifier 192 is approximately
100 W, and the peak power from the power amplifier 193 is
approximately 10 to 15 kW.
[0147] Without the use of the reflector 196, the apparatus 190 was
found to suffer from catastrophic failures. A failure mechanism is
characterized by a short length (up to 100 mm) of ytterbium-doped
optical fibre within the power amplifier 193 destructing. The short
length of fiber turns into a white powder. Another failure
mechanism is a fusion splice at an end of the power amplifier
failing. These and other failures can occur both during
manufacturing and test, but also in a fully tested product after
several months of operation.
[0148] FIG. 20 shows the optical power 201 of the forward going
pulse 1719 (measured at the output of the power amplifier 193) and
the output power 202 as measured by the back reflection detector
198 in normal operation. The optical powers 201 and 202 are plotted
on different scales since the output power 201 has a much higher
power than the optical power 202. The output power 202 includes a
first reflection 203 and a second reflection 204. The first
reflection 203 originates from the splice (not shown) between the
tap 197 and the power amplifier 193. The second reflection 204
originates from a reflection from the output of the fiber optic
cable 1711. The pulse shapes as shown in FIG. 20 are typical of the
vast majority of pulses emerging from the apparatus 190.
[0149] FIG. 21 shows two examples of backward traveling pulses 211,
212. The pulses 211 and 212 include the optical powers 203 and 204
as before, but also contain additional spikes 213 and 214. These
additional spikes 213 and 214 occur infrequently and are random in
both occurrence and in magnitude. The frequency of occurrence can
be varied by changing the operating conditions of the seed laser
192 (such as temperature, drive current and pulse shape). At a
pulse repetition frequency of 25 kHz, the additional spikes have
been observed at a rate of between approximately 1 measured over a
weekend through to 30,000 measured over a five minute period.
Additionally, the rate of occurrence can be varied by using
different seed lasers 192 supplied by different or the same
manufacturer. Although additional spikes 213, 214 were observed in
the backward traveling direction, no evidence is observable in the
forward going pulse shape 201. It is contended that the spikes 213
and 214 are evidence of stimulated Brillouin scattering (SBS).
[0150] FIG. 22 shows an example of a giant pulse 221 superimposed
on a normal looking pulse 201 that occurred in the forward
direction. A related pulse 222 measured by the back reflection
detector 198 has a complex shape and has a magnitude several orders
higher than the pulse 202 shown in FIG. 20. Indeed the optical
powers 203 and 204 are not visible on this scale. The pulse 222 has
a spike 223, a trailing edge 224, and a dip 225. These data were
obtained by setting up the apparatus 190 so that the backward
spikes 213 and 214 were occurring at around 100 Hz. At this
repetition frequency, the giant pulses 221 and associated pulses
222 were observed occurring at around 1 every five minutes. In
other words, the giant pulses 221 are much more infrequent than the
backward traveling pulses 213 and 214, and occur on a random
basis.
[0151] It is contended that the dip 225 is again evidence of SBS.
The backward traveling pulse 222 has sufficient energy to pump a
forward going pulse via SBS. This results in the giant pulse 221 in
the forward going direction which therefore extracts energy from
the backward traveling pulse 222 resulting in the dip 225. All the
above pulses are additionally amplified by the active gain medium
in the power amplifier 193.
[0152] Referring to FIG. 22, the amplitude 226 of the giant pulse
221 shown is approximately twice the amplitude 227 of the pulse 201
(without the giant pulse 221 superimposed). The amplitude 226
varies randomly, and can be several times the amplitude 227 of the
pulse 201. It is contended that the amplitude of the giant pulse
221 can be sufficient to exceed the optical damage threshold of the
fiber within the power amplifier 193, and it is this, probably with
additional energy caused by the acoustic wave that is associated
with SBS propagation, that caused the random and unpredictable
catastrophic failures described above. However this explanation is
based on theory, and our attempt to describe a possible failure
mechanism is not intended to limit the scope of the present
disclosure.
