U.S. patent application number 11/721873 was filed with the patent office on 2009-12-31 for not temperature stabilized pulsed laser diode and all fibre power amplifier.
This patent application is currently assigned to VECTRONIX AG. Invention is credited to Ulrich Drodofsky, Marcel Zeller.
Application Number | 20090323734 11/721873 |
Document ID | / |
Family ID | 35283678 |
Filed Date | 2009-12-31 |
United States Patent
Application |
20090323734 |
Kind Code |
A1 |
Drodofsky; Ulrich ; et
al. |
December 31, 2009 |
NOT TEMPERATURE STABILIZED PULSED LASER DIODE AND ALL FIBRE POWER
AMPLIFIER
Abstract
So as to establish laser light with a desired characteristic
downstream a laser light source (151), the light is amplified by an
amplifier (107) which is gain modulated (E.sub.107G).
Inventors: |
Drodofsky; Ulrich; (Berneck,
CH) ; Zeller; Marcel; (Balgach, CH) |
Correspondence
Address: |
ANTONELLI, TERRY, STOUT & KRAUS, LLP
1300 NORTH SEVENTEENTH STREET, SUITE 1800
ARLINGTON
VA
22209-3873
US
|
Assignee: |
VECTRONIX AG
HEERBRUGG
CH
|
Family ID: |
35283678 |
Appl. No.: |
11/721873 |
Filed: |
September 30, 2005 |
PCT Filed: |
September 30, 2005 |
PCT NO: |
PCT/CH2005/000568 |
371 Date: |
September 17, 2009 |
Current U.S.
Class: |
372/6 ;
372/29.02 |
Current CPC
Class: |
H01S 5/0612 20130101;
H01S 2301/02 20130101; G01S 7/4814 20130101; G01S 7/499 20130101;
H01S 3/06754 20130101; G01S 7/4818 20130101; H01S 3/094076
20130101; G01S 7/484 20130101; H01S 3/1305 20130101; H01S 3/1302
20130101; H01S 3/0078 20130101; H01S 3/09415 20130101; G01S 17/10
20130101; H01S 3/005 20130101 |
Class at
Publication: |
372/6 ;
372/29.02 |
International
Class: |
H01S 3/067 20060101
H01S003/067; H01S 3/13 20060101 H01S003/13 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 16, 2004 |
EP |
04029867.1 |
Jan 14, 2005 |
EP |
05000669.1 |
Claims
1. A method for producing laser light with a desired
characteristic, comprising the steps of: generating laser light in
a spectrum range; amplifying said laser light with an active fibre
amplifier; modulating the gain of said amplifying so as to achieve
and maintain said desired characteristic.
2. The method of claim 1 wherein said desired characteristic is at
least one of intensity of laser light dependent on said laser light
generated, signal-to-noise ratio of said dependent laser light and
wall-plug efficiency of said dependent laser light.
3. The method of claim 1, wherein said modulating is performed
within a negative feedback control loop for said desired
characteristic.
4. The method of claim 1, wherein said modulating is performed by
at least one of pumping light intensity for said amplifying,
spectrum of pumping light for said amplifying, pulse-width of
pulsed pumping light for said amplifying, shift of spectral
position of an optical filter characteristic, length of active
fibre for said amplifying.
5. The method of claim 1, wherein said laser light is generated as
pulsed laser light and comprising time-synchronizing at least a
part of said modulating with said pulsed laser light.
6. The method of claim 1, further comprising optical filtering
laser light amplified by said amplifying.
7. The method of claims 6, further comprising performing said
filtering with a filter characteristic which is shiftable with
respect to spectral location.
8. The method of claim 7, further comprising performing shift of
spectral location of said filter characteristic in dependency of a
temperature.
9. The method of claim 8, further comprising selecting said
temperature dependent on temperature of a laser source generating
said laser light.
10. The method of claim 1, wherein said generating laser light
comprises generating laser light by means of a laser diode.
11. The method of claim 1, further comprising stabilizing said
laser light generated by a stabilizing filter and filtering said
laser light after said amplifying by a downstream filter,
respectively with a stabilizing filter characteristic and a
downstream filter characteristic, both filter characteristics being
shiftable with respect to spectral position and matching shift of
spectral positions of said stabilizing filter characteristic and
downstream filter characteristic.
12. The method of claim 1, further comprising filtering said laser
light at least one of before and after said amplifying by means of
optical fibre filter, said filtering before said amplifying
including stabilizing a stabilizing filter.
13. The method of claim 1, further comprising generating said laser
light as pulsed laser light.
14. The method of claim 1, further comprising emitting laser light
dependent on said generated laser light amplified and receiving
laser light dependent on said emitted laser light at one common
laser input/output port.
15. The method of claim 1, further comprising guiding said
generated laser light up to a laser output port substantially
exclusively in optical fibres.
16. The method of claim 1, further comprising guiding laser light
dependent from said laser light generated by means of an optical
fibre to a transmitter optic and determining the divergence of a
beam of said dependent laser light by conception of an end of said
fibre towards said optic.
17. The method of claim 16, wherein said transmitter optic is a
transmitter and receiver optic.
18. The method of claim 16, wherein said optical fibre is an active
optical fibre.
19. A method of laser range finding or of laser target designating
comprising: generating laser light with a desired characteristic in
a pulsed manner by steps of: generating laser light in a spectrum
range; amplifying said laser light with an active fibre amplifier;
modulating the gain of said amplifying so as to achieve and
maintain said desired characteristic; directing laser light
dependent on said laser light generated towards a target.
20. The method of laser range finding of claim 19, further
comprising evaluating multiple laser light pulses received.
21. A laser system comprising: a laser light source with an output;
an active fibre optical amplifier having an input, the output of
the laser light source being operationally coupled to the input of
the active fibre optical amplifier; and wherein said active fibre
optical amplifier has a gain modulation control input.
22. The laser system of claim 21, further comprising a negative
feedback control loop, wherein said gain modulation control input
is operationally connected into the negative feedback control loop
for a desired characteristic of laser light downstream said active
fibre amplifier acting as an adjusting member within said negative
feedback control loop.
23. The laser system of claim 21, wherein said gain modulation
control input is an input for pumping light.
24. The laser system of claim 23, further comprising a pumping
light source, wherein the pumping light source is operationally
connected to said modulation control input whereby said pumping
light source comprises a control input for adjusting at least one
of light intensity, spectral range of pumping light, and
pulse-width of pulsed pumping light.
25. The laser system of claim 21, further comprising a pulsed laser
light source, a pulsed pumping source for said active fibre
amplifier and a synchronization interconnected between said pulsed
laser light source and said pulsed pumping source.
26. The laser system of claim 21, further comprising at least one
optical filter having an input, the input of the filter being
operationally connected with the output of said active fibre
amplifier, the filter characteristic of said optical filter being
spectrally shiftable.
27. The laser system of claim 26, wherein a temperature sensing
element is operationally connected to said optical filter,
controlling said shift of spectral location of said filter
characteristic.
28. The laser system of claim 27, wherein said temperature sensing
element is arranged adjacent to or at said laser light source.
29. The laser system of claim 28, wherein said temperature sensing
element is formed by said optical filter.
30. The laser system of claim 21, wherein said laser light source
comprises a laser diode.
31. The laser system of claim 21, further comprising a stabilizing
filter and a further filter element having an input, the input of
said further filter element being operationally connected to an
output of said active fibre optical amplifier, said stabilizing
filter and said further filter element having respective filter
characteristics the spectral position thereof being controllably
shiftable in a mutually matched manner.
32. The laser system of claim 21, further comprising an optical
fibre filter at least one of upstream and downstream said active
fibre amplifier whereby a filter upstream said amplifier including
a stabilizing filter.
33. The laser system of claim 32, wherein a spectral position of
filter characteristic of at least one of said optical fibre filters
is controllably shiftable.
34. The laser system of claim 33, wherein said filter
characteristic is determined by at least one geometric entity at
said optical fibre filter and said geometric entity is controllably
varied to establish said shift.
35. The laser system of claim 21, further comprising a circulating
unit having an input and an output, a detector unit having an
output/input, and a laser light transmitter and receiver port,
wherein said active fibre optical amplifier has an output which is
operationally connected to the input of the circulating unit, the
output of the circulating unit being operationally connected to the
detector unit, the output/input of the detector unit being
operationally connected to the laser light transmitter and receiver
port.
36. The laser system of claim 21, wherein laser light in the system
is substantially exclusively guided in optical fibres.
37. The laser system of claim 36, further comprising a laser light
emitting port and a laser light reception port and an evaluation
unit operationally connected to said laser light reception
port.
38. The laser system of claim 37, wherein said emitting port and
said reception port are one common port.
39. The laser system of claim 37, wherein said laser light source
is operated in pulsed mode and said evaluation unit performs
multiple pulse evaluation.
40. The laser system of claim 21, further comprising a transmitter
optic having an input, and an optical fibre having one end
connected to the input of the transmitter optic, said one end of
said optical fibre being conceived to determine divergence of laser
light transmitted by said transmitter optic.
41. The laser system of claim 40, wherein said transmitter optic is
also a receiver optic for said system.
42. The system of claim 40, wherein said optical fibre is an active
optical fibre.
43. The laser system of claim 21, wherein the system is at least a
part of a laser range finder system.
44. The laser system of claim 21, wherein the system is at least a
part of a laser target designator system.
45. The laser system of claim 21, wherein the system is integrated
in a portable or handheld device.
46. The laser system of claim 45, wherein the system is at least a
part of a laser range finder system for target distances of at
least 1 km throughout up to ranges of at least 10 km.
47. The laser system of claim 21, wherein the system is part of a
vehicle.
Description
[0001] The present invention departs from the object to construe a
laser system which is highly compact, low power consuming and
robust to environmental hazards so as to be applicable for portable
or even handheld devices. The invention especially departs from
such an object to be resolved for a laser system integrated into a
laser range finder device or target designator device e.g.
incorporated in an observation instrument. Thereby, in addition to
the addressed requirements with respect to compactness, power
consumption and robustness such a laser system, as for long
distance range findings and target designation, must be of
relatively high output power and must allow accurate evaluation of
target reflected laser light.
[0002] One problem which is especially addressed in the present
application is the control of a characteristic of output laser
light especially of at least one of intensity,
signal-to-noise-ration, wall-plug efficiency, departing from a
laser system as addressed above. Nevertheless, the solution of this
object may be applied more generically on laser systems where
especially constructional compactness and power consumption as well
as accurate evaluation are prevailing considerations.
[0003] Thus the present invention is directed on a method for
producing laser light with a desired characteristic of the output
laser light. This is accomplished according to the present
invention in that there is generated laser light in a spectrum
range. The laser light is amplified with an active fibre amplifier.
The gain of such amplifying is modulated so as to achieve and
maintain the addressed desired characteristic.
[0004] Instead of providing stabilizing measures within a laser
system so as to properly control e.g. keep constant, parameters
which do affect the addressed characteristic of output laser light,
which measures customarily necessitate significant constructional
efforts and do consume additional power as e.g. for cooling,
negative feedback-controlling purposes, the desired characteristic
is achieved and maintained by appropriately modulating the
addressed gain of amplifying.
[0005] Further, providing the addressed amplifying with an active
fibre amplifier significantly improves constructional compactness
on one hand as well as output power of laser light on the other
hand.
[0006] In one embodiment of the method according to the present
invention as a desired characteristic at least one of intensity of
laser light which depends on the laser light generated, of
signal-to-noise ratio of such dependent laser light and of
wall-plug efficiency of the dependent laser light is selected.
[0007] In a further embodiment of the method according to the
invention modulating is performed within a negative feedback
control loop for the addressed desired characteristic. As perfectly
known to the skilled artisan such a negative feedback control loop
comprises sensing the addressed characteristic as momentarily
prevailing, comparing such momentarily prevailing characteristic
with a desired characteristic or with a desired time-course of such
characteristic and acting upon the system by adjusting so as to
match the momentarily prevailing characteristic as closely as
desired with the desired characteristic. Such adjusting in the
control loop is performed by acting on the gain modulation of the
amplifying.
[0008] In one embodiment modulating of the gain of amplifying by
means of the active fibre amplifier is performed by at least one of
intensity of pumping light for such amplifying, spectrum of such
pumping light, pulse-width of pulsed pumping light for such
amplifying, shift of a spectral position of an optical filter,
length of active fibre for such amplifying.
[0009] A significant improvement especially with an eye on
signal-to-noise ratio is achieved at output laser light, by
generating pulsed laser light and time-synchronizing at least a
part of the addressed gain modulating with the laser light as
pulsed. Just as an example it thereby becomes possible to increase
the gain of amplifying just during time periods in which the laser
light pulses are "ON". Thereby the noise during "OFF"-periods of
the laser light pulses is reduced.
[0010] By providing the addressed amplifying, noise may be
generated by amplified spontaneous emission ASE which may
significantly contribute to the overall noise in the output laser
light. Therefore in one embodiment there is performed optical
filtering laser light which has been amplified by the addressed
amplifying. Thereby normally such optical filtering will comprise
narrow pass-band filtering, on one hand to pass the desired
spectral band of laser light and on the other hand to reduce light
components which are located spectrally aside the desired laser
light spectral band. Clearly such filtering may be performed by
transmissive pass-band or by reflective pass-band type filters.
[0011] In a further embodiment the just addressed filtering is
performed with a filter characteristic which is controllably
shiftable with respect to spectral location. This significantly
improves the possibility to cope with effects in the laser system
which provide for undesired variations of the desired
characteristic as e.g. of output laser light intensity,
signal-to-noise ratio or wall-plug efficiency. Due to the fact that
the addressed filter characteristic is controllably shiftable with
respect to its spectral location, in fact additional gain
modulation ability is introduced. This ability as being based on
spectral shift is especially suited to cope with any spectral shift
of the spectral band at which laser light downstream the addressed
filtering is generated. As an example if, due to temperature
influences, the spectral band at which laser light is generated is
shifted and a narrow pass-band filtering is provided downstream the
addressed amplifying, as for noise reduction, and if the filtering
characteristic of such filtering is kept spectrally at a constant
position, the addressed spectral shift of the laser light spectral
band will lead to an overall change of amplification due to mutual
shift of such spectral band relative to the stationar filter
characteristic. If, as proposed in one embodiment, the spectral
location of the addressed filter characteristic is controllably
shifted and the control of such shift is performed to match the
shift with the shift of the addressed spectral band, then the
spectral shift of the laser light spectral band e.g. caused by
temperature variation at the laser light source will not cause an
undesired change of overall amplification.
[0012] With an eye on temperature caused variation of the desired
characteristic to be achieved, in one further embodiment the
addressed shift of spectral location of the filter characteristic
is performed dependent from a temperature.
[0013] As one of the main sources for temperature caused variation
of a desired characteristic is the temperature variation at the
laser source generating the laser light, in a further embodiment
the addressed temperature is selected to be dependent on the
temperature of the laser source.
[0014] By the fact that generating laser light comprises generating
laser light by means of a laser diode, on one hand the requirement
of constructional compactness is further dealt with and on the
other hand a lasering element is introduced which has a spectral
shift of the spectral band of emitted laser light, which depends on
temperature. Therefore, combining the use of a laser diode with
controlled spectral shift of the filter characteristic as was
addressed above is to be closely considered.
[0015] A further embodiment of the method according to the present
invention comprises stabilizing the laser light by a stabilizing
filter and filtering the laser light after amplifying. Thereby
there is applied on one hand a stabilizing filter characteristic
and on the other hand a downstream filter characteristic. Both
filter characteristics are controllably shiftable with respect to
spectral position. The addressed shifts of spectral position of the
stabilizing filter characteristic and of the downstream filter
characteristic are matched.
[0016] Stabilizing filtering which is done by narrow pass-band
reflective filtering (see definition of stabilizing filter) governs
the spectral band of laser light applied to the amplifying and to
the downstream filtering. Whenever the filter characteristic of
stabilizing filtering is spectrally shifted, this causes the
spectral band of laser light to the amplifier to be shifted.
Filtering downstream the addressed amplifying which is again
normally done by narrow pass-band filtering, reduces, as was
addressed, noise thereby improving signal-to-noise ratio. The
addressed shift of spectral position of the spectral band of laser
light caused before amplifying, will affect the overall
amplification due to the downstream filtering if latter is not
spectrally shifted as well. Therefore both, namely stabilizing and
downstream filtering are matched with respect to spectral location
of their filter characteristics. Again such shifting may be
controlled in dependency of temperature.
[0017] In a further embodiment all the addressed filtering, be it
for stabilizing purposes or for removing spectral components, is
performed by means of optical fibre filter. Thereby with an eye on
constructional flexibility and compactness an additional
improvement is reached.
[0018] It has to be noticed that optical filters and especially
optical fibre filters which are controllably shiftable with respect
to spectral location of their filter characteristic may be
conceived by providing the filter characteristics of such filters
governed by at least one geometric entity of a respective filter
element, be it spatial location of a material interface, be it
thickness of dielectric layers, be it width of gratings etc.
Shifting of the addressed spectral location is thereby effected in
one embodiment by mechanically acting upon such entity which is
decisive for the spectral location of the filter characteristic. In
the case of making the addressed spectral shift controlled in
dependency of a temperature the temperature is sensed remote from
such filter element and by temperature to mechanical conversion a
respective mechanical signal is applied to the addressed filter
element. Alternatively the filter element is exploited itself as a
temperature sensing element in that temperature caused variations
of at least one geometric entity and/or of at least one optical
parameter of a material which is or are decisive for the addressed
spectral location, are exploited so as to vary as a function of
temperature as desired.
[0019] In a further embodiment the laser light as generated is
generated as pulsed laser light.
[0020] Still in a further embodiment of the method according to the
present invention, laser light which is dependent on the generated
laser light is amplified and emitted and laser light which is
dependent on the emitted laser light is received at a common laser
input/output port. Providing a common input/output port for
transmitting as well as for receiving laser light additionally
contributes to constructional compactness of an overall laser
system performing the method according to the present
invention.
[0021] By guiding the generated laser light up to a laser output
port substantially exclusively in optical fibres utmost flexibility
is reached with respect to placing different components of an
overall laser system and opens the possibility to construe such
system in a highly compact manner.
[0022] In one embodiment laser light dependent from laser light as
generated is guided by an optical fibre to a transmitter optic.
Thereby the divergence of the laser beam output from the
transmitter optic is determined by appropriately conceiving the end
of the fibre adjacent to the transmitter optic. Different
approaches to do so are addressed in the detailed description part.
By doing so a significant saving of lenses is achieved which leads
to further advantages with respect to compactness, robustness and
price of a respective laser system.
[0023] In a further embodiment the transmitter optic is also a
receiver optic for laser light and, still in a further embodiment,
the addressed optical fibre is an active optical fibre.
[0024] Under a further aspect of the present invention there is
proposed a method of laser range finding or of laser target
designating which comprises generating laser light according to the
method for producing such laser light as has been addressed above,
whereby such laser light is generated in a pulsed manner. Laser
light dependent on the laser light as generated, thus pulsed too,
is directed towards a target. In a further embodiment of the just
addressed method especially for laser range finding, it further
comprises evaluating multiple laser light pulses as received.
[0025] Still under a further aspect, the present invention proposes
a laser system with a laser light source, the output thereof being
operationally coupled to an input of an active fibre optical
amplifier. The active fibre optical amplifier has a gain modulation
control input.
[0026] Further embodiments of such laser system are defined by the
claims 22 to 47. The respective topics reached by such embodiments
become perfectly clear to the skilled artisan reading on one hand
the respective comments above with respect to the method according
to the present invention as well as the following description
wherein the invention under all its aspects is exemplified with the
help of the figures.
[0027] Attention is drawn on the fact that the content of the
European application no. 05 000 669.1 dated Jan. 14, 2005 as well
as the content of the European application no. 04 029 867.1 dated
Dec. 16, 2004 upon which the present application resides with
respect to priority, is considered as a part integrated by
reference to the present disclosure.
[0028] The inventions under all their aspects and combinations
shall now be exemplified by means of figures which show:
[0029] FIG. 1 a signal-flow/functional-block diagram of an
all-fibre laser system as realized today for portable range finder-
or target designator-applications;
[0030] FIG. 2 schematically and simplified the occurrence and
result of relative laser wavelength shift relative to a downstream
optical filter characteristic;
[0031] FIG. 3 in a schematic and simplified representation the
principle of controlling spectral shift of a filter characteristic
matched to laser wavelength shift;
[0032] FIG. 4 in a simplified schematic representation, controlled
spectral shifting of the stabilized laser wavelength and of the
spectral position of a downstream filter characteristic;
[0033] FIG. 5 simplified and schematically, "active" shifting of a
filter characteristic;
[0034] FIG. 6 in a representation in analogy to that of FIG. 5
passive spectral shifting of a filter characteristic;
[0035] FIG. 7 by means of a simplified signal-flow/functional-block
diagram a laser system with matched laser wavelength and filter
characteristic both shifting as a function of temperature;
[0036] FIG. 8 the matching technique according to FIG. 7 applied to
a laser system according to FIG. 1;
[0037] FIG. 9 a controllably spectrally shiftable pass-band optical
filter in a simplified and schematic representation as applicable
in the embodiment of FIG. 8;
[0038] FIG. 10 a simplified signal-flow/functional-block
representation of a laser system with a transmission filter;
[0039] FIG. 11 by means of a part of the laser system as of FIG. 1
a possible form of realizing the principle as of FIG. 10 at the
laser system as of FIG. 1;
[0040] FIG. 12 by means of a simplified
signal-flow/functional-block diagram a laser system with gain
modulated optical amplifier;
[0041] FIG. 13 purely qualitatively, pulsed laser light (a),
modulated gain (b) of an amplifier for the addressed laser light
and laser light resulting from gain modulated gain (c);
[0042] FIG. 14 pulsed laser light (a) amplified by
pulse-width-modulated gain (b) of an optical amplifier and the
result laser light (c);
[0043] FIG. 15 a part of the laser system as of FIG. 1, whereat
pulse-width-modulation as of FIG. 14 is applied;
[0044] FIG. 16 an all-fibre coupling device in a simplified and
schematic representation for bi-directional laser
emission/reception and as integratable in the system of FIG. 1.
DESCRIPTION OF THE INVENTION
[0045] The present invention will first be described by means of a
today's realized embodiment. This under the title of "1. Today's
realized embodiment".
[0046] As in this embodiment, various features are considered per
se inventive and may be realized in different variants, may further
be combined with other laser systems different from the today's
realized embodiment, subsequent to the description of today's
realized embodiment, those specific features possibly with their
variants, their applicability to laser systems different from the
today's realized will be addressed under separate titles namely
under "2. Temperature shift matching", "3. Modulatable Amplifier",
"4. Bi-directional coupler".
1. Today's Realized Embodiment
[0047] The today's embodiment as shown in FIG. 1 is a laser range
finder for cooperative or non-cooperative targets or applied as a
laser target designator. The laser system as shown is of a size,
construction and power consumption which allows integration into a
handheld device and is fully autonome. It may also be applied for
other fields of applications where similar requirements are valid
with respect to size or compactness, power consumption and
robustness.
[0048] A master laser unit 1 comprises a single mode DFB
(distributed feedback) laser diode 3 emitting light pulses of a
wavelength within a predetermined bandwidth. The spectral
temperature drift of the wavelength of emitted laser light of such
DFB diode is typically of the order of 0.1 nm/K and below. Such a
DFB laser diode is e.g. a diode of Series FOL 15DCWD as available
from Fitel, Furukawa Inc. The light emitted from the DFB diode 3 is
coupled from an output A.sub.1 of the master laser unit 1, possibly
via an optical fibre 5, to the input E.sub.7 of a first amplifier
stage 7. The length of the optical fibre 5 is primarily selected
according to the mutual positioning of the unit 1 and unit 7 and is
omitted for optimum packaging density and for minimum optical loss
from output A.sub.1 to input E.sub.7.
[0049] The first amplifier stage 7 comprises, as an actively
amplifying element, an active fibre 9 which is optically pumped by
light input at pumping input PE.sub.7. Thus the output laser light
of the master laser unit 1 is coupled into and amplified by the
active fibre 9.
[0050] The active fibre is an Er/Yb co-doped fibre having a gain
spectral band between 915 nm and 1500 nm. More generically the
active fibre is doped with metallic ions as e.g. ions of Erbium
and/or of Ytterbium and/or of Neodymium and/or of Praesodymium
and/or of Chromium. The spectral band of light output at
A.sub.1--is within the gain band of amplifier stage 7.
[0051] The pumping light energy input to input PE.sub.7 is
generated at an output A.sub.11 of a pumping unit 11 comprising a
pumping diode 13. Diode 13 is a Fabry-Perot Pump-Laser diode having
a typical temperature dependency of the emission wavelength of 0.3
nm/K and having its 20.degree. C. centre wavelength at about 945
nm. Such a diode is e.g. a diode QOFP-975-3 from QPhotonics,
LLC.
[0052] Thus by selecting the centre wavelength of the pumping diode
13, at about a centre temperature of a temperature range expected
at the pumping diode 13, within the gain spectrum band of the first
and, as will be described later, of a second and possibly a third
amplifier, and the expected temperature shift of that centre
wavelength covered by the gain absorption spectral bands of the
amplifier stages, no temperature stabilization of the pump laser
diode 13 is necessary. Thereby a first substantial saving of
constructional space and of electric power is already achieved.
[0053] Depending on intended constructional positioning of pumping
unit 11 and first amplifier stage 9 an optical fibre 15 is
interconnected between output A.sub.11 and input PE.sub.7.
[0054] Due to the high gain G of the first fibre amplifier stage 7
there is present at its output A.sub.7 optical noise especially due
to amplified spontaneous emission ASE, that is emitted in a broad
spectral band and which increases with the gain value of the
amplifier stage 7. Amplified spontaneous emission ASE results in
broadband light emission out of the first high gain amplifier stage
7 independent from and superimposed on the amplified laser light
wavelength .lamda..sub.L. Because the energy of the ASE has to be
taken into account for qualification into certain laser safety
classes, and, in addition, adds to the noise level of the output
light at .lamda..sub.L and finally at and from an illuminated
target, a fibre-optical ASE filter unit 29 with input E.sub.29 and
output A.sub.29 is coupled, possibly via an optical fibre 31, to
the output A.sub.7 of the first amplifier stage 7. The ASE filter
unit 29 is a fibre narrow band-pass filter. The central pass
wavelength .lamda..sub.F of ASE filter unit 29 accords with the
wavelength .lamda..sub.L of laser light generated by the master
laser 1. To prevent the narrow pass-band of the ASE filter unit 29
and thus .lamda..sub.F and the wavelength .lamda..sub.L of laser
light to become offset due to temperature variations at the laser
source 51 and/or the ASE filter unit 29, a temperature shift
matching is established as will be discussed also under a more
generic aspect in "2. Temperature shift matching".
[0055] By such shift matching it is achieved that .lamda..sub.F
shifts spectrally substantially equally as does .lamda..sub.L.
[0056] Thereby, no cooling or temperature control is to be provided
at the laser source 51 which leads to a second substantial saving
of constructional space and power consumption.
[0057] In FIG. 1 the ASE filter unit 29 although represented rather
to operate in transmissive band-pass mode may also be conceived to
operate in reflective band-pass mode as schematically shown by dash
line at the filter output A.sub.29r.
[0058] The output A.sub.29 (or A.sub.29r) of fibre ASE filter unit
29 is coupled, possibly via an optical fibre 33, to an input
E.sub.25 of a second fibre-optical amplifier stage 25, which is
conceived at least similar to the first fibre amplifier stage 7 and
which has an output A.sub.25 and is pumped at an input PE.sub.25.
The output A.sub.25 is coupled via an optical fibre 35 to the input
E.sub.37 of a fibre based circulator 37, as e.g. available from JDS
Uniphase as polarization-intensive fiber optic circulator.
[0059] The circulator 37 has an input/output EA.sub.37. According
to the arrow direction shown, light input at E.sub.37 is output at
EA.sub.37 and isolated from an output A.sub.37. Light input at
EA.sub.37 is isolated from E.sub.37 and output at A.sub.37. The
EA.sub.37 is coupled via an optical fibre 39 to the transceiver
optics 41. Output A.sub.37 is coupled to a detector unit 43 via
optical fibre 45. In the detector unit 43 optical to electrical
conversion is performed and the respective electric signals are fed
to an evaluation unit 47 which generates the desired result
information as e.g. target distance, target speed, target
trajectory etc.
[0060] In spite of the fact, that fibre 39 as shown may be realized
as a third fibre amplifier stage pumped at PE.sub.39, in the
today's realized embodiment it is a "passive" optical fibre.
[0061] By the fibre based circulator 37 and the optical fibres 35,
39 and 45 there is realized a fibre output/input coupler unit 49
comprising the circulator device 37 for polarised or unpolarized
laser light.
[0062] Thereby fibre 45 and 39 are of few-mode type. Fibre 35 is
optimized with respect to the laser source up to A.sub.25 e.g. with
respect to laser light intensity.
[0063] As fibre 39 is selected short i.e. up to at most 10 cm and
is not bended, coupling from the fundamental to higher order modes
in that fibre is neglectable. Because manufacturers of commercially
available circulating devices as of 37 do impose fibre parameters,
fusion splicing of the fibres 35, 39 and 45 to the fibres of the
device 37 is performed to minimize losses. For such fusion splicing
we refer to Electron. Let. Vol. 22 No. 6; pp. 318, 1986; "Low-loss
joints between dissimilar fibres by tapering fusion splices".
[0064] The connector at the end of fibre 39 towards the transceiver
optics 41 adapts the mode field diameter MFD to the transceiver
optics 41 acting as emitter and receiver optics and determines the
divergence of the emitted light beam. The coupler unit 49 with
transceiver optics 41 is considered per se inventive and is more
generically addressed in "3. Bi-directional-coupler."
[0065] If there is provided, separately, a transmitter optic
41.sub.T as shown in dash line and a receiver optic 41.sub.R also
shown in dash line, obviously the circulator 37 is omitted. Then
the end of that fibre, as of active fibre from amplifier stage 25
adapts the MFD to the optic 49.sub.T and thereby determines the
divergence of the emitted laser beam. By determining this
divergence by appropriate conceiving the addressed fibre end,
significant structural savings at the respective optics 41.sub.I,
41.sub.T as with respect to lenses are achieved.
[0066] If the unit with fibre 39 is to be conceived as an amplifier
stage, instead of an active fibre a doped body of glass as e.g. a
rod of doped glass may be provided.
[0067] In spite of the fact that it might be possible to pump all
the amplifier stages 7, 25 and possibly 39 with a single pump diode
13, it has to be understood, that the pumping unit 11 which is
shown in FIG. 1 to pump the first 7, second 25 and possibly further
fibre amplifier stages comprises the number of decentralized
pumping diodes necessary to provide the pumping power as requested.
Thus the "one unit" representation as in FIG. 1 has been selected
merely for simplifying reasons.
[0068] The laser source 51 incorporating master laser unit 1 and at
least the first fibre amplifier stage 7 is a fibre
Master-Oscillator-Power-Amplifier laser source, a fibre MOPA laser
source.
DEFINITION
[0069] We understand under "optical fibre", be it "passive" or
active as for amplifying purposes, coaxial- as well as
strip-waveguides. As it becomes more and more possible to
manufacture low-loss waveguides by strip coating plastic material
substrates allowing high waveguide package density and flexible
mount, we believe that in the rather near future it will become
possible to construe the optical fibres also for the present system
by this strip-technique.
[0070] In the embodiment of FIG. 1, a double stage or possibly
triple stage fibre amplifier system is used. Today such systems are
limited to single pulse energies of approx. 100 .mu.J, which is not
enough for single pulse laser ranging on non-cooperative targets at
distances of several kilometres. Therefore a multi-pulse
integrating evaluation method is today used.
[0071] Multi-pulse direct range finding or target designating
comprises--as known in the art--detection of the time-variant light
signal reflected from the target 27 and according to FIG. 1
collimated by the transceiver optics 41 or 41.sub.R.
[0072] The signal is converted into an electronic signal, digitised
and stored e.g. in evaluation unit 47. By integrating in the
evaluation unit the electric digital signals representing reflected
light of multiple pulses the signal-to-noise-ratio is
increased.
[0073] Various known methods of digital signal processing can be
applied to identify the time-of-flight of the laser multi-pulses
emitted from the laser system, reflected form the target 27,
detected and evaluated by the receiver detector and evaluation
units 43 and 47 which methods are not described in the frame of the
present inventions under all its aspects.
[0074] As may be seen schematically in FIG. 1 the laser diode 3 of
master laser unit 1 is controlled by a pulse control unit 53. The
pumping diode or diodes 13 of pumping unit 11 are operated in
pulsed mode too, whereby under one aspect considered inventive per
se, and addressed under "3. Modulatable Amplifier" pulsing of the
pumping diode or diodes 13 is synchronised with pulsing of the
laser diode 3. Thus there is established a predetermined or
adjustable phasing of pulsating control of the pumping diodes 13
with respect to pulsing control of the laser diode 3. Nevertheless
such phasing needs further not be equal for respective pumping
diode or diodes pumping different fibre amplifier stages and needs
not be constant in time. The synchronisation is phase locked by
respective negative feedback phase lock control loops (not shown in
FIG. 1). Pulsating power applied from the pumping diodes 13 to
their respective fibre amplifier stages 7, 25, possibly 39 may be
said to be a pulse modulation of the gain G of these stages.
Parameters of such gain modulation as especially gain value, duty
cycle, on/or gain ratio may be adjusted or negative feedback
controlled to optimize stability and signal-to-noise ratio of the
overall system.
[0075] As addressed above the ASE fibre filter unit 29 is conceived
so that its pass-band with .lamda..sub.F has substantially the same
shift as a function of temperature and in a predetermined
temperature range as the wavelength .lamda..sub.L of the laser
light emitted from master laser unit 1. This is achieved by
"passive" matching fibre ASE filter unit 29 realized as exemplified
in FIG. 9 and explained under "2. Temperature shift matching". The
master laser unit 1, the fibre ASE filter unit 29 as well as
possibly the fibre amplifier stages 7, 25 and possibly 39 are
thermally tightly coupled, so that they experience substantially
the same temperature variations over time. This simplifies the
addressed matching.
[0076] In context with FIG. 1 there has been described a fibre MOPA
Laser System in context with a non-coherent direct multi-pulse
detection method for laser-range finding on cooperative or
non-cooperative targets or for target designator purposes by
portable or even handheld instruments.
[0077] Instruments including the system as has been described with
the help of FIG. 1 are compact, show maximum detecting ranges
dependent from installed laser power from 1 km far above 10 km
distance on non-cooperative end even small sized targets, exhibit
low power consumption, provide an emitted laser beam of extremely
low divergence--due to fibre-end MFD adaptation--even with short
focal length collimators and are easy to integrate into optical
systems. Due to the all fibre design, this laser system is rugged
or robust without the need of stable construction elements to fix
discrete optical components that could misalign during vibration,
temperature cycling or temperature shocks. An in-fibre output beam
has several advantages for place-independent application. The
flexibility of packaging of the components of the fibre MOPA laser
system within the housing leads to reduced form factors when
integrated into optical systems, like portable observation
instruments and surveying instruments, handheld distance meters or
ship-, sub-marine-, space craft-, aircraft-land vehicles-based
systems as tanks, where available space is limited.
2. Temperature Shift Matching
[0078] With the help of FIG. 1 matching of temperature shift of the
spectral location of the characteristic of filter unit 29 with
temperature shift of laser wavelength .lamda..sub.L was addressed.
More generically, a laser source with a downstream optical filter
especially having a narrow pass-band characteristic removing
unwanted spectral components from the light emitted from the laser
source, shall now be considered.
[0079] Without providing in the laser source as of 51 of FIG. 1 a
temperature stabilization at least for the active laser light
generating devices e.g. by high capacity cooling or by negative
feedback temperature control, dependent also from the environmental
temperature conditions to which the laser source is exposed in
operation, the varying temperature leads to a shift of the laser
light wavelength .lamda..sub.L. The signal-to-noise ratio (S/N)
downstream a narrow band-pass filter unit, as of 29 in FIG. 1,
increases with diminishing width of the pass-band of the filter
unit at stationar, timeinvariant conditions. On the other hand the
smaller than the pass-band width is selected, the more shifting of
the laser light wavelength .lamda..sub.L will lead to reduced S/N.
Especially for laser systems whereat compactness, low-power
consumption and high S/N are predominant requirements, the
necessity of temperature stabilizing the laser source establishes
serious problems. This is especially true for substantially all
fibre laser sources, especially MOPA laser sources as of 51 of FIG.
1 with downstream filter unit 29 whereat the filter unit 29 is
especially provided to reduce ASE noise.
[0080] Whenever the temperature shift of the laser light wavelength
.lamda..sub.L per se is not of significant harm but the resulting
decrease of S/N is, the principal approach according to one aspect
of the present invention is not to stabilize the wavelength of the
laser light by stabilizing the temperature but to match the
temperature dependency of the spectral location of the filter
characteristic of the downstream filter with the temperature
dependency of the laser light wavelength.
[0081] Thereby in a laser system whereat downstream of a laser
source there is provided an optical filter, temperature
stabilization of the laser wavelength .lamda..sub.L is superfluous
and thus omitted.
[0082] By means of a functional-block/signal-flow diagram according
to FIG. 2 the generic solution according to the one aspect of the
present invention shall be described.
[0083] The laser source 51.sub.g emits laser light at a wavelength
.lamda..sub.LO given a temperature .theta..sub.O of the laser
source, with an eye on FIG. 1 especially of the laser diode 3. As
qualitatively shown within the block representing laser source
51.sub.g the wavelength .lamda..sub.L shifts as a function of
temperature .theta..sub.51 according to a wavelength/temperature
characteristic (a).
[0084] The laser light emitted at the output A.sub.7g, as of output
A.sub.7 of FIG. 1, is operationally connected to the input
E.sub.29g of filter unit 29.sub.g which has at least one
characteristic wavelength .lamda..sub.F of the filter
characteristic. This characteristic may, in the most general case,
be a low-pass and/or a high-pass or a band-pass characteristic. The
filter unit 29.sub.g may act in transmission or reflection with
respect to input and output light at output A.sub.29g.
[0085] Generically, the addressed characteristic wavelength
.lamda..sub.F of filter unit 29.sub.g characterizes that part of
the filter characteristic which is exploited to remove undesired
spectral bands from the output light. The filter characteristic may
define for more than one characteristic wavelength .lamda..sub.F.
The filter characteristic defined by the one or more than one
characteristic wavelengths .lamda..sub.F may shift as a function of
filter temperature .theta..sub.29 as qualitatively shown in FIG. 2
by characteristic (b).
[0086] According to the addressed aspect of the present invention,
instead of stabilizing .theta..sub.51 e.g. on the working point
temperature .theta..sub.o at the laser source 51.sub.g and either
selecting a filter unit 29.sub.g whereat spectral shift of the
filter characteristic as a function of temperature is neglectable
or stabilizing the temperature .theta..sub.29 at the filter unit
29.sub.g as well, as on e.g. .theta..sub.o as shown in FIG. 2, the
temperature shift of the characteristic filter wavelengths
.lamda..sub.F is tailored to closely match with the temperature
shift of the laser light wavelength .lamda..sub.L at least in a
predetermined temperature range .DELTA..theta.. This is facilitated
by establishing thermally narrow coupling between the laser source
51.sub.g and the filter unit 29.sub.g as represented schematically
by coupling 60.
[0087] Assuming the laser light output at A.sub.7g has a desired
wavelength .lamda..sub.L and has noise energy in the spectral
ranges adjacent to .lamda..sub.L. As .lamda..sub.L shifts with
temperature, at the output A.sub.29g filtered output light is thus
present with a shifted wavelength .lamda..sub.L and with a
substantially unaffected S/N. Thereby, a significant reduction of
temperature dependency of S/N is achieved. Due to the fact that no
temperature stabilization, in the sense of keeping temperature
constant, is necessary as e.g. a negative feedback temperature
control, the overall arrangement is significantly simplified which
leads to improved compactness as well as to reduced power
consumption. Also dependent on the intensity of the laser light
emitted by the laser source 51.sub.g and thereby on thermical
loading of the optical filter unit 29.sub.g different techniques
may be used as known to the skilled artisan to realize an optical
filter unit 29.sub.g first considered without additional measures
for providing the controlled shift of spectral location shift of
its characteristic in dependency of temperature.
[0088] Such filters may be e.g. [0089] interference filters
comprising a layer system of thin dielectric layers [0090] optical
surface and/or volume gratings [0091] Bragg gratings [0092]
spectrally selective mirrors all in transmissive of reflecting
operation mode.
[0093] All or at least practically all optical filters which may be
used for the addressed purpose reside on the geometry of filter
structures e.g. on layer thickness, grating width, which are
decisive for the characteristic wavelengths of such filters as well
as on optical parameters as on index of refraction of materials
involved.
[0094] Such residing on geometry is exploited according to the
present aspect of the invention by generating at the respective
filter a mechanical loading which may--in one case--be realized
directly by loading the respective filter structure thermally and
exploiting material inherent geometric variations as a function of
temperature or--in another case--by applying externally a
mechanical load generated by on appropriate thermal-to-mechanical
conversion, Thereby also taking temperature dependent variation of
optical material parameters into account. In fact in both cases
there is exploited a thermal-to-mechanical conversion be it by
respective thermal behaviour of a material or be it by applying
externally a mechanical load as a function of a temperature. Thus
under a most generic aspect there is exploited a
thermal-to-mechanical conversion.
[0095] Generically and according to FIG. 3 there is provided a
temperature to mechanical converter 62 the mechanical output signal
A.sub.62 being operationally connected to a mechanical input
E.sub.29g of filter unit 29.sub.g which unit acts as a mechanical
to optical converter, in that the filter characteristic with
.lamda..sub.F is spectrally shifted by the mechanical loading and,
resulting therefrom, geometric variation. Thereby the spectral
location of the filter characteristic with .lamda..sub.F of the
filter unit 29.sub.g in dependency of input temperature .theta. is
matched with the temperature dependency of laser wavelength
.lamda..sub.L.
[0096] According to the embodiment of FIG. 3, the combined
temperature to mechanical and mechanical to optical conversion has
to be matched with the temperature dependency of the wavelength
.lamda..sub.L of the laser source 62.
[0097] If the laser source, as of laser source 51 of FIG. 1,
comprises an active laser device, as of the laser diode 3, which
emits light in a broader spectral band as e.g. a Fabry-Perot diode
it is customary to stabilize the laser source output by loading the
lasering device with an optical resonator. Such a resonator may be
optically delimited by an optical filter acting as a narrow-band
reflective filter. The center wavelength of the filter-structure
pass-band substantially defines for the wavelength at which the
lasering device operates and is thus stabilized.
DEFINITION
[0098] We call a filter structure as a part of an optical resonator
which loads an active laser device, and which filter structure
operates as a narrow-pass-band reflective filter, the center
wavelength thereof stabilizing the addressed device to operate in a
narrow wavelength-band, ideally on a laser-wavelength, a
stabilizing filter. In this case one possibility of realizing
substantially equal temperature shifts of the emitted laser light
wavelength .lamda..sub.L and of the filter characteristic with
wavelength .lamda..sub.F of the downstream filter unit is to
establish for substantially equal spectral temperature shifts of
the stabilizing filter and of the downstream filter. This is shown
in FIG. 4 schematically.
[0099] According to FIG. 4 the active lasering device 64, in the
specific embodiment of FIG. 1 laser diode 3, emits in its operation
light in a relatively broad spectral band B.sub.64. A stabilizing
oscillator 65 with stabilizing filter 66 has a resonance wavelength
substantially determined by the central wavelength .lamda..sub.F1
of the pass-band of stabilizing filter 66. The stabilizing filter
66 is conceived as a mechanical to optical converter. A mechanical
load, as a sharing-, compressing-, pulling- or moving-action,
applied thereon, results in a spectral shift of the center
wavelength .lamda..sub.F1. Thus in dependency of a mechanical
signal m applied to the stabilizing filter 66 the wavelength
.lamda..sub.L on which the device 64 is stabilized is varied.
[0100] Especially due to additional optical stages as of amplifier
stages according to amplifier stage 7 of FIG. 1, at the output of
stabilized laser source 51.sub.S the emitted light comprises also
energy at wavelength different from .lamda..sub.L=.lamda..sub.F1
(m) which is considered as noise.
[0101] There it is provided, in analogy to FIG. 3, a filter unit
29.sub.g simultaneously acting as a mechanical to optical
converter. The spectral location of the filter characteristic of
unit 29.sub.g, specified by one or more than one characteristic
wavelengths .lamda..sub.F, is controllably shifted in dependency of
an applied mechanical load signal. In the case of a narrow
pass-band characteristic of filter unit 29.sub.g, the pass-band
central wavelength .lamda..sub.F2 is selected equal to
.lamda..sub.F1 of stabilizing filter 66. The spectral shifts of
.lamda..sub.F1 and of .lamda..sub.F2 respectively in dependency of
input mechanical load signals m is tailored to be as equal as
possible.
[0102] If the stabilizing filter 66 and the filter 29.sub.g are
equal and a temperature to mechanical converter 68 provides to both
filters 66 and 29.sub.g the same mechanical load signal m, then the
temperature shift of .lamda..sub.F2 and of .lamda..sub.F1 will be
substantially equal. As .lamda..sub.F1 governs the laser light
wavelength .lamda..sub.L, the temperature .theta. does not affect
the gain of laser light in spite of the varying wavelength
.lamda..sub.L(.theta.) as would be caused by a shift of
.lamda..sub.L with respect to the characteristic filter wavelength
.lamda..sub.F2.
[0103] It is not necessary that the two filters 66 and 29.sub.g
have the same mechanical to optical conversion characteristic. If
these characteristics are different, and as schematically shown in
FIG. 4 by respective weighting units 70.sub.66 and 70.sub.29g, the
different characteristics are taken into account by applying for
the same temperature .theta. different mechanical loadings to the
filters 66 and 29.sub.g.
[0104] In the embodiment according to FIG. 3 the overall conversion
characteristic of temperature .theta. to spectral shift of the
filter characteristic with .lamda..sub.F is to be matched with the
spectral temperature shift of the laser wavelength .lamda..sub.L.
In the embodiment according to FIG. 4, this is achieved by matching
the downstream filter 29.sub.g with the stabilizing filter 66. In
both embodiments as of FIG. 3 and of FIG. 4 we have discussed
controlled temperature dependent shift of the spectral location of
the filter characteristic of one or more than one optical filters
so as to avoid the wavelength of laser light becoming offset from a
desired spectral filter band.
[0105] As was already addressed, two approaches are to be
considered with respect to mechanical control of optical filter
characteristics. In a first approach that we call "active" the
optical filter is subjected to a mechanical load signal as e.g. to
a force which is generated in dependency of temperature by an
external converter. A second possibility is to exploit mechanical
and/or optical characteristics e.g. index of refraction, which vary
in dependency of temperature at the optical filter itself. Such
material characteristics may be thermal expansion, compression,
bending index of refraction etc. The filter characteristic is then
controlled by the geometric and material layout and the
thermical/mechanical and thermical/optical characteristics of
material which governs the filter characteristic in dependency of
temperature. We call this approach the "passive" approach.
[0106] The "active" and the "passive" approaches for realizing
temperature control of filter units as of unit 29.sub.g and/or
stabilizing filter 66 of FIGS. 3 and 4 and, with an eye on FIG. 1,
of filter unit 29, are schematically shown respectively in the
FIGS. 5 and 6. According to FIG. 5 a filter unit 72 as has been
addressed is realized e.g. by grating 72.sub.a e.g. applied within
the volume of material M.sub.O. An external drive unit comprises a
temperature to electric converter 74 e.g. a temperature probe. The
output of converter 74 acts on an electrical to mechanical
converter unit 76 as e.g. on a Piezo-material device. The
electrical to mechanical converter unit 76 acts as e.g. by pressure
on the filter unit 72 with the grating 72.sub.a. Thereby the
grating 72.sub.a is mechanically deformed which results in a
spectral shift of the transmitted or reflected spectrum with
wavelength .lamda.(m).
[0107] In the "passive" embodiment as schematically shown in FIG. 6
the grating 72.sub.p is realized in the interface between two
different materials M.sub.1 and M.sub.2 or possibly within the
volume of single material. Due to temperature dependent geometric
and optical variation of the one material or of the different
materials, the spectral location of the filter characteristic is
shifted. Thus in the "passive" embodiment as schematically
exemplified in FIG. 6 the material structure of the filter element
per se acts as a temperature to mechanical converter as of 62, 68
of the FIG. 3 or 4 and, additionally, as a mechanical to optical
converter and, with respect to optical material characteristics as
thermical to optical converter.
[0108] In FIG. 7 there is schematically shown by means of a
signal-flow/functional-block diagram one realization form of the
embodiments as have been principally explained with help of FIGS. 2
to 6.
[0109] The output A.sub.80 of a laser source 80 is operationally
connected to input E.sub.82 of circulator 82. The input/output
EA.sub.82 of circulator 82 is fed to input/output EA 84 of
bi-directional optical amplifier unit 84. The output/input
AE.sub.84 of amplifier unit 84 is operationally connected to
input/output EA.sub.86 of a narrow-band reflecting unit 86. The
reflected spectral band of unit 86 is controllably shiftable via
mechanical load input signal mE.sub.86. A temperature to mechanical
converter unit 88 has a mechanical output mA.sub.88 which is
operationally connected to the mechanical input mE.sub.86 of narrow
band reflecting unit 86. As evident to the skilled artisan laser
light at A.sub.80 is led via circulator 82 and amplifier unit 84
onto the narrow band reflecting unit 86 and is there reflected. The
reflected light is fed via amplifier unit 84 and EA.sub.82 of
circulator 82 to the output A.sub.82. Temperature .theta..sub.2 of
laser source 80 is sensed by temperature to mechanical converter
88, resulting in shifting the spectral position of the narrow-band
reflected spectrum of the reflecting unit 86. Thereby the spectral
position of the filter characteristic reflecting unit 86 is matched
to the temperature shift of laser light wavelength
.lamda..sub.L.
[0110] This embodiment described up to now accords with the
embodiment as was described with the help of FIG. 3, thereby
exploiting "active" matching according to FIG. 5. As shown in dash
lines in a further embodiment there may be provided a stabilizing
filter 89 according to stabilizing filter 66 of FIG. 4 so that the
filter characteristic of unit 86 is spectrally shifted matched with
the spectral shift of laser wavelength .lamda..sub.L transmitted
due to the stabilizing filter 89.
[0111] Both embodiments i.e. with or without stabilizing filter 89
may thereby also be realized in "passive" form. This according to
FIG. 6 and as shown in FIG. 7 by temperature .theta..sub.1,
directly affecting unit 86 and its geometric and/or optical
parameters decisive for the spectral location of filter
characteristic at unit 86. The same "passive" technique may be
applied to stabilizing filter 89. In one embodiment the stabilizing
filter 89 is conceived at least similar to the narrow band
reflecting unit 86 as of same type and material so as to facilitate
spectral shift matching. As further schematically shown in FIG. 7
by the mechanical signal m e.g. the tilting angle .phi. of a
mirroring surface may controllable be varied, "passively" or
"actively", thereby varying controllably the spectral location of
the reflected pass-band.
[0112] In certain cases and with applying a stabilizing filter 89,
mixed type realization may be adequate e.g. "active" operation of
stabilizing filter 89 and "passive" operation of filter unit 86 or
vice-versa.
[0113] As we have already addressed, matching the spectral
positions of filter characteristics of filter units downstream the
laser source with the laser wavelength shift, in dependency of
temperature, is especially suited for highly compact, low-power
laser systems. Such a laser system is especially one which is at
least in a substantial part conceived in optical fibre technique.
Thereby and as shown in FIG. 7 e.g. the amplifier unit 84 may be
realized by an "active" optical fibre 84.sub.a whereby in such case
the narrow band reflecting unit 86 is advantageously realized in
optical fibre technology, too.
[0114] Several possibilities for realizing a reflecting unit 84a
exist: [0115] An optical filter unit consisting of thin layers of
dielectric materials and operating as an interference reflecting
device. The layers are applied e.g. by gluing or coating on the end
AE.sub.84a of the "active" optical fibre 84.sub.a or are provided
in a separate optical element which is butt-coupled or coupled via
a separate coupling device to the addressed fibre end. The
dielectric coatings are conceived to result in a spectral shift of
the reflected narrow-band spectrum when mechanically stressed or
when directly thermically loaded. [0116] A further possibility is
to provide surface and/or volume gratings as e.g. spatially
periodic structures at the/or adjacent to the end AE.sub.84a of the
"active" optical fibre 84a. Here too the gratings are conceived
e.g. so as to be geometrically varied by mechanical stress applied
thereto being "actively" or "passively" as was explained. [0117] A
further possibility is to apply fibre Bragg gratings, uniform
apodized or chirped or coated fibre Bragg gratings, fibre Bragg
gratings in different fibre compositions or structures such as e.g.
on polymer fibres, germanosilicate fibres or photonic crystal
fibres. Here too geometric variations and/or variations optical
parameters of material provide for spectral shift of the filter
characteristics.
[0118] Laser systems which are temperature matched as describe and
realized in fibre technique--at least in part--are highly suited
for handheld or at least portable systems, for systems where space,
power consumption and robustness are predominant requirements. Such
systems may e.g. be submarines, ships, spacecrafts, aircrafts,
landvehicles as tanks. A laser system especially suited for such
applications was described in context with FIG. 1.
[0119] FIG. 8 shows a part of the system of FIG. 1 which is
realized according to FIG. 7 in fibre technique. The same reference
numbers are used for elements which have already been described to
facilitate understanding. The output of laser diode 1 of FIG. 1 is
operationally connected to circulator 82 of FIG. 7. The amplifier
stages 7 and 25 of FIG. 1 are realized by the pumped bi-directional
fibre amplifier stage 84a as of FIG. 7 and the ASE filter unit 29
is realized by a narrow band reflecting fibre unit 86 as has been
explained in context with FIG. 7. The output of circulator 82, with
an eye on FIG. 1, may directly operationally be connected to the
input E.sub.37 of circulator 37. Amplifier stage 7, ASE filter 29
and second amplifier stage 25 as of FIG. 1 are realized by the
fibre bi-directional, pumped amplifier stage 84.sub.a and the fibre
narrow band reflecting unit 86. Clearly for temperature matching
all the possibilities which have already been addressed as of
"passive" control, "active" control, additional provision of a
stabilizing filter as of 89 of FIG. 7 may be applied also in the
embodiment of FIG. 8.
[0120] The embodiment of FIG. 8 is a double-pass MOPA laser system
configuration with a narrow band ASE filter which is matched with
the master laser as concerns temperature shift of laser wavelength
and spectral location of the pass-band of the ASE filter.
[0121] The narrow band reflecting unit 86 of FIG. 7 and according
the ASE filter unit 29 of FIG. 8 may e.g. be realized as was
addressed in context with FIG. 7.
[0122] In FIG. 9 there is schematically exemplified one realization
form of unit 86 especially to be linked to an upstream optical
fibre as to the active fibre amplifier 84a of FIG. 8. Unit 86
comprises a low-pass grating filter stage 87 followed by a
high-pass grating filter stage 88, at a reference temperature
.theta..sub.O, both with corner wavelengths at about .lamda..sub.L
of the laser light. A fibre Bragg grating 90 acts as reflecting
element. Mechanical control especially of the corner wavelengths of
the stages 87 and 88 is e.g. performed by "active" compression or,
"passively", by providing the respective grating in a material
which has a desired volume versus temperature shrinking
characteristic. With an eye on FIG. 7 it is evident that the
stabilizing filter 90 may be provided with grating filter stages
similar to the stages 87 and 88 to provide for matched shift of
laser wavelength .lamda..sub.L and filter pass-band.
[0123] The laser system as has been exemplified in the FIGS. 7, 8
and 9 are operating with reflective filter units 86.
[0124] In analogy to FIG. 7, FIG. 10 exemplifies schematically a
laser system whereat the narrow pass-band filter unit operates as a
transmissive unit.
[0125] According to FIG. 10 the output A.sub.92 of laser source 92
is operationally connected to the input E.sub.94 of an optical
amplifier unit 94. The output A.sub.94 is operationally connected
to the input E.sub.96 of a narrow pass-band filter unit 96. The
wavelength .lamda..sub.L of the laser source 92 shifts with
temperature .theta. as shown in block 92. The filter characteristic
with the centre wavelength .lamda..sub.F of the narrow pass-band
filter unit 96 is shifted in dependency of temperature .theta.
substantially equally as .lamda..sub.L. Thereby, again "active" or
"passive" control of temperature dependent spectral shift of the
filter characteristic may be realized.
[0126] Both "passive" and "active" control have become clear to the
skilled artisan from previous explanations so that in FIG. 10 both
possibilities are addressed merely by the mechanical loading signal
m.sub.1 (.theta.).
[0127] The principle of the system of FIG. 10 is e.g. realized in
the system of FIG. 1 as shown in FIG. 11. Thereby the ASE filter
unit 29 is conceived with a fibre grating low-pass stage 87 and a
fibre grating high-pass stage 88 in analogy to FIG. 9. Again,
"passive" or "active" control may be applied so as to spectrally
shift the pass-band centre frequency in dependency of temperature
.theta. matched with the temperature shift of laser wavelength
.lamda..sub.L. Clearly here too, and with an eye on FIG. 7 or FIG.
4 a stabilizing filter may be provided and temperature shift of
that filter matched with temperature shift of ASE filter 29.
[0128] We have described in this chapter according to one aspect of
the present invention a technique by which the impact of laser
light wavelength temperature shift is remedied not by stabilizing
the temperature at the laser source but by matching the addressed
temperature shift and the temperature shift of the spectral
location of downstream filter characteristics. Due to the fact that
the addressed matching technique may make cooling or temperature
control circuits superfluous it is most apt to be applied for laser
systems whereat high compactness, low power consumption and
robustness is a predominant requirement. These requirements are
especially encountered for laser systems which are at least in part
conceived by optical fibre on one hand, to be most flexible in
construction leading to increased compactness and which are, due to
this advantage, most suited for handheld or portable equipment
which also require low power consumption and high robustness. A
high advantage with respect to compactness is thereby achieved by a
substantially all optical fibre laser system as has been disclosed
in context with FIG. 1, specifically but not exclusively suited for
portable laser range finders or target designators. Nevertheless
the addressed matching technique may also be used more generically
and as was described for all kind of laser systems where a relative
shift of laser wavelength and spectral position of a downstream
filter characteristic is a problem and where the wavelength shift
per se is acceptable.
3. Modulated Amplifier
[0129] In context with the laser system as realized today and as
has been described with a help of FIG. 1 we have addressed pulsing
operation of the laser diode 3 and pulsing pumping of the optical
fibre amplifier stages 7, 25 and possibly 39, whereby pumping of
the addressed fibre amplifier stages is synchronized with pulsing
of the laser diode 3.
[0130] We consider more generically the technique of pulsing
operation of a laser source and of pulsing pumping of a downstream
optical amplifier thereby synchronizing such pulsing operations.
These aspects shall further be exemplified in this chapter.
[0131] Varying pulsed amplifier pumping as for synchronizing
purposes may be considered under a broader aspect namely of gain
modulating the optical amplifier on one hand, on the other hand
doing so at least in part synchronized with pulsing the laser
source. Thereby such a technique may be applied per se to a laser
system or in combination with one or more than one of the other
aspects considered inventive.
[0132] According to FIG. 12 a laser source 151 is operated to emit
pulsed laser light which is controlled by a pulse-control unit 153
via a pulse control input E.sub.3P to laser source 151. The pulsed
laser light emitted at the output A.sub.151 is operationally fed to
the input E.sub.107 of an optical amplifier stage 107. The
amplifier stage 107 is gain modulated. Gain modulation is
controlled by a modulation control unit 113 via gain control input
E.sub.107G to amplifier stage 107. At the output of amplifier stage
107 there is emitted gain modulated pulsed laser light as indicated
in FIG. 12 by G(t)i wherein i is the pulsed laser light emitted
from laser source 151. Thereby operation of the gain control unit
113 i.e. variation of the gain G(t) at the amplifier stage 107 is
at least in part synchronized with pulsed operation of laser source
151 as shown in FIG. 12 by the synchronizing unit 114.
[0133] The modulated gain G(t) may be a composite gain signal
consisting of a possibly time varying gain component G.sub.O(t)
which is not synchronized with the pulsed light emitted from laser
source 151 and with a component G.sub.S(t) which is synchronized
with the addressed pulsed operation.
[0134] In FIG. 13, purely as an example, there is shown pulsed
laser light i (a), a qualitative gain-course
G(t)=G.sub.O(t)+G.sub.S(t) as modulated at the amplifier stage 107
and (c) the resulting pulsed light G(t)i.
[0135] As may be seen from FIG. 13 gain modulation comprises an
unsynchronized gain component G.sub.O(t) and, superimposed thereon,
a synchronized component G.sub.S(t). Synchronization is e.g. based
on the rising edge r of the laser pulses i and is set by the
phasing O(t). The synchronizing phase O(t) may thereby be
time-invariant or may be varying in time. As may be seen from FIG.
13 by the controlled synchronized modulation of the gain G of the
optical amplifier stage 107 the time course of laser pulses at the
output of the amplifier stage may be most flexibly varied.
[0136] These are different reasons for time-varying energy of the
laser pulses emitted from laser source 151. In chapter "2.
Temperature shift matching" we have discussed how relative spectral
shifts between the wavelength .lamda..sub.L of the laser light and
a filter characteristic e.g. of a narrow pass-band optical filter,
may significantly affect the energy of output laser light at
.lamda..sub.L and S/N. There we have discussed the approach of
temperature shift matching of the wavelength .lamda..sub.L of laser
light and spectral position of downstream filter-characteristic so
as to cope with the addressed problem. Instead of this approach or
in addition thereto, the output laser energy downstream the
amplifier stage 107 as schematically shown in FIG. 12 may be
watched and a undesired decrease or increase of such energy e.g.
due to the addressed mutual shifts may be compensated. Thereby the
technique considered here namely of gain modulation allows to cope
more generically with undesired output energy variations
irrespective of their upstream origin.
[0137] Further targets which may be aimed at by the addressed gain
modulation technique are maximum S/N, optimized output pulse-energy
versus electrical input power, i.e. optimized wall-plug
efficiency.
[0138] With respect to modulating gain of the optical amplifier
stage different possibilities may be applied in dependency of the
type of such optical amplifier stage.
[0139] Commonly an optical amplifier for laser light is a pumped
amplifier as was already addressed in context with FIG. 1. Thereby
at a pumped optical amplifier the addressed gain modulation may be
controlled by controlling pump light energy and/or pump light
wavelength. A further possibility for gain control is to provide at
the optical amplifier an optical filter characteristic and to
perform gain modulation by spectrally shifting the filter
characteristic as was discussed for various optical filters in
chapter "2. Temperature shift matching" especially in context with
the "active" mode. It is perfectly clear to the skilled artisan
that by providing within the amplifier stage 107 an optical filter
as was described in the addressed chapter and controllably
spectrally shifting its filter characteristic the gain of the
amplifier stage 107 may be controllably modulated. Further for
pumped amplifiers, the optical length of excited "active" material
may be modulated which length directly affects the gain of the
amplifier stage.
[0140] In FIG. 12 there is further shown a sensing arrangement 115
which senses, downstream the gain-modulatable amplifier stage 107
one or more than one parameters of the pulsed laser light. Such
sensing arrangement 115 may e.g. sense actual S/N, pulse energy or
averaged pulse energy. The sensed actual value of interest
represented by an electric signal at output A.sub.115 is compared
at a comparator unit 117 with a desired value of interest or a
respective time course pre-established in storage unit 119. At the
output A.sub.117 of comparator unit 117 a signal-difference .DELTA.
is generated which controls, via a controller-unit 121, modulation
of the gain of amplifier stage 107 at modulation control input
E.sub.113mod and/or controls the gain value G.sub.O(t), i.e. the
non-synchronized part of amplifier gain G(t). Thereby a negative
feedback control for the desired entity at the laser light
downstream amplifier stream 107 is established. Clearly instead of
providing negative feedback control of the addressed parameters in
the laser light downstream the amplifier stage 107 it is also
possible to provide open-loop control by adjusting the synchronized
component of the gain modulation at E.sub.113mod and/or by
adjusting the un-synchronized gain modulation G.sub.O(t).
[0141] As we have already addressed, providing a gain modulatable
optical amplifier stage downstream the laser source allows to
substantially compensate temperature caused variations of laser
output energy and of S/N. Thereby similarly to the effects of the
previously addressed temperature shift matching technique,
significant efforts for temperature stabilization especially of the
laser source are avoided. This improves the overall laser system
with respect to compactness and power consumption. Such
requirements prevail especially for portable or even handheld
equipment whereat such a laser system is integrated.
[0142] We have already addressed such a laser systems in context
with FIG. 1 as well as--more generically--in context with laser
systems at least in part conceived in optical fibre technique which
especially comprise one or more than one pumped optical fibre
amplifier stages. The technique addressed here of gain modulating
an optical amplifier stage downstream the laser source is
especially suited for such highly compact and low power consumption
laser systems with pumped optical fibre amplifier stages.
[0143] This is addressed in FIG. 12 by the dash line representation
of pumped optical fibre amplifier 107.sub.a. Thereby and as was
already mentioned gain modulation of such pulsed optical fibre
amplifier stage 107 may be achieved by means of varying the
intensity of pumping light and/or varying the spectrum of pumping
light and/or shifting spectrally the filter characteristic of an
optical filter within the amplifier stage and/or varying the length
of actively amplifying material instead or additionally to
modulating the addressed gain by pump-pulse-width modulation.
[0144] In FIG. 14 there is shown qualitatively pulse width
modulated pumping of the optical amplifier stage as of 107 or 107a
of FIG. 12. In analogy to the representation in FIG. 13 "i" denotes
the laser light pulses emitted at the output A.sub.151 of FIG. 12.
The amplifier stage 107 or 107a is pumped in that pumping light
pulses are applied to gain control input E.sub.107G. Thereby the
pumping pulses as of (b) in FIG. 14 are synchronized with the laser
light pulses "i" as e.g. with varying time lag O(t) (see FIG. 13)
based on the rising edge r of the laser light pulses "i". Gain
modulation is performed by pulse-width-modulation of the pumping
pulses whereby as shown in (b) of FIG. 14 the duty-cycle defined by
the on-time T.sub.ON to the pulse repetition period .tau. is
controllably varied. The resulting laser light pulses are shown in
(c). As further shown in FIG. 14 gain modulation may additionally
to pulse-width-modulation be controlled by pumping pulse intensity
I.sub.ON and/or I.sub.Off, spectrum of the pumping light
represented in FIG. 14 by the wavelength .lamda..sub.P and/or as
shown in FIG. 12, by geometric variation of the length of absorbing
material 5.
[0145] In FIG. 15 there is shown a part of the laser system as of
FIG. 1. Thereby pumping of the one or more than one of the
amplifier stages 7, 25 and possibly 39 is performed in
pulse-width-modulation technique as it was addressed in context
with FIG. 14. Thereby and synchronized with the laser control
pulses from unit 53, separate pulse-width-modulation units 14a, 14b
. . . control the pulsed pumping of the fibre amplifier stages 7,
25 and possibly 39 via pumping diodes 13a, 13b, etc.
[0146] The pulse-width-modulation at the respective units 14 may
thereby be open-loop adjusted or, with an eye on FIG. 12, negative
feedback controlled from a sensing unit 115. The
pulse-width-modulation control is done by a respective control
signal to the modulation control inputs E.sub.14mod. Thereby, the
pulse-width-modulation for the respective pumping of the amplifier
stages may be set differently as addressed by the separate
modulation units 14a, 14b assigned to the pumping diodes 13a, 13b .
. . . The difference between setting of the pulse-width-modulations
takes into account e.g. different locations of the pulsed amplifier
stages along the laser light path. The difference may be with
respect to synchronization phasing .phi.(t) as of FIG. 13 as well
as with respect to gain control parameters. Instead of pumping
diodes 13a, b . . . other pumping sources as e.g. pumping laser
sources may be used. Further instead of a diode laser source 1
other laser source types may be used as e.g. solid state laser
sources.
[0147] By means of the modulatable gain G of the optical amplifier
as described in this chapter it most generically becomes possible
to counter-act laser light intensity variations which are due e.g.
to temperature influence or to aging of the system. The addressed
technique is most suited to be integrated in the laser system as of
FIG. 1, more generically for laser systems as addressed namely for
portable or even handheld equipment as for handheld laser range
finders and target designators which have already been
addressed.
4. Bi-Directional Coupler
[0148] In context with FIG. 1 we have addressed a coupler unit 49
which his considered under a further aspect of the present
invention as inventive per se.
[0149] Such coupler unit 249 is more generalized shown in FIG. 16.
It comprises an input optical fibre or waveguide 135 to an input
E.sub.137 of a circulator 137. The input fibre 135 is to be
connected to a laser source. The output A.sub.137 of circulator 137
is connected to an output optical fibre 145 to be connected to a
detector unit as to a unit 43 of FIG. 1. The input/output
EA.sub.137 of circulator 137 is connected via fibre 139 to the
objective of a laser device. Laser light from the laser source is
coupled by the circulator 137 as output light O to fibre 139 and to
the objective whereas the laser light R received at the objective
e.g. reflected from a target is coupled by circulator 137 from
fibre 139 via fibre 145 to the detector unit.
[0150] Different possibilities exist for the selection of the
fibres 135, 139 and 145.
[0151] In one embodiment all these fibres are standard single mode
fibres at the wavelength .lamda..sub.L of the laser light from the
laser source. Thereby the overall losses are minimized. The laser
light is only guided in the core of the fibres. Thereby the
aperture of the light emitting and of the light receiving optics of
the objective is selected equal. The optimum aperture width F/# of
the objective may be adapted to the divergence of the fibre 139.
Further the detection surface of the detector unit may be adapted
to the mode filed diameter MFD of fibre 145.
[0152] In a further embodiment wherein all the fibres 135, 139 and
145 are selected as standard single-mode fibres at the laser
wavelength .lamda..sub.L, the emitted light O is only guided in the
core of fibre 135 and 139. The received light R is guided in the
core as well as in the cladding of fibres 139 and 145. Thereby
especially fibres 139 and 145 are selected short so as to minimize
losses in the claddings to a negligible amount. The detection
surface of detector unit downstream fibre 145 is to be adapted to
the cladding size of that fibre. Coupling losses of the received
light R is minimized. The numerical aperture of the emitter is
selected different from the numerical aperture of the receiver at
the objective.
[0153] In a further embodiment fibre 135 is optimized with respect
to the laser source and fibres 139 and 145 are few mode. As the
length of fibre 139 is selected short and this fibre is
substantially un-bended, coupling from the fundamental to higher
order modes can be neglected and optimum beam quality is achieved.
Still in a further embodiment fibre 135 is optimized with respect
to the laser source and fibre 139 is a double clad fibre which has
the same core MFD as fibre 135. Fibre 145 is optimized to collect
the light guided in the cladding and in the core of fibre 139.
[0154] In a further embodiment the fibres 135, 139 and 145 are
multi-mode fibres.
[0155] If the laser source is a source of polarized laser light, in
a further embodiment the fibres are selected as polarization
maintaining fibres. This simplifies separation of emitted -O- and
received -R- light.
[0156] In a further embodiment photonic crystal fibres, single or
double-clad, are used which allows high flexibility with respect to
the choice of the MFD parameters for emitted -O- and received -R-
light.
[0157] Commercially available un-polarized circulator units 137 may
be adapted to the different fibres as mentioned. Often
manufacturers of circulators impose the parameters of fibres to be
applied. Therefore, as was already addressed in context with FIG.
1, fusion splicing of the optimum fibres to the circulator fibres
is to be performed in order to minimize losses.
[0158] The circulator unit 137, in one embodiment is a polarization
independent circulator which separates the received light R from
the transmitted light O and thereby additionally removes background
light by filtering.
[0159] The all-fibre coupler unit 149 or 49 of FIG. 1 has the
advantage that it may be applied with un-polarized laser light as
especially suited for the addressed range finder and target
designator portable applications. No detection limitation due to a
coaxial surface ratio, defined as emitter or receiver surface, to
total objective surface or due to polarization state of the
received light is present.
[0160] The application of MFD adaptation at the fibre -139- end of
the all-fibre device allows realizing optimal beam divergence of
the device with the coupler unit 149 or 49 as of a range finder or
a target designator without providing additional lenses. An
increase of MFD increases reliability at the end of fibre 139.
[0161] The MFD of the fibre 139 directly determines the numerical
aperture at that fibre end and is influenced by the geometry and/or
refractive index of the wave guiding fibre. The numerical aperture
of the fibre end determines the beam side output by the objective
and thus the divergence of the laser beam emitted by the device as
by a range finder or by a target designator device. Therefore the
choice of MFD at the end of fibre 139 influences the performance of
such device. In spite of the fact that optimum emitted beam
divergence may be achieved by placing optical lenses downstream the
end of fibre 139 in one embodiment of the coupler 149 and 49--as
was mentioned--adaptation of the MFD is performed at the end of
fibre 139 opposite to circulator 137 which allows the omission of
additional lenses. Different techniques are known to alter and thus
optimize the MFD of such fibre 139:
[0162] An increase of MFD can be achieved by diffusion of dopants
obtained by heating the fibre in a flame according to J. of Appl.
Phys.; Vol. 60 No. 12 pp. 4293, 1986, K. Shigihara et al. or J.
Lightwave Technol. Vol. 8 No. 8 pp. 1151, 1990, K. Shiraishi et al.
or Electron. Lett. Vol. 24 No. 4 pp. 245, 1988; J. S. Harper et
al.
[0163] Another known possibility is irradiating the fibre with a
CO.sub.2 laser according to Appl. Opt. Vol. 38 No. 33, pp. 6845,
1999; T. E. Dimmick et al.
[0164] Still a further known possibility to increase MFD of single
mode fibres is to reduce the core diameter by tapering the fibre,
Electron. Lett. Vol. 20 No. 15 pp. 621, 1984; Keil, R.
[0165] Further cladding modes have a higher beam diameter than core
modes. Therefore coupling the core mode near the end of fibre 139
into a cladding mode allows significant changes in the numerical
aperture. This effect has been investigated in Opt. Commun. Vol.
183 pp. 377, 2000; Y. Li et al.
[0166] Lensed fibre ends are presented in the publication of
Jarmolik et al. Optik Vol. 110, No. 1, pp. 37 1999, A. Jarmilik et
al. lensed fibre ends.
[0167] Generically an increase of the emitted beam diameter allows
the applications of higher peak power.
[0168] A further technique to increase MFD at the end of fibre 139
is UV-irradiation of a photo-sensitive cladding at a fibre `Spot
size expander using UV-trimming of trilayer photosensitive fibres`;
OECC/I00C 2001, Conference Incorporating ACOFT, Sydney, pp. 408,
2001; R. A. Jarvis et al. or `High-energy and high-peak-power
nanosecond pulse generation with beam quality control in 200 .mu.m
core highly multimode Yb-doped fibre amplifiers`; Opt. Lett. Vol.
30 No. 4 2005; pp. 358; Cheng et al. It has further to be noticed
that core-less fibre end caps may be applied to the end of fibre
139 so as to completely eliminate surface damages, as known from
US-20040036957 (A. Galvanauskas et al.).
[0169] Thus the coupler unit 149 or 49 as of FIG. 16 provides
single channel laser light emission and reception for polarized or
un-polarized laser light. It is ideally suited to be combined with
diode or solid state laser sources making use of optical fibre
coupling technique as especially for an all-fibre laser system as
of an all-fibre MOPA laser system as was described with a help of
FIG. 1. Thereby optical fibre based laser systems guarantee an
increased stability and robustness with respect to environmental
disturbances in comparison to systems with free space parts. Such
laser systems may have a very high compactness and the availability
of the output beam as well as of the reception beam in a fibre tail
allows substantially independent location of the input/output laser
port at a respective device with such laser system. Single channel
emitting/receiving optics further increase compactness allowing for
high system stability. Thereby the all-fibre reception channel to
the detector diode couples only light which is present within the
fibre to such diode whereby stray-light impinging upon such diode
is reduced.
[0170] We have described a today's realized embodiment of an
all-fibre laser system wherein different features are realized in
combination. All these features as especially temperature shift
matching, gain modulation of optical amplifiers and bi-directional
optical coupler unit are considered per se inventive as being
applicable per se or in any combination to laser systems which may
differ from the system as realized today.
* * * * *