U.S. patent application number 09/725129 was filed with the patent office on 2002-05-30 for low cost amplifier using bulk optics.
Invention is credited to Duck, Gary S., Nyman, Bruce, Teitelbaum, Neil.
Application Number | 20020063952 09/725129 |
Document ID | / |
Family ID | 22611320 |
Filed Date | 2002-05-30 |
United States Patent
Application |
20020063952 |
Kind Code |
A1 |
Nyman, Bruce ; et
al. |
May 30, 2002 |
Low cost amplifier using bulk optics
Abstract
The present invention relates to an in-line optical amplifier
that can be coupled to optical fiber, wherein the amplifying medium
has a substantially larger mode field than the optical fiber to
which it is coupled. The present invention has realized a design to
utilize a very high power pump launching a multimoded signal having
approximately 1 W of pump power into a block of erbium doped glass
having a mode field diameter orders of magnitude larger than the
mode field diameter of erbium doped fiber. This invention provides
a relatively inexpensive optical amplifier that is compatible for
use in an optical fiber telecommunications system or for other
uses. Advantageously, this invention provides a device that does
not require unwieldy lengths of erbium doped fiber to form an
amplifier. By using a block of glass having a rare earth therein,
packaging, temperature stabilizing and temperature tuning of the
amplifier also become practicable. Furthermore, a cylindrical block
of glass having planar ends, lends itself to applying coatings or
filters thereto, thereby forming selective filters at ends of the
erbium doped block to allow the pump light in, and the signal light
in at opposite ends, while preventing light at the pump wavelength
to propagate out with the amplified signal.
Inventors: |
Nyman, Bruce; (Freehold,
NJ) ; Duck, Gary S.; (Nepean, CA) ;
Teitelbaum, Neil; (Ottawa, CA) |
Correspondence
Address: |
LACASSE & ASSOCIATES, LLC
1725 DUKE STREET
SUITE 650
ALEXANDRIA
VA
22314
US
|
Family ID: |
22611320 |
Appl. No.: |
09/725129 |
Filed: |
November 29, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60168391 |
Dec 2, 1999 |
|
|
|
Current U.S.
Class: |
359/342 |
Current CPC
Class: |
H01S 3/06754 20130101;
H01S 3/063 20130101; H01S 3/1608 20130101; H01S 3/0602 20130101;
H01S 3/06708 20130101; H01S 3/094 20130101; H01S 3/061
20130101 |
Class at
Publication: |
359/342 |
International
Class: |
H01S 003/00 |
Claims
What is claimed is:
1. An optical amplifier comprising: an optical waveguide for
carrying an optical signal to be amplified, the optical waveguide
having an output end for launching the optical signal; a
substantially collimating lens optically coupled with the output
end of the optical waveguide for receiving the optical signal and
for providing a substantially collimated beam to be amplified, the
substantially collimated beam having a substantially larger mode
field diameter than the optical signal being carried by the optical
waveguide; a block of light transmissive material sized to carry
the substantially collimated beam for amplification, the block of
light transmissive material being comprised of a gain medium doped
with a rare-earth element, the block being disposed to receive the
substantially collimated optical beam; and, a high power pump
disposed to impart optical energy to the block; and, an output
optical waveguide disposed to couple focused light of the optical
signal after it has been amplified within the block of light
transmissive material.
2. An optical amplifier as defined in claim 1, wherein the pump
energy is of a wavelength that is substantially different than the
wavelength of the signal to amplified.
3. An optical amplifier as defined in claim 2, wherein the block is
adapted to guide the substantially collimated beam.
4. An optical amplifier as defined in claim 3, wherein the block is
adapted to guide the optical energy imparted by the high power
pump.
5. An optical amplifier comprising: a first optical waveguide for
providing a signal to be amplified, the waveguide having an average
mode field diameter d.sub.1; a second optical waveguide optically
coupled with the first waveguide for receiving the signal after it
has been amplified, the second waveguide having a mode field
diameter d.sub.2, where d.sub.1 and d.sub.2 are substantially
smaller than d.sub.3; a light transmissive amplifying medium for
guiding a beam having a mode field diameter of at least d.sub.3,
said light transmissive amplifying medium being disposed to receive
light from the first optical waveguide and to provide amplified
light to the second optical waveguide; a pump optically coupled
with the light transmissive amplifying medium for providing pump
energy to the amplifying medium.
6. An optical amplifier as defined in claim 5, wherein the light
transmissive amplifying medium is a block of glass, doped with a
rare earth element.
7. An optical amplifier as defined in claim 6, wherein the pump and
the first and second optical waveguides are disposed at different
ends of the light transmissive amplifying medium.
8. An optical amplifier as defined in claim 7, wherein the pump and
the first and second optical waveguides are disposed at opposite
ends of the light transmissive amplifying medium.
9. An optical amplifier as defined in claim 8, wherein the block of
glass is provided with means for maintaining or varying the
temperature of the block.
10. An optical amplifier as defined in claim 8, further comprising
filters disposed at the opposite ends of the block of glass, the
filters having different output responses.
11. An optical amplifier as defined in claim 10, wherein one of the
filters disposed adjacent the pump has a pass band at a central
wavelength of the pump energy, and wherein the other of the filters
disposed adjacent the first and second waveguides has a pass band
at a wavelength corresponding to a wavelength of the signal to be
amplified.
12. An optical amplifier as defined in claim 11, further comprising
a lens disposed between the two waveguides and the filter having a
passband corresponding to the wavelength of the signal to be
amplified for providing collimated light to the block and for
providing a focused beam to the second waveguide.
13. An optical amplifier as defined in claim 6, wherein the glass
block is sized to carry a beam having a mode field diameter of at
least 100 .mu.m, the block having a filter at an end thereof for
passing the pump beam and for substantially preventing the optical
signal to be amplified from passing therethrough, and having a
filter at another end thereof, for passing the signal to be
amplified and for substantially preventing the pump beam from
passing therethrough.
14. An optical amplifier as defined in claim 13, wherein the light
transmissive medium includes a coating to promote total internal
reflection within the medium.
15. An optical amplifier as defined in claim 13, wherein the light
transmissive medium has optical power.
16. An optical amplifier as defined in claim 13, wherein the glass
block has a graded index forming a lens.
17. An optical amplifier as defined in claim 13, further comprising
an optical element formed on an end of the glass blockselected from
at least one of: a diffraction element, a lens and a filter.
18. An optical amplifier as defined in claim 13 further comprising
optical fibres disposed to provide light to and receive amplified
light from the end of the glass block having the filter that
prevents the pump beam from passing therethrough.
19. An optical amplifier as defined in claim 18, wherein the fibres
are polarization maintaining fibres.
20. An optical amplifier as defined in claim 12 wherein the lens
for substantialily expanding a mode field diameter is a GRIN lens
and wherein the input optical fibre and the output optical fibre
are both housed within a ferrule, the amplifier further comprising
a light transmissive spacer element disposed between the ferrule
and the GRIN lens.
21. An optical amplifier as defined in claim 20, further comprising
a lens disposed between the pump source and the pump medium for
substantially collimating pump light from the pump source
22. A method of amplifying an optical signal comprising the steps
of: launching a beam carrying the optical signal from an optical
fibre; substantially increasing a mode field diameter of the beam
and providing the beam to an amplifying medium; pumping optical
energy having a different wavelength from the optical signal into
the amplifying medium, and receiving an amplified optical signal
from the amplifying medium and, decreasing the mode field diameter
of the amplified signal and coupling the amplified signal to an
output optical fibre.
Description
FIELD OF THE INVENTION
[0001] This invention relates generally to optical amplifiers and
more particularly, to an in-line optical amplifier that can be
coupled to optical fibre, wherein the amplifying medium has a
substantially larger mode field diameter than the optical fibre to
which it is coupled.
BACKGROUND OF THE INVENTION
[0002] There is considerable interest in using rare earth doped
fiber amplifiers to amplify weak optical signals for both local and
trunk optical telecommunications networks. The rare earth doped
optical amplifying fibers exhibit low-noise, have relatively large
bandwidth with low polarization dependence, substantially reduced
crosstalk problems, and low insertion losses at the relevant
operating wavelengths which are used in optical communications.
Furthermore, rare earth doped optical fiber amplifiers can be
coupled end-to-end to a transmission fiber, and coupled, through a
directional coupler, to a laser diode pump. The directional coupler
is designed to have a high coupling ratio at the pump wavelength
and a low coupling ratio at the signal wavelength so that maximum
pump energy is coupled to the amplifier with minimal signal loss.
When the amplifying medium is excited with the pump laser, signal
light traversing the amplifier experiences gain. The pump energy
may be made to propagate either co-directionally or
counter-directionally relative to the signal energy, selected for
higher power efficiency or better noise performance
[0003] To date, erbium fiber amplifiers appear to have the greatest
potential for the high amplification necessary to overcome the
signal losses. Erbium doped fiber amplifiers (EDFAs) operate at
1550 nm which is of particular interest for optical communication
systems because, in this wavelength region, the amplifiers exhibit
low insertion loss, broad gain bandwidth (approximately 30 nm) and
relatively polarization insensitive gain. Such amplifiers, pumped
with light having a wavelength of 980 nm can have a gain as high as
26 dB but require as much as 76 mW of launched pump power. It has
generally been desired to achieve a higher gain together with a
lower value of pump power coupled into a fiber, and such
optimization of EDFAs has been a goal. The pump required to launch
a signal into a single mode fibre is quite costly.
[0004] The present invention has realized a design to utilize a
very high power pump launching a multimoded signal having
approximately 1 W of pump power. Currently, high power optical pump
lasers are commercially available at a relatively low cost. Such
high power pumps are not compatible for use with erbium doped fibre
in the manufacture of EDFAs. However, this invention provides a
relatively inexpensive optical amplifier that is compatible for use
in an optical fibre telecommunications system or for other
uses.
[0005] This invention also provides a device that does not require
unwieldy lengths of erbium doped fibre to form an amplifier. In
contrast, the instant invention uses a block of glass having a mode
field diameter orders of magnitude larger than the mode field
diameter of erbium doped fibre.
[0006] By enlarging the mode field of the signal beam, greater pump
energy can be applied without the significant difficulty and loss
which are present when coupling pump energy into a single mode
fiber amplifier.
[0007] By using a block of glass having a rare earth therein,
packaging, temperature stabilizing and temperature tuning of the
amplifier become practicable.
[0008] Furthermore, a cylindrical block of glass having planar
ends, lends itself to applying coatings or filters thereto, thereby
forming selective filters at ends of the erbium doped block to
allow the pump light in, and the signal light in at opposite ends,
while preventing light at the pump wavelength to propagate out with
the amplified signal.
SUMMARY OF THE INVENTION
[0009] In accordance with the invention there is provided, an
optical amplifier comprising: an optical waveguide for carrying an
optical signal to be amplified, the optical waveguide having an
output end for outcoupling the optical signal;
[0010] a substantially collimating lens optically coupled with the
output end of the optical waveguide for receiving the optical
signal and for providing a substantially collimated beam to be
amplified, the substantially collimated beam having a substantially
larger mode field diameter than the optical signal being carried by
the optical waveguide;
[0011] a block of light transmissive material sized to carry the
substantially collimated beam for amplification, the block of light
transmissive material being comprised of a gain medium doped with a
rare-earth element, the block being disposed to receive the
substantially collimated optical beam; and,
[0012] a high power pump disposed to impart optical energy to the
block; and, an output optical waveguide disposed to couple focused
light of the optical signal after it has been amplified within the
block of light transmissive material.
[0013] In accordance with the invention there is further provided,
an optical amplifier comprising: a first optical waveguide for
providing a signal to be amplified, the waveguide having an average
mode field diameter d.sub.1;
[0014] a second optical waveguide optically coupled with the first
waveguide for receiving the signal after it has been amplified, the
second waveguide having a mode field diameter d.sub.2, where
d.sub.1 and d.sub.2 are substantially smaller than d.sub.3;
[0015] a light transmissive amplifying medium for guiding a beam
having a mode field diameter of at least d.sub.3, said light
transmissive amplifying medium being disposed to receive light from
the first optical waveguide and to provide amplified light to the
second optical waveguide;
[0016] a pump optically coupled with the light transmissive
amplifying medium for providing pump energy to the amplifying
medium.
[0017] In accordance with the invention there is further provided,
an optical amplifier for amplifying an incoming optical signal
comprising a glass block in the form of a light transmissive medium
sized to carry a beam having a mode field diameter of at least 100
.mu.m, the block being doped with a rare earth for amplifying light
passing therethough when the rare earth is excited by a pump beam,
the light transmissive medium having a filter at an end thereof for
passing the pump beam and for substantially preventing the optical
signal to be amplified from passing therethrough, and having a
filter at another end thereof, for passing the signal to be
amplified and for substantially preventing the pump beam from
passing therethrough.
[0018] In accordance with the invention there is provided, an
optical amplifier comprising:
[0019] an input optical fibre for providing a signal to be
amplified;
[0020] an amplifying medium comprising a light transmissive
material having a diameter substantially greater than the diameter
of the input optical fibre, for receiving the signal to be
amplified;
[0021] a lens for substantially expanding a mode field diameter of
a beam of light of the signal to be amplified, optically coupled
between the input waveguide and the amplifying medium;
[0022] a pump source for providing high intensity optical pump
energy to the amplifying medium; and,
[0023] an output optical fibre for receiving an amplified optical
signal from the amplifying medium.
[0024] In accordance with another aspect of the invention a method
of amplifying an optical signal is provided, comprising the steps
of:
[0025] coupling the optical signal from an optical fibre into an
amplifying medium having a diameter a plurality of orders of
magnitude greater than a mode field diameter of the signal
propagating with the optical fibre such that the mode field
diameter of the signal is converted to a substantially larger
collimated beam than the signal propagating with the optical
fibre;
[0026] pumping optical energy having a different wavelength from
the optical signal into the amplifying medium, and receiving the
amplified optical signal from the amplifying medium.
[0027] In accordance with the invention there is further provided,
a method of amplifying an optical signal comprising the steps
of:
[0028] launching a beam carrying the optical signal from an optical
fibre;
[0029] substantially increasing a mode field diameter of the beam
and providing the beam to an amplifying medium;
[0030] pumping optical energy having a different wavelength from
the optical signal into the amplifying medium, and receiving an
amplified optical signal from the amplifying medium and,
[0031] decreasing the mode field diameter of the amplified signal
and coupling the amplified signal to an output optical fibre.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] Exemplary embodiments of the invention will now be described
in conjunction with the drawings in which:
[0033] FIG. 1 is a conventional erbium doped fibre optical
amplifier;
[0034] FIG. 2 is a schematic illustration of an amplifier in
accordance with the present invention;
[0035] FIG. 3 is a schematic illustration of an alternative
embodiment of the present invention;
[0036] FIG. 4 is a schematic illustration of an alternative
embodiment of the present invention in which the amplifying medium
has optical power;
[0037] FIG. 5 is a schematic view of the amplifying medium
illustrating angular separation of input and output signals;
[0038] FIG. 6a is a schematic view of the amplifying medium
illustrating total internal reflection of the pump signal;
[0039] FIG. 6b is a schematic view of the amplifying medium
illustrating a collimated pump signal; and
[0040] FIG. 7 is a schematic illustration of a further embodiment
of the present invention in which the amplifying medium is enlarged
to accommodate a plurality of input and output fibres.
DETAILED DESCRIPTION
[0041] Rare earth doped fibers for amplifying weak signals for both
local and trunk optical telecommunications networks have been of
interest for some time now, because of their low insertion loss,
broad gain bandwidth and low polarization sensitivity. In use, the
doped optical fiber is normally coupled to a pump so that a weak
optical input signal at some wavelength within the rare earth gain
profile experiences a desired amplification. Pump light which can
be coupled into the optical fiber via a directional coupler may
propagate either co-directionally or counter-directionally within
the fiber relative to the signal. The directional coupler can have
a high coupling ratio at the pump wavelength and a low coupling
ratio at the signal wavelength.
[0042] When the fiber is not pumped, the signal experiences loss
due to ground state absorption by the rare earth ions. As the pump
power that is applied to the fiber is increased, the loss due to
ground level absorption decreases (i.e., gain is negative but
increasing) until, at some value of pump power, there is no net
signal absorption (i.e. the gain is zero). This is referred to as
the transparency state. Thereafter, as the pump power in the fiber
is increased, a higher proportion of rare earth ions are in their
excited state and the stimulated emission from the upper lasing
state to the ground state becomes stronger than the absorption from
the ground state to the upper lasing state, resulting in a net
positive gain at various wavelengths. Thus, the optical amplifier,
when pumped so as to populate the upper lasing level, produces a
net positive gain above the pump threshold level and the fiber acts
as an amplifier.
[0043] Pumping is effected by a separate laser or lamp which emits
photons of an appropriate energy which is higher than that which
corresponds to the signal wavelength. The electrons are excited
from the ground state to one or more pump bands, which are above
the upper lasing level. It is important that the spontaneous
lifetime of the upper lasing level exceed that of the pump bands by
a significant margin to allow heavy population of the upper level.
When a photon at the laser wavelength interacts with an excited ion
in the upper lasing state, stimulated emission can occur. The
photon can come from either previous spontaneous emission,
stimulated emission, or an input signal.
[0044] This invention utilizes a cylindrical block of erbium doped
glass as an amplifying medium. In contrast to erbium doped optical
fibre amplifiers, the cylindrical block has a cross section orders
of magnitude greater than the cross section of optical fibre.
Furthermore a very high power pump laser is utilized to provide a
required amount of energy to the erbium-doped block. Essentially,
the mode field diameter of a beam propagating within an optical
fibre is expanded to propagate through and traverse the cylindrical
block.
[0045] Turning now to FIG. 1, a prior art erbium doped optical
fibre amplifier 10 is shown having a pump 12 coupled with an
incoming optical signal 20 to be amplified. A laser diode 12
provides an output signal having a wavelength of 980 nm that is
coupled with an incoming signal 20 to be amplified having a
wavelength of 1550 nm. A laser diode pump at 1480 nm can
alternatively be used. A coupler 14 couples the two signals
together to be output on a suitable length of erbium doped optical
fibre 16.
[0046] Turning now to FIG. 2, a block of glass 22 in the form of a
rod having a diameter of approximately 350 .mu.m is shown; the
block 22 is doped with erbium. A suitable glass is commercially
available under the name MM-2, an erbium doped phosphate laser
glass produced by Kigre, Inc. This material includes high dopant
percentages and provides high gain. A typical length of block 22
required for a net gain of 20 dB is in the range of 1 cm.
Notwithstanding, the overall performance depends upon many
variables; the same physical principles used in current optical
fiber based amplifiers apply. The block 22 is disposed between two
substantially quarter pitch GRIN lenses 24a and 24b which are
disposed between two optical fiber sleeves 23a and 23b housing
input optical fibre 20a and output optical fibre 20b. In operation,
light to be amplified of a wavelength of approximately 1550 nm is
launched into optical fiber 20a and is output on optical fibre 20b.
After the light enters the GRIN lens 24a it is collimated and the
mode field of the beam is expanded to a diameter that can be
supported by the erbium doped glass block 22. Hence the beam
diameter is expanded to occupy most of the block 22, as it
traverses the block. Simultaneously a laser 25 optically coupled
with the block 22 having a wavelength of 980 nm outputs and pumps
the block medium 22 with a high power 1 watt signal that is
distributed across and into the block by the lens 27 disposed
between the block 22 and the laser 25. As the signal passes through
it gains energy from the excited medium 22 and becomes amplified.
The mode gain of the medium is calculated by the following
equations:
G=exp(g.multidot.L)
g=.sigma.N
[0047] where g is the gain coefficient, L is the length of the gain
medium, and .sigma. is the emission cross section, and N is the
Er.sup.3+ ion density. Conditions of strong inversion with high
pump power are assumed for the calculation. The resulting gain
coefficient can reach 22 dB/cm, given an Er.sup.3+ concentration of
10.sup.21 cm.sup.3, and emission cross section of
5.times.10.sup.-<cm.sup.2.
[0048] Referring now to FIG. 3, a preferred embodiment of the
invention is shown, wherein both input and output optical fibres
are coupled into the same end of the device. This type of
arrangement is preferred and offers advantages when providing
hermetic devices. A block 22 similar to the one shown in FIG. 2 is
provided having a first optical filter 34 and a second optical
filter 32 at opposing ends. The filter 34 is designed to pass light
having a wavelength of 1550 nm while reflecting light having a
wavelength of 980 nm generated by the pump laser. Conversely, the
filter 32 is designed to pass light having a wavelength of 980 nm
and reflect light incident thereon having a wavelength of 1550 nm.
The pump laser 38 is optically coupled to the erbium doped block 22
via a lens 24b. Both input and output optical fibres 30a and 30b
respectively are disposed with an optical fibre ferrule 23 and are
coupled optically coupled to the block of rare earth doped medium
22 via a light transmissive spacer element 36 and a GRIN lens 24a.
For optimum coupling, it is preferred that the optical path length
of the spacer is equal to the optical path length of the block 22,
such that the beam traversing both elements traverses equal path
lengths.
[0049] In operation, a signal light having wavelength 1550 nm is
launched from input optical fibre 30a and is collimated to a
substantially larger beam with a substantially larger mode field
diameter as it traverses the GRIN lens 24a. The light then passes
through the filter 34 and enters and substantially fills the erbium
doped block of glass 22. Simultaneously, the high power laser 38
provides a pump signal having a wavelength of 980 nm to the block
22 after being substantially collimated by lens 24b. Amplified
light having a wavelength of 1550 nm is reflected by filter 32 and
passed through the filter 34 to couple into the output fibre
30b.
[0050] If the rare earth doped block 22 is dimensioned to absorb
substantially all the pump energy, the filter 34 is not necessary.
In addition, the pump laser 38 can be coupled through a different
lens, than a GRIN lens, or no lens at all. In particular, the rare
earth medium 22 can be formed with a taper to a 100 micron diameter
and coupled directly to the laser 38.
[0051] In addition to filters 34 and 32 at the end surfaces of the
block 22, advantageously in accordance with the present invention,
additional optical elements can be formed on the block 22, such as
diffraction elements or additional filters, or lenses, by etching,
depositing or adhering to the end faces of the block 22. Input
fibres 20a, 30a and output fibres 20b, 30b can advantageously be
polarization maintaining fibre pigtails to provide a polarization
maintaining amplifier.
[0052] An alternative embodiment of the invention is shown in FIG.
4 wherein two quarter pitch focusing/collimating glass GRIN lenses
are doped with erbium and form an optical amplifier. A first GRIN
lens 44a is coated on an input/output end with a 1550 nm bandpass
filter 54; at an opposite end of the lens is a coating 52 that
serves as a 980 nm bandpass filter. A second lens 44b is disposed
directly against the filter 52. At an outwardly facing end of the
second GRIN lens 44b is a laser pump 38; The operation of this
device is substantially the same as the amplifier described in FIG.
3, however amplification takes place inside the lens.
[0053] FIG. 5 illustrates the input signal 30a and output signal
30b launched at a small angle, for instance of approximately 1.5
degrees in order to easily separate the input signal from the
output signal.
[0054] As shown in FIGS. 6a and 6b, the amplifying medium 122 can
be formed to provide waveguiding for the pump energy without
guiding the signal Coating the block 22, for example with a metal
cladding 120 will keep pump light within the block to assist in
inducing the maximum pump light absorption. With the pump source 38
coupled directly to the metal cladded block 122 total internal
reflection causes the pump light to reflect from the sidewalls
within the block 122. As shown in FIG. 6b, with a collimating lens
is 124 coupling the laser pump 38 to the block 122, an expanded
beam of pump light is launched through the medium 122.
[0055] Of course, due to the symmetry of a GRIN lens, multiple
groups of input and output fibres 130, 230 can be disposed to
amplify more than one signal at a time, as shown in FIG. 7. A
larger block diameter is required to prevent the multiple signal
beams from overlapping and interacting within the block 222.
* * * * *