[0153] FIG. 23 shows a typical spectrum 230 from the seed laser
192. The centre wavelength 231 is approximately 1062 nm, and it is
an overall bandwidth 232 of approximately 6 nm. The spectrum 230
includes approximately 150 laser lines 233 (not all shown) that are
separated by approximately 0.045 nm. The spectrum 230 includes
three families 234, 235, 236 of the laser lines 233 that have been
observed to vary as the pulse 1719 evolves. In particular, the
first family 234 is dominant at the first stages of the pulse
duration 177 (during which the pulse 1719 chirps), and the central
family 235 becomes more dominant thereafter.
[0154] FIG. 25 shows the design of the fibre Bragg grating 250 (not
shown) used as the reflector 196 in the apparatus of FIG. 19. FIG.
25 shows the reflectivity 251 (left axis) and group delay 252
(right axis) plotted as a function of wavelength 253. The group
delay 252 is offset from zero. The offset is arbitrary as the group
delay 252 depends upon the position at which it is measured. There
is noise on both reflectivity 251 and group delay 252 curves which
is mathematical in nature owing to computations involving very
small numbers. The grating 250 is chirped with a bandwidth 255 of
approximately 4 nm. Its central wavelength 254 was designed to be
1061 nm, aligned approximately within the central family 235 of
FIG. 23. FIG. 26 shows the measured reflectivity 261 of the grating
250. The central wavelength 262 is approximately 1060.7 nm, and the
bandwidth 263 is approximately 2 nm. The bandwidth 263 is less than
the design bandwidth 255 because of detuning effects in the
manufacture of the grating 250. These effects are known by those
skilled in the art of fiber grating manufacture. Techniques to
design and manufacture such gratings are described in U.S. Pat.
Nos. 6,445,852 and 6,072,926, hereby incorporated herein by
reference.
[0155] It should be noted here that the purpose of adding the
grating 250 was not to stabilize the emission from the laser diode
2. Narrowband gratings (bandwidth less than around 0.5 nm, and
often less than around 0.1 nm) are often incorporated into laser
diode packaging in order to lock the emission wavelength, to
prevent mode partition noise, to reduce the amplitude of side modes
(so-called side-mode suppression), and/or to narrow the linewidth.
All such purposes would tend to result in a more stable emission
which (it is contended) would promote SBS and increase the
frequency of occurrence of the giant pulses, and would therefore
not avoid the catastrophic failures of the apparatus 190.
[0156] FIG. 24 shows the spectrum 240 of the laser diode 2 measured
with the grating 250 in place. The central wavelength 241 is
approximately 1061 nm. The spectrum 240 has a bandwidth 242 that is
less than the bandwidth 232 of the spectrum 230. The bandwidth 242
is approximately 2 nm, being determined by the optical feedback
from the grating 250. In accordance with the teaching of U.S. Pat.
No. 6,433,306, such a narrowing of the bandwidth would be expected
to degrade the SBS performance. Surprisingly, the use of the
grating 250 has eliminated both the backward traveling random
pulses 213 and 214, and also the associated and more random
forward-traveling giant pulses 221. The apparatus 190 has been
found to be failure free. Additional experiments with other laser
diodes, and with and without the depolarizer, have confirmed the
beneficial effects of including the reflector 196 in the apparatus
190. The grating 250 has shown to provide reliable operation for
pulsed lasers having at least 10% of the optical radiation in a
single transverse mode and having peak powers in the range 1 kW to
40 kW.
[0157] It is contended that higher peak powers can be achieved up
to 100 kW or more without SBS causing a problem.
[0158] It is contended that the grating 250 has destabilized the
seed laser 192 (in this case a laser diode). That is, the
individual laser lines 233 are broadened, chirped or are being
modulated such that the conditions for SBS generation in the power
amplifier 193 are avoided. Other methods to avoid SBS generation
are to implement the reflector 196 as a broadband reflector such as
a partially reflecting mirror or surface, which can be coupled in
via an optical fiber coupler, or a grating having a bandwidth
greater than 0.5 nm, preferably greater than 1 nm, and more
preferably greater than 2 nm. The grating 250 is preferably (but
not necessarily) chirped. The chirped grating 250 can provide a
wavelength dependent effective external cavity length which is
contended to contribute further to the spectral broadening of the
seed laser 192. The chirped grating 250 is preferably inscribed in
a polarization maintaining optical fiber with its polarization axes
aligned or at an angle to the polarization axes of the seed laser
192 polarization maintaining fiber pigtail. The grating 250 can
equally be inscribed in a non-polarization maintaining fiber. Also,
the seed laser 192 pigtail need not necessarily be polarization
maintaining fiber. The reflector 196 can be positioned to reflect
laser radiation into either the front or back facets (not shown) of
the seed laser 192. The reflector 196 can also include dielectric
coatings. The reflectivity of the reflector 196 can be between
approximately 0.1% and 10%. Results showing no pulses 213, 214 have
been obtained with reflectivities as high as 34%; however pulse
distortion was observed at these levels. Preferably the
reflectivity is approximately 2% to 6%. The exact number will
depend upon the specific laser diode being, used and can be found
by experimentation. It is contended that the reflectivity of the
reflector 196 should be higher than the reflectivity used in the
front facet of the seed laser 192. It is also contended that the
bandwidth of the seed laser 192 is preferably comparable to or
smaller than the bandwidth of the reflector 196. This was not the
case in the experiments described above, and therefore the
operating conditions (temperature and drive current) of the seed
laser 192 needed to be set up for different batches of seed lasers
192. Alternatively or additionally the bandwidth of the grating
reflector 196 can be increased to include the entire bandwidth of
the seed laser 192 and manufacturing tolerances of the emission,
wavelengths. Note also, the provision of the tap 197 enables such a
set up since the complete absence of backward traveling pulses is a
reliable indication of reliable operation.
[0159] The grating 250 was positioned between 40 cm and 1 m from
the seed laser 192. No difference in performance was observed. It
is therefore contended that the exact position is not critical,
provided that the distance is not so large that there is
insufficient time to stabilize the seed laser 192 with reflective
feedback.
[0160] The seed laser 192 and the pumps 199 and 194 are controlled
by the controller 25. Further information on the use of the
controller to control the pulse shape of the optical pulses 1719,
and the application and use of the apparatus in materials
processing is described in co-pending U.S. Provisional Patent
application Ser. No. 60/812,164, which is hereby incorporated
herein by reference.
[0161] The preamplifier 192 and power amplifier 193 can be
core-pumped or cladding pumped. The preamplifier 192 can be a
core-pumped preamplifier. A core-pumped preamplifier is a shorter
device than a cladding-pumped preamplifier and is more efficient.
Alternatively, the preamplifier 192 can be a cladding-pumped
preamplifier. This is preferable in order to reduce cost.
Preferably (but not necessarily) the power amplifier 193 is
cladding pumped. Such arrangements provide for efficient devices
that can be produced with low cost. The design and construction of
cladding pumped amplifiers are described in U.S. Pat. No.
6,826,335, which is hereby incorporated herein by reference.
[0162] The optical fibres contained within the preamplifier 192 and
power amplifier 193 can each be solid core fibres or so-called
holey fibres. They are preferably (but not necessarily) doped with
rare-earth dopant such as ytterbium, erbium, neodymium, holmium,
thulium or praseodymium. Preferably (but not necessarily) the
optical fibres are resistant to photodarkening. Such fibres are
described with reference to FIGS. 1 to 16.
[0163] Referring to FIGS. 17 and 19, the apparatus 190 of FIG. 19
can be used to replace the light source 172 and amplifier 173 of
FIG. 19. As detailed below, the controller 174 controls the seed
laser 192 and the pumps 194 that pump the pre-amplifier 192 and
amplifier 193 such that the average power 178 and peak power 179
are maintained at levels sufficient to process materials 1714 over
a range of pulse repetition frequencies 176.
[0164] FIG. 27 shows output pulses 271, 272, 273 and 274 versus
time 275 for pulse repetition rates of 115 kHz, 46 kHz, 30 kHz and
24 kHz respectively. The corresponding optical pulses 175 emitted
by the seed laser 192 are shown as pulses 281, 282, 283 and 284
respectively in FIG. 28. Output pulses 271 to 274 and optical
pulses 281 to 284 are reproduced in FIGS. 29 and 30 respectively
where the respective pulses are superimposed. FIG. 31 shows the
variation of peak power 179 and pulse energy 1710 with pulse
repetition frequency 176. The peak power 179 is maintained above 5
kW and the pulse energy 1710 is maintained above 0.1 mJ for pulse
repetition frequencies 176 between 24 kHz and 115 kHz. Moreover,
since the average power 178 is the product of pulse repetition
frequency 176 and pulse energy 1710, the average power 178 can be
seen to be greater than approximately 10 W for pulse repetition
frequencies 176 between 24 kHz and 115 kHz. The results shown in
FIGS. 27 to 31 are very significant because it has proven possible
to achieve peak powers of 5 kW with pulse energies of 0.1 mJ at a
pulse-repetition frequency of greater than 100 kHz with only two
stages of amplification. The MOPA 190 will have great utility in
increasing processing speeds compared to other fibre-optic pulse
laser systems of comparable average output power 178.
[0165] FIG. 32 shows the output power 320 from the MOPA 190 with
increased power from the pumps 194. A peak power of 35 kW is
achieved with a pulse width of around 10 ns (measured at half the
peak power). FIG. 33 shows the optical spectrum 331 of the pulse
which is shown plotted versus wavelength 332. The signal wavelength
333 of the pulse from the seed laser 192 was approximately 1062 nm.
There is a significant proportion of the power at wavelengths
longer than the signal wavelength 333, which power has been
wavelength shifted due to non-linear processes such as stimulated
Raman scattering. Referring back to FIG. 32, the output power 321
was measured using an optical filter that attenuated wavelengths
greater than 1070 nm. It is seen from FIGS. 32 and 33 that output
powers greater than approximately 8 kW are shifted in wavelength.
The 8 kW output power can be defined as a non-linear threshold 322.
Use of the MOPA 190 for the processing of materials can thus
require the use of wide band optics within the processing head 1712
of FIG. 17. By "wide band optics" it is meant optics that can
transmit and focus signal wavelengths having a bandwidth greater
than approximately 100 nm. Nevertheless, the MOPA 190 operating in
this mode has great utility for the processing of materials that
require peak powers 179 greater than around 8 kW.
[0166] The controller 174 can be used to reduce the proportion of
the output pulses 1719 that is wavelength converted by reducing the
power emitted by the pumps 194 and 199, or by controlling the shape
of the output pulses 1719. The proportion that is wavelength
converted can be reduced to less than 50%, and preferably less than
10%. Preferably (but not necessarily) substantially none of the
output pulses 1719 are wavelength converted over a wide range of
pulse repetition frequencies 176 such as was demonstrated by
reference to FIGS. 27 to 31. The control function of the controller
174 can be effected either by open loop control (for example
achieved through experimentation and characterization), or be
closed loop control in which a measurement of power (average power
or peak power or a wavelength shifted power) is taken from a point
within the MOPA 190 (for example at the input or output of the
amplifier 193 or elsewhere), compared with a desired value, and the
difference between the measurement and the desired value used to
modify the control function provided by the controller 174.
[0167] FIGS. 34 to 37 show the effect of varying the pulse
repetition frequency (prf) 176 and pulse width 177 has on the shape
of the output pulse 1719 in a MOPA 190. All four figures are
plotted on the same scales with the output power being plotted in
arbitrary units. The controller 174 controlled the pumps 194 and
199 such that the average power 178 was maintained at 12 W. The
controller 174 also set the pulse repetition frequency 176 to its
desired value. The controller 174 then varied the pulse width 177
(i.e., the overall pulse width of each pulse 1719) to set the peak
power 179 at desired values.
[0168] FIG. 34 shows the output power 340 versus time 275 measured
with a pulse repetition frequency 176 of 25 kHz, a pulse energy
1710 of 0.6 mJ, and an average power 178 of 12 W. The pulse width
343 (i.e., the overall pulse width as controlled by the controller
174) was approximately 200 ns. The output power 340 has a peak
power 341, and a full width half maxima pulse width 342 of
approximately 35 ns.
[0169] FIG. 35, shows the output power 350 and 351 versus time 275
measured for two different pulse widths 355 and 356 respectively.
The pulse repetition frequency 176 was 50 kHz, the pulse energy
1710 was 0.24 mJ, and the average power 178 was 12 W. The full
width half maxima pulse width 354 for the output power 350 was
approximately 35 ns. The peak powers 352, 353 reduce with
increasing pulse widths 355, 356.
[0170] FIG. 36, shows the output power 360, 361 and 362 versus time
275 measured for three different pulse widths 367, 368 and 369
respectively. The pulse repetition frequency 176 was 100 kHz, the
pulse energy 1710 was 0.12 mJ, and the average power 178 was 12 W.
The full width half maxima pulse width 126 for the output power 360
was approximately 35 ns. The peak powers 363, 364, 365 reduce with
increasing pulse widths 367, 368 and 369.
[0171] FIG. 37, shows the output power 370 and 371 versus time 275
measured for two different pulse widths 375 and 376 respectively.
The pulse repetition frequency 176 was 200 kHz, the pulse energy
1710 was 0.06 mJ, and the average power 178 was 12 W. The full
width half maxima pulse width 374 for the output power 370 was
approximately 20 ns. The peak powers 372 and 373 reduce with
increasing pulse widths 375 and 376.
[0172] By varying the shape of the seed laser pulse 175 in the
apparatus of FIG. 19, the peak power 179 has been maintained above
the 5 kW level over a range of pulse repetition frequencies 176
from 1 Hz to 200 kHz, while providing pulse energies 1710 in excess
of 0.1 mJ. The peak power 179 has been maintained above the 3 kW
level over a range of pulse repetition frequencies 176 from 1 Hz to
500 kHz, while providing pulse energies 1710 in excess of 0.04 mJ.
Significantly, both these results were achieved without any
evidence of SBS and with only two amplification stages.
[0173] By controlling the shape of the seed laser pulse 175 in the
apparatus of FIG. 19, increasing the gain of the amplifiers 192,
193 and controlling their relative gain, we further demonstrated an
average power 178 between 20 to 25 W, pulse energy 1710 in the
range 0.8 to 1 mJ, full width half maxima pulse width 374 in the
range 15-20 ns, peak power 179 in the range 18-26 kW, pulse
repetition frequency 176 of 1 Hz to 25 kHz. Significantly, these
results were also achieved without any evidence of SBS and with
only two amplification stages.
[0174] In the system under study, the semiconductor diode 386
emitted 1 W peak power at 1090 nm. The power amplifier fibre was
Yb3+ doped with Aeff=245 .mu.m.sup.2 and a pump absorption 1.8
dB/m@915 nm. The fibre 1 contained the pedestal 4, was arranged in
a configuration similar to that shown in FIG. 4, and made according
to U.S. Pat. No. 5,930,435, which is hereby incorporated by
reference herein.
[0175] In this disclosure we have reported in detail our finding
regarding the effects of pulse reshaping and the impact on the
performance of pulsed fibre MOPAs. It is shown that properly
designing the active fibre and appropriately defining its
saturation energy can control the pulse reshaping. As a result,
high peak powers and maximum possible energy can be achieved
without the onset of SRS.
[0176] We have also discovered a pulsed fibre MOPA configuration,
which maintains the peak power over a 5 kW level for repetition
rates up to and exceeding 200 kHz. The MOPA configuration includes
a directly modulated 1080 nm laser diode seed, followed by a
two-stage all-fibre amplification unit and an additional delivery
fibre. The average power is in excess of 12 W, the pulse energy
lies in the 0.1-0.5 mJ range, the pulse duration varies between 10
ns and 200 ns, while the peak power remains constant at about the 5
kW or 10 kW level for rep rates in the range 10 kHz to >200
kHz.
[0177] Finally, we have demonstrated, using accelerated aging
tests, that it is possible to develop Yb3+ doped optical fibres
that do not photodarken under maximum inversion conditions.
[0178] Embodiments provided for herein are also in the form of a
materials processing machine (e.g., one for cutting, welding,
drilling, marking or processing materials) including one or more of
the lasers and controllers described above.
[0179] Embodiments provided for herein also include a method of
marking which method includes the step of providing a reflector
within a marking laser.
[0180] Embodiments provided for herein also include a materials
processing machine (e.g., one for cutting, welding, drilling, or
processing materials) including one or more of the lasers and
controllers described above.
[0181] It is to be appreciated that the embodiments described above
with reference to the accompanying drawings have been given by way
of example only and that modifications and additional components
can be provided to enhance performance.
[0182] The embodiments described herein extend to the above
mentioned features taken singly or in any combination.
* * * * *