U.S. patent application number 13/518457 was filed with the patent office on 2013-03-21 for laser system for the marking of metallic and non-metallic materials.
This patent application is currently assigned to DATALOGIC AUTOMATION S.R.L.. The applicant listed for this patent is Fabio Cannone, Orazio Svelto, Marco Tagliaferri. Invention is credited to Fabio Cannone, Orazio Svelto, Marco Tagliaferri.
Application Number | 20130068733 13/518457 |
Document ID | / |
Family ID | 42244932 |
Filed Date | 2013-03-21 |
United States Patent
Application |
20130068733 |
Kind Code |
A2 |
Tagliaferri; Marco ; et
al. |
March 21, 2013 |
LASER SYSTEM FOR THE MARKING OF METALLIC AND NON-METALLIC
MATERIALS
Abstract
A laser system for the marking of metallic and non-metallic
materials comprising a laser oscillator, characterized in that said
laser oscillator comprises: an active optical means of the crystal
laser type, a laser pump to provide a pump energy to said active
optical means; a mirror disposed upstream said active optical
means; an optical switch, apt to provide a pulsed laser beam,
disposed downstream said active optical means; a mode adaptor
coupled to said optical switch; a predetermined length single-mode
optical fibre, coupled to said mode adapter; a Bragg Grating type
reflector coupled to said optical fibre.
Inventors: |
Tagliaferri; Marco; (Taino
(VA), IT) ; Cannone; Fabio; (Melzo (MI), IT) ;
Svelto; Orazio; (Segrate (MI), IT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Tagliaferri; Marco
Cannone; Fabio
Svelto; Orazio |
Taino (VA)
Melzo (MI)
Segrate (MI) |
|
IT
IT
IT |
|
|
Assignee: |
DATALOGIC AUTOMATION S.R.L.
MONTE SAN PIETRO, BOLOGNA
IT
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20120255936 A1 |
October 11, 2012 |
|
|
Family ID: |
42244932 |
Appl. No.: |
13/518457 |
Filed: |
December 20, 2010 |
PCT Filed: |
December 20, 2010 |
PCT NO: |
PCT/EP2010/007800 PCKC 00 |
371 Date: |
June 22, 2012 |
Current U.S.
Class: |
219/121.6 |
Current CPC
Class: |
H01S 3/115 20130101;
H01S 3/08009 20130101; H01S 3/08013 20130101; H01S 3/117 20130101;
H01S 3/1611 20130101; H01S 3/08031 20130101; H01S 3/094053
20130101; H01S 3/1673 20130101; H01S 3/08 20130101; H01S 3/08045
20130101; H01S 3/09415 20130101 |
Class at
Publication: |
219/121.6 |
International
Class: |
B23K 26/00 20060101
B23K026/00 |
Claims
1. A laser system for the marking of metallic and non-metallic
materials comprising a laser oscillator, characterized in that said
laser oscillator comprises: an active optical means of the crystal
laser type, a laser pump to provide a pump energy to said active
optical means; a mirror disposed upstream said active optical
means; an optical switch, apt to provide a pulsed laser beam,
disposed downstream said active optical means; a mode adaptor
coupled to said optical switch; a predetermined length single-mode
optical fibre, coupled to said mode adapter; a Bragg Grating type
reflector coupled to said optical fibre.
2. The laser system according to claim 1, characterized in that
said first mirror and said reflector define the cavity of said
laser oscillator and the length of said optical fibre determines
the duration of the pulses of said pulsed laser beam.
3. The laser system according to claim 1, characterized in that
said optical switch comprises an acoustic-optical modulator.
4. The laser system according to claim 1, characterized in that it
presents a fibre optical amplifier coupled to said laser oscillator
and an optical insulator disposed between said laser oscillator and
said amplifier.
5. The laser system according to claim 1, characterized in that
said pump laser is a semiconductor laser.
6. The laser system according to claim 1, characterized in that it
presents a focusing lens of said pulsed laser bean to couple said
pulsed laser beam at the entrance of said optical fibre.
7. The laser system according to claim 1, characterized in that
said active optical means comprises a Neodymium-doped yttrium
Orthovanadate crystal.
8. The laser system according to claim 1, characterized in that it
comprises a second optical fibre at the exit of said amplifier to
provide a laser beam to a laser head for marking metallic and
non-metallic materials.
9. The laser system according to claim 1, characterized in that
said system provides at the exit thereof a polarized single-mode
laser beam.
10. A laser oscillator comprising an active optical means of the
crystal laser type; a pump laser to provide a pump energy to said
active optical mean; a mirror located upstream said active optical
means; an optical switch, apt to provide a pulsed laser beam,
located downstream said active optical means; an optical fibre
having a predetermined length, coupled to said optical switch; a
Bragg Grating type reflector coupled to said optical fibre.
11. A method for varying the optical pulses duration in a laser
system according to claim 1, characterized in that it comprises the
step of varying the length of the optical fibre.
12. A laser system according to claim 1, characterized in that it
further comprises a non linear crystal and a dichroic mirror to
make the harmonic duplication effect.
Description
[0001] This application is the national stage of PCT/EP2010/007800,
filed Dec. 20, 2010, which claims priority from Italian Application
No. BG2009A000067, filed Dec. 23, 2009.
FIELD OF THE INVENTION
[0002] The present invention relates to a laser system for the
marking of metallic and non-metallic materials and to a method for
varying the optical pulses duration in laser systems. In
particular, it refers to a laser system for the marking of metallic
and non-metallic materials in solid state fibre.
BACKGROUND OF THE INVENTION
[0003] The sector of industrial marking using laser is in
considerable expansion subsequent to the possibility of marking
metallic and non-metallic materials with techniques such as surface
or deep engraving of the material or through colour change of the
material. In this sector solid state lasers excited by laser diodes
(Diode-Pumped Solid-State, or DPSS) are conventionally used, with
average powers generally below 100 W and operating with light pulse
repetition obtained through the "Q-switching" technique. The laser
beam, produced by diodes, is then sent, through suitable
collimation optics, to the work surface. This transfer can take
place in two ways: by moving the laser beam on a fixed sample using
a system of galvanometric mirrors, or by moving the sample using a
system of axes x-y-z on a fixed laser beam. The DPSS laser source
is typically constituted by discrete optical components such as:
mirrors, crystals, lenses and prisms. A laser diode for excitation,
also called power pump diode, excites a suitably doped optical
crystal. When population inversion takes place inside the crystal,
a coherent and monochromatic electromagnetic radiation is generated
at the wavelength corresponding to the emission transition of the
doped crystal. This radiation is amplified inside the resonant
laser cavity delimited by two mirrors: a mirror through which the
pump beam is sent to the crystal, known as High Reflection Mirror
(HR) and an Output Coupler (OC) mirror, thus giving rise to the
laser radiation. Deconvolution of the emission spectrum of the
crystal with the reflection and pass bands of said mirrors produces
a highly monochromatic radiation (<0.1 nm). An acoustic-optical
modulator, i.e. based on sound-light interaction, positioned inside
the cavity produces a pulsed signal giving rise to Q-Switching.
However, this solution presents some problems.
[0004] It is known that DPSS lasers are affected by the problem of
thermal lens that causes a variation in the quality of the laser
mode as a function of the intensity of the pump diode with which
the crystal of the active material is irradiated. Consequently, the
quality of the output beam, and therefore the quality of the
industrial marking, depends on the output power. Moreover, the
quality of the laser beam (known as Beam Quality Parameter,
M.sup.2) depends on the optical length of the resonator, defined as
the distance between the HR and OC mirrors. That is, by varying the
optical length, the quality of the beam changes. The time duration
of the pulses also depends on this length. It is possible to define
the time duration of the laser pulse by varying the aforesaid
length. By increasing this length the time duration of the pulses
increases. This variation is not random, but is dependent on the
stability parameters of the laser, which in turn are dependent on
the optical properties of the OC and HR mirrors and on the length
of the thermal lens. However, when obtaining a stable laser cavity,
such as to guarantee pulses of long duration, the problem of
optical stability (optical alignment) of the cavity must be
tackled. We can conclude that with DPSS technology it is not
possible to obtain a laser source with a long pulse duration and
simultaneously high mode quality. Moreover, the reliability of the
laser source is limited as it is constituted by discrete elements
and the cost of the whole system is relatively high and cannot be
significantly decreased, as the cost of the discrete components
cannot be further reduced, given that they cannot be mass
produced.
[0005] For industrial machining operations, the DPSS laser source
must be kept close to the work surface, in a single and relatively
voluminous system, as transfer of the beam from the laser to the
surface occurs through propagation in the free space. An
alternative technology to the above relates to the use of a pulsed
fibre laser. In a particularly common configuration, the laser is
constituted by an oscillator that pumps a power amplifier, both
made completely in optical fibre. In this architecture, the laser
wavelength is not generated by pumping an optical crystal, as is
the case in DPSS technology, but an optical fibre, known as active
fibre, suitably doped with rare earths. There are two types of
fibre laser architectures that allow pulsed laser beams to be
produced. The first uses a low power seeder diode (a few tens of
mW) whose signal, electronically pulsed, must be amplified several
times to reach a sufficient power value. In the second type the
laser beam, again emerging from a lower power diode, is pulsed
using a Q-Switch, connected with the fibre. The beam output from
this chain is conveyed, through a further fibre, to the work
surface, guaranteeing remote positioning of the laser beam.
Compared with the DPSS laser, the fibre laser has undoubted
advantages. The quality of the beam and consequently of the marking
does not depend on the output power and on the repetition rate,
i.e. the fibre laser is not dependent on the effect of the thermal
lens. Unlike DPSS lasers, the high quality of the laser beam
(M.sup.2.apprxeq.1) is not dependent on the laser power and is
established by the single mode of the fibre.
[0006] Optical fibre components already available and with
relatively low costs are used, as these are used in the
telecommunications sector and are readily available. The source is
more reliable, as the laser beam always propagates in optical fibre
and no discrete optics are involved, as instead is the case in DPSS
lasers. Finally, the source can be positioned remotely with respect
to the work surface, as the beam is conveyed towards it directly
from the optical fibre, while in DPSS lasers the beam is propagated
to the work surface in the air. However, it must be stated that
this latter solution also has undeniable disadvantages or problems.
The intensity and the shape of the laser pulse emerging from a
fibre laser is greatly influenced by non-linear phenomena such as:
Scattering, i.e. Stimulated Brillouin Scattering (SBS),
Photodarkening, Amplified Spontaneous Emission (ASE), which occur
in the optical fibre and, principally, in the oscillator, phenomena
that are completely absent in DPSS lasers as the laser radiation is
produced in a crystal. In particular, phenomena of scattering, such
as Stimulated Brillouin Scattering, compete largely with the laser
efficiency giving rise to pre- and post-pulses that are amplified
in the subsequent amplification chain reducing the efficiency of
the laser and producing signals that can counterpropagate in the
chain of the fibre laser colliding with and damaging sensitive
elements such as the pump diodes. It is not possible to compensate
this loss of efficiency by increasing the length of the fibre, as
the intensity of Stimulated Brillouin Scattering depends on this
parameter. Therefore, the longer the active fibre is, the more
probable all non linear phenomena are, as for example in fibre
laser architecture composed of a plurality of amplification
stages.
[0007] Moreover, fibre laser architecture which involves the use of
the Q-Switch is complicated by the need to launch the signal
emerging from the fibre in the Q-Switch and subsequently receive it
in this fibre. This type of architecture has a complex optical
design, the objective of which is to guarantee maximum launching
and collection efficiency in the single-mode fibre. This complexity
is inexistent in a DPSS cavity as the Q-Switch must be proportioned
only to the dimension of the laser mode. The output light is not
linearly polarized, as is the case in some DPSS lasers, a fact that
prevents possible and effective frequency duplication of the output
beam, particularly useful in the case of micromachining operations,
such as the formation of solar cells. Moreover, this radiation has
a spectral width greater than one nanometre (FWHM Full Width at
Half Maximum) not suitable for harmonic conversion, which is
instead possible with laser radiation emitted from a DPSS
source.
SUMMARY OF THE INVENTION
[0008] The object of the present invention is to provide a laser
system for the marking of metallic and non-metallic materials
capable of simultaneously overcoming the drawbacks of DPSSL
architecture and of fibre lasers.
[0009] In accordance with the present invention, these and other
objects are achieved by a laser system for the marking of metallic
and non-metallic materials comprising a laser oscillator,
characterized in that said laser oscillator comprises: an active
optical means of the crystal laser type; a laser pump to provide a
pump energy to said active optical means; a mirror disposed
upstream said active optical means; an optical switch, apt to
provide a pulsed laser beam, disposed downstream said active
optical means; a mode adaptor coupled to said optical switch; a
predetermined length single-mode optical fibre, coupled to said
mode adapter; a Bragg Grating type reflector coupled to said
optical fibre, and optionally, a non linear crystal for frequency
duplication.
[0010] These objects are also achieved by a laser oscillator
comprising an active optical means of the crystal laser type; a
laser pump to provide a pump energy to said active optical means; a
mirror disposed upstream said active optical means; an optical
switch, apt to provide a pulsed laser beam, disposed downstream
said active optical means; a predetermined length single-mode
optical fibre, coupled to said optical switch; a Bragg Grating type
reflector coupled to said optical fibre.
[0011] These objects are also achieved by a method for varying the
optical pulses duration in a laser system according to claim 1,
characterized in that it comprises the step of varying the length
of said optical fibre.
[0012] Further characteristics of the invention are described in
the dependent claims.
[0013] The object of this invention is a laser system for
industrial machining, such as marking, constituted by an
oscillator, of extremely small dimensions, based on an optical
crystal as active means, that uses a Q-Switch, a single-mode fibre
comprising an output mirror of the Bragg Grating type, and followed
by an amplifier all in fibre.
[0014] In turn the optical beam is conveyed through a fibre, which
allows transport of the laser beam directly on the work surface.
Here the fibre is connected to a marking head that can be a
galvanometric head (the beam is moved through galvanometric mirrors
on the fixed sample to be processed) or a plotter (the laser beam
is fixed and a motorised system is used for one, two or three
dimensional movement of the sample). In this invention the
oscillator practically does not depend on the effect of the thermal
lens and produces pulses without non linear phenomena, which can
therefore be efficiently amplified in the amplifier producing a
laser pulse with high mode quality.
[0015] This invention has various important elements.
[0016] A hybrid laser based on DPSS and fibre laser technology,
i.e. a solid state fibre laser, which overcomes the limits of both
architectures achieving an innovative laser structure.
[0017] An oscillator based on an active crystal and an
acoustic-optical modulator that produces pulses substantially
without non linear phenomena as the signal is not generated in the
fibre but in the crystal.
[0018] An oscillator optically designed so as to achieve efficient
mode matching between the mode of the active means and the mode of
the fibre and reduced mechanical dimensions to guarantee
mechanical/optical and thermal stability.
[0019] An oscillator having a discrete HR mirror, typical of DPSS
architecture and a fibre OC mirror, i.e. a Fiber Bragg Grating,
typical of fibre laser architecture. In this invention the Fiber
Bragg Grating does not have the job of defining the wavelength of
the laser radiation, as the emission wavelength is defined mainly
by the narrow emission band of the active material (<0.1
nm).
[0020] An oscillator, which although based on an active crystal,
has a pumped mode that is invariant in the fibre guaranteeing that
it is insensitive to the thermal lens as the OC mirror is a Fiber
Bragg Grating.
[0021] An oscillator that produces a laser pulse whose time
duration depends on the length of the fibre, therefore selectable
on the basis of the industrial machining requirements, varying only
the length of this fibre without changing the physical dimensions
of the oscillator, without the need for complex control
electronics, and always maintaining a high mode quality of the
beam, as it is propagating in the fibre. That is, an oscillator
which produces laser pulses of the required time duration always
with excellent beam quality.
[0022] A laser oscillator in which it is possible to introduce non
linear crystals that allow the phenomenon of harmonic generation to
be produced.
[0023] An amplification of simple configuration that can amplify
the signal emerging from the oscillator without limits dictated by
the presence of non linear phenomena, as the laser radiation is not
produced in fibre but in a crystal.
[0024] A laser system with laser beam quality which is always ideal
(M.sup.2=1), regardless of the output power and of the repetition
rate.
[0025] A laser system that produces spectrally limited laser pulses
with high peak power, because of the narrow emission band of the
active material used (<0.1 nm), therefore suitable for harmonic
duplication for production of frequency duplicated lasers, also
considering that the laser system produces linearly polarized light
signals.
[0026] A modular laser system in which pump diode, oscillator,
amplifier, and the transport system of the optical beam are
connected through power connectors and therefore easily
interchangeable without having to act on an alignment of the
system.
[0027] A laser system that allows interfacing with a beam scanning
system on a fixed machining sample or vice versa.
BRIEF DESCRIPTION OF THE DRAWING FIGURES
[0028] The characteristics and advantages of the present invention
are apparent from the following detailed description of a practical
embodiment thereof, illustrated by way of non-limiting example in
the accompanying drawings, wherein:
[0029] FIG. 1 schematically shows a laser oscillator according to
the present invention;
[0030] FIG. 2 schematically shows a laser oscillator connected to
an optical amplifier, according to the present invention;
[0031] FIG. 3 schematically shows a laser oscillator connected to
an optical amplifier, in turn connected, through an optical fibre,
to a marking head composed either of a galvanometric scanning head
or of a plotter, according to the present invention;
DETAILED DESCRIPTION OF THE INVENTION
[0032] With reference to the accompanying figures, a laser
oscillator 1 according to the present invention, comprises a pump
source 10, connected to a multi-mode optical fibre that provides a
laser pump to a collimation and focusing lens 12 and then to an
active means 13, which are positioned along an axis known as pump
axis.
[0033] The pump source 10 is constituted by a single emitter at 808
nm with maximum power of 8 W operating in thermally stabilized
steady state. The electromagnetic radiation is sent to the
collimation and focusing lens 12 through a multi-mode fibre 11 with
core 200 .mu.m having numeric aperture 0.22. The connection between
the fibre 11 and the lens 12 takes place through an industrial
connector that allows rapid replacement of the diode without having
to act on the inner section of the oscillator. Naturally, the
maximum power of the pump source 10 is chosen on the basis of the
power level to be produced by the oscillator 1.
[0034] The collimation and focusing lens 12 is constituted by two
lenses, respectively capable of collimating and focusing the beam
coming from the fibre 11 in the crystal of the active means 13,
guaranteeing a pump mode, .omega..sub.p, of 200 .mu.m. The laser
pump enters the active means 13 through a mirror 14, known as High
Reflection (HR) mirror, provided with a highly reflecting
dielectric coating (typically 99%) at the laser wavelength (1064
nm), on the side facing the inside of the active means 13 and with
antireflective dielectric coatings at the pump wavelength of 808
nm, on both sides. The distance between the HR mirror 14 and the
crystal 13 of the active means is 3 mm.
[0035] A Neodymium-doped yttrium Orthovanadate crystal (Nd:YVO4)
with absorption coefficient of .alpha.=12.45 cm.sup.-1 is
preferably used as active means 13. The use of this type of crystal
guarantees a polarized beam. Alternatively, other types of crystal
can be used as active means, such as Yb:YVO4, which manifests a
wide absorption band so as to absorb fluctuations, due to thermal
instability, of the pump radiation. It is known to those skilled in
the art that variation of the type of active material causes
variation of the emission wavelength of the pump system (10).
[0036] The wavelength of the crystal is fixed at 0.8 cm so as to
guarantee complete absorption of the pump diode in the whole length
of the crystal.
[0037] The active means 13 under the action of the pump laser beam
generates a laser mode 15, having a radius .omega..sub.0=400 .mu.m,
sent to an active acoustic-optical modulator 16, which is a Quartz
crystal, i.e. a Q-Switch, which behaves as variable attenuator
electronically controlled by a circuit, not shown. An acoustic wave
propagating transversely in the crystal generates periodicity,
therefore this flat wave causes a refraction of the light if the
Bragg condition is satisfied. Alternatively, the modulator 16 can
be of electro-optical type.
[0038] In this way, this device is able to generate pulsed laser
beams and to completely extinguish the laser radiation for a
certain time interval.
[0039] The laser mode 15 continues, in sequence, towards a focusing
system 17, and an optical mode adapter 18 which is connected to a
predetermined length single-mode optical fibre 19. Here the laser
mode, generated in the crystal 13 passes from propagation in air to
propagation confined in fibre.
[0040] A reflector 20 of Fiber Bragg Grating (FBG) type, is
positioned at the end of the fibre 19 and preferably produced
inside this fibre. This has the function of an output mirror of the
Output Coupler (OC) type.
[0041] An optical connector 21 is connected to the fibre 19 through
a splice.
[0042] The focusing system 17 is constituted by a lens with high
transmission efficiency at 1064 nm, mounted on a device that allows
an alignment in x-y-z and .theta..sub.x, .theta..sub.y. The focal
length, f, of this lens is 25 mm so as to produce in the adapter 18
a mode with a radius .omega..sub.f=30 .mu.m.
[0043] The adapter 18, having an input radius of 35 .mu.m, allows
the laser mode focused by the lens 17 to be conveyed in the
single-mode fibre 19. It can be schematized as a funnel that
maintains the brightness of the beam.
[0044] The adapter 18 is preferably a mode adapter but a ball lens
can also be used.
[0045] The optical fibre 19 (core=6.+-.0.3 .mu.m and
cladding=125.+-.0.5 .mu.m) is a single-mode fibre with Mode-Field
of 2.95 .mu.m and maximum attenuation of 1.5 dB/Km. It has a
V-number=2.4, therefore is single-mode. It allows conversion of
cavity mode into a single-mode beam.
[0046] The distance between active crystal 13 and focusing lens 17
and the focal length of the lens 17 have been designed so as to
couple the laser mode with the mode of the fibre with maximum
efficiency.
[0047] In order to reduce cavity losses, the surface of the optical
adapter 18 has an antireflective coating at 1064 nm.
[0048] The percentage of reflectivity at 1064 nm of the reflector
20 (50%-60%) is chosen so as to reduce power losses in the
oscillator and to lower the power threshold of the laser.
[0049] The optical connector 21 is a common fibre FC/PC connector
used for connection to the subsequent amplifier. Use of this
connector allows the oscillator to be exchanged with an amplifier
and vice versa without having to act on both systems. The entire
sequence of fibre components (from 18 to 20) are preferably
produced on the same fibre so as to avoid connections and,
consequently, losses due to connection.
[0050] The spectral measurements show the absence of Amplified
Spontaneous Emission. For this reason no ASE filter is inserted
between the oscillator 1 and the amplifier 2.
[0051] The oscillator 1 therefore provides a laser beam at the
wavelength of 1064 nm and spectral width 0.1 nm, with a power of
500 mW, with a pulse repetition frequency of 20 kHz, and a pulse
duration that can vary from 50 to 350 ns, selectable only by
varying the length of the fibre 19 present in the oscillator, from
0.1 m to 5 m. In particular, using a fibre 30 cm in length pulses
of a duration of 80 ns, with repetition frequency 20 kHz and mean
output power of 400 mW, are typically obtained. With a fibre 60 cm
in length pulses of a duration of 100 ns are typically obtained,
and with a fibre 150 cm in length pulses of a duration of 190 ns
are typically obtained.
[0052] The distance between the mirror 14 and the reflector 20
defines the length of the cavity of the laser oscillator 1, and
therefore the pulse duration.
[0053] The reflector 20 is not used to define the wavelength of the
laser as it is defined by the spectral properties of the active
crystal 13.
[0054] The distance between the mirror 14 and the focusing lens 17,
in an example of embodiment, is equal to 183 mm, and the length of
the fibre varies from 0.1 m to 5 m. Therefore, this distance can be
deemed negligible with respect to the length of the fibre, and the
width of the pulses is substantially defined by the length of the
fibre 19.
[0055] The length of the fibre 19 is therefore the parameter that
is varied to select the time duration of the pulses.
[0056] In a variant of the oscillator 1 described above, it is
possible to produce intracavity duplication by inserting a non
linear crystal and a dichroic mirror with an angle of incidence of
0.degree. in the laser cavity. In this way, it is possible to
produce visible laser radiation (e.g. 532 nm). The non linear
crystal allows frequency duplication of the radiation at 1064 nm,
while the dichroic mirror allows the cavity of the duplicated
frequency to be delimited. The properties of this mirror are AR at
the wavelength of 1064 nm and HR at the wavelength of 532 nm and
the crystal is a Lithium Borate Oxide (LBO) Type I Crystal, i.e.
1064.0(o)+1064.0(o)=532.0(e). The Fiber Bragg Grating must be HR at
the wavelength 1064 nm and AR at the wavelength 532 nm. The crystal
is maintained at the optimum temperature to maximize conversion
efficiency. The dichroic mirror is aligned so as to launch the beam
at 532 nm produced in the fibre 18 always through the lens 17.
Naturally, the single-mode fibre 19 must be a fibre suitable to
transmit radiation at 532 nm, for example, Germanium-free. Knowing
this example, it is clear that the insertion of further crystals
allows further harmonic orders to be produced.
[0057] An amplifier 2 is connected to the oscillator 1, through the
connector 21. The amplifier 2 comprises, in sequence, after the
connector 21, an optical isolator 30, a single-mode optical adapter
31, followed by an active fibre 32 and then a combiner 33. The
amplifier comprises only components in fibre.
[0058] From the element 21 to the element 31 the fibre has
specifications identical to the fibre 19 present in the
oscillator.
[0059] The optical isolator 30 is an integrated component in fibre
that is connected by splices to the fibre of the amplifier. It has
an insulation degree of 30 dB, and allows propagation of the
radiation at 1064 nm only in oscillator-amplifier direction.
[0060] The optical adapter 31 acts as an inverse funnel with
respect to the adapter 18. In fact, the dimension of the fibre core
passes from 6 .mu.m (diameter) to 25 .mu.m (diameter). Considering
that the dimension of the cladding of the fibre 19 is 125 .mu.m, in
order to preserve brightness, the active fibre 32 is a Large Mode
Area Ytterbium Doped Fiber with an absorption coefficient at 940 nm
of 1.7 dB/m core and cladding dimensions respectively 25 .mu.m and
400 .mu.m.
[0061] Consequently, to produce a 10 dB-20 dB amplifier, suitably
considering all the various sources of loss, the length of the
active fibre 32 is 5-12 m. The active fibre 32 is pumped from a set
of diodes 34.
[0062] The set of diodes 34 is composed of 6 single emitters with a
power of 8 W at 940 nm, each connected with a fibre having core and
cladding respectively of 105 .mu.m and 125 .mu.m, mutually combined
through a combiner.
[0063] The diodes 34 emit radiation at 940 nm as the active fibre
32 has a wide absorption band at 940 nm. The fibre 32 has a
cross-section more or less constant between 920 nm and 960 nm,
therefore a variation in emission wavelength of the diodes 34 due
to thermal effects (typically 0.3 nm/K) and has no effects in
absorption of the fibre 32. The development of new types of fibres
will allow increasing improvement of the efficiency of the
amplifier, thereby allowing a reduction in the number of diodes
present in 34.
[0064] The laser pump generated by the pump 34 at 940 nm reaches
the combiner 33 and provides the laser beam, received by the
oscillator 1, amplified, to a connector 35.
[0065] The active fibre 32 is connected to the passive fibre of the
combiner 33 through a splice. The coupling efficiency is maximum as
the active/passive fibres have the same properties.
[0066] The amplifier is connected to the optical beam transport
system through the connector 35. The optical beam transport system
allows remote positioning of the beam. It is composed of a fibre 38
of a length variable from 1 m to 10 m with cladding of 400 .mu.m
coated with an industrial sheath capable of withstanding bending
and pressures without damage to the internal fibre. The fibre 38 is
connected to a marking head 3 through an optical isolator, not
shown.
[0067] The optical isolator is connected directly to an optical
collimator (not shown) which allows collimation of the beam
emerging from the laser system. The optical isolator acts as
suppressor of retroreflection and scattering that can come from the
sample being machined. Its transmission efficiency is greater than
90% with an isolation level >30 dB.
[0068] The marking head 3 can provide movement of the beam through
two galvanometric mirrors or movement of the sample through a
plotter. In the first case, the laser beam is moved on the fixed
sample and vice versa in the second case. The marking head 3 can be
replaced with a suitably designed optical focusing system, capable
of focusing the beam emerging from the fibre 38 on a sample moved
through a plotter.
* * * * *