U.S. patent application number 09/370735 was filed with the patent office on 2002-08-08 for l band amplifier with distributed filtering.
Invention is credited to MINELLY, JOHN D., YADLOWSKY, MICHAEL J..
Application Number | 20020105720 09/370735 |
Document ID | / |
Family ID | 23460942 |
Filed Date | 2002-08-08 |
United States Patent
Application |
20020105720 |
Kind Code |
A1 |
MINELLY, JOHN D. ; et
al. |
August 8, 2002 |
L BAND AMPLIFIER WITH DISTRIBUTED FILTERING
Abstract
An L-band optical amplifier has a rare earth doped gain medium
including a filter distributed over a finite physical portion of
the gain medium. The filter is distributed over between about 25%
to substantially the entire length of the gain medium. The
distributed filter substantially eliminates out-of-band light
emission (C-band ASE, 1520 nm-1565 nm) and thus improves the
performance of L-band amplification (1565 nm-1620 nm). Examples of
distributed filters include discrete type filters such as long
period gratings, or continuous type filters such as rare earth
doped, twin core fibers, non-adiabatically tapered fibers and
coaxial resonant ring fibers.
Inventors: |
MINELLY, JOHN D.;
(RESIDENCE, XP) ; YADLOWSKY, MICHAEL J.;
(RESIDENCE, XP) |
Correspondence
Address: |
CORNING INCORPORATED
SP-TI-3-1
CORNING
NY
14831
|
Family ID: |
23460942 |
Appl. No.: |
09/370735 |
Filed: |
August 9, 1999 |
Current U.S.
Class: |
359/341.1 |
Current CPC
Class: |
H01S 3/1608 20130101;
H04B 10/291 20130101; H01S 3/10023 20130101; H01S 3/06754 20130101;
H01S 2301/02 20130101 |
Class at
Publication: |
359/341.1 |
International
Class: |
H01S 003/00 |
Claims
We claim:
1. An optical amplifier for optical fiber telecommunications lines
operating with an in-band transmission signal in a longer
wavelength, tail region of a gain spectrum associated with the
amplifier, comprising: a rare earth doped gain medium providing a
first gain stage for the amplifier; a source of pump power
connected to the gain stage; and a filter distributed over the gain
medium, wherein the filter attenuates light associated with
amplified spontaneous emission (ASE) in the amplifier, such that
the transmission signal in the longer wavelength region of the gain
spectrum is amplified.
2. The amplifier of claim 1 wherein the rare earth dopant includes
erbium, further wherein the gain spectrum associated with the
amplifier extends from about 1520-1620 nm, the ASE extends from
about 1520-1565 nm, and the longer wavelength, tail region of gain
spectrum extends from about 1565-1620 nm.
3. The amplifier of claim 1 wherein the filter is distributed over
a finite physical portion of the gain medium ranging from between
about 25% thereof to substantially the entire gain medium.
4. The amplifier of claim 3 wherein the filter includes a plurality
of discrete filters.
5. The amplifier of claim 4 wherein the discrete filters have an
inter-filter spacing corresponding to less than or equal to about a
20 dB gain length at a peak gain wavelength of the gain spectrum of
the amplifier.
6. The amplifier of claim 4 wherein the discrete filters are long
period gratings.
7. The amplifier of claim 4 wherein the long period gratings are
written into the gain medium.
8. The amplifier of claim 1 wherein the gain medium comprises a
silicate based fiber.
9. The amplifier of claim 1 wherein the gain medium comprises a
phospho-silicate based fiber.
10. The amplifier of claim 3 wherein the filter includes a
continuous filter.
11. The amplifier of claim 10 wherein the continuous filter
comprises a doped waveguiding core that is unpumped so as to absorb
the ASE light, and a doped waveguiding core that is pumped so as to
amplify the transmission signal, further wherein the unpumped
waveguiding core and the pumped waveguiding core exhibit waveguide
dispersions such that coupling from the pumped core to the unpumped
core occurs substantially only for light in the ASE gain
spectrum.
12. The amplifier of claim 10 wherein the continuous filter
comprises an axial doped waveguiding core and one of a doped and an
undoped annular waveguiding core that is coaxial with the axial
core.
13. The amplifier of claim 10 wherein the distributed filter
comprises a non-adiabatic taper waveguide.
14. The amplifier of claim 1 wherein the filter has a bandwidth
that is substantially coincident with the spectral bandwidth of the
ASE, and a depth that is approximately equal to, or greater than, a
peak gain coefficient in the gain spectrum of the amplifier.
15. The amplifier of claim 1, further comprising another gain
medium providing another gain stage coupled to the first gain
stage.
16. The amplifier of claim 15, wherein the other gain stage is
coupled closer to an output of the first gain stage than to an
input thereof.
17. The amplifier of claim 15 further wherein an ASE filter is
distributed over the other gain medium.
18. The amplifier of claim 15, wherein the first and second gain
stages each comprises a rare earth doped fiber gain medium.
19. The amplifier of claim 18 wherein the rare earth doped fibers
have different host glass compositions.
20. The amplifier of claim 19 wherein at least one of the rare
earth doped fibers comprises a phospho-silicate host glass
composition.
21. An erbium doped fiber amplifier having a known gain bandwidth
for providing a signal amplification in a long wavelength region of
the known gain bandwidth from about 1560 nm to 1620 nm, comprising:
a first gain stage comprising an erbium doped fiber exhibiting a
first gain spectrum within the known gain bandwidth extending from
about 1520 nm to 565 nm; a source of pump energy coupled to the
first gain stage; a second gain stage comprising an erbium doped
fiber serially connected to the first gain stage exhibiting a
useful gain spectrum within the known gain bandwidth extending from
about 1565 nm to 1620 nm, wherein the first gain stage further
comprises a filter distributed along the first gain stage, said
filter having a depth and a bandwidth sufficient to filter an
out-of-band light emission having an emission spectrum
substantially coincident with the first gain spectrum to the extent
that any unfiltered out-of-band light emission in the first gain
stage is insufficient to self-saturate the first gain stage,
further wherein the useful gain spectrum exhibits a peak gain that
is the peak gain of the known gain bandwidth.
22. The amplifier of claim 21 wherein the filter comprises two
erbium doped waveguiding cores in the erbium doped fiber of the
first gain stage, further wherein no pumping energy is delivered to
one of the doped cores such that said unpumped core attenuates the
out-of-band light emission coupled into said unpumped core.
23. The amplifier of claim 21 wherein the filter comprises a
non-adiabatic taper.
24. The amplifier of claim 21 wherein the filter comprises a
plurality of long period gratings written into the erbium doped
fiber of the first gain stage.
25. The amplifier of claim 21 wherein the filter comprises a doped
axial core in the erbium doped fiber of the first gain stage and a
coaxial annular core in the erbium doped fiber of the first gain
stage.
26. The amplifier of claim 21 wherein the first gain spectrum is
substantially constant and has a neutral gain magnitude, and the
useful gain spectrum includes a peak gain of the known gain
bandwidth.
27. A method of operating an optical amplifier for amplification of
a signal in a longer wavelength tail region of a known gain
bandwidth of the amplifier, comprising the steps of: distributing a
filter over a finite physical length portion of a gain medium of a
first gain stage of the amplifier wherein the filter has a depth
and bandwidth sufficient to attenuate a wavelength spectrum
associated with amplified spontaneous emission from the
amplifier.
28. The method of claim 27, wherein the step of distributing a
filter over the medium of the amplifier comprises distributing a
continues filter over the gain medium.
29. The method of claim 27, wherein the step of distributing a
filter over the gain medium of the amplifier comprises distributing
a plurality of discrete filters over the gain medium.
30. The method of claim 29, comprising spacing the discrete filters
apart by a distance corresponding to less than or equal to about a
20 dB gain length at a peak gain wavelength of the amplifier.
31. The method of claim 27, further comprising distributing a
filter over a second gain medium of a second gain stage of the
amplifier wherein the filter has a depth and bandwidth sufficient
to attenuate an out-of-band light emission associated with
amplified spontaneous emission from the amplifier.
32. A method of operating an optical amplifier for amplification of
a signal in a longer wavelength tail region of a known gain
bandwidth of the amplifier, comprising the steps of: distributing a
filter over a gain stage of the amplifier to attenuate an
out-of-band light emission associated with amplified spontaneous
emission from the amplifier such that an average inversion value
associated with the amplifier is higher than the average inversion
value associated with the amplifier without a distributed
filter.
33. The method of claim 32 wherein the filter is distributed over
an input gain stage of the amplifier.
34. The method of claim 33 wherein the filter is distributed over a
second gain stage of the amplifier coupled to the first gain
stage.
35. The method of claim 32 wherein the amplifier is an erbium doped
optical amplifier, the longer wavelength tail region of a known
gain bandwidth is from about 1565 nm-1620 nm, the known gain
bandwidth is from about 1520 nm-1620 nm, and the out-of-band light
emission is from about 1620-1565 nm.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates generally to the field of
optical communications and in particular to a device and method for
providing optical signal amplification in the longer wavelength or
tail region of a given gain spectrum.
[0003] 2. Technical Background
[0004] Rare earth doped optical amplifiers and particularly
erbium-doped fiber amplifiers (EDFAs) are used extensively and
almost exclusively to amplify optical signals in today's
communications systems and networks. The well-known benefits of
rare earth doped optical amplifiers include cost effectiveness,
good noise performance, a relatively broad polarization insensitive
bandwidth, low insertion loss, and improved crosstalk performance
over other amplifier technologies. EDFAs are increasingly being
used in wavelength division multiplexed (WDM) optical
communications systems and networks.
[0005] As service providers strive to keep up with the ever-growing
demand for capacity, attention has been focused on providing as
many WDM optical channels as possible within a given WDM system. As
such, broadband optical amplifiers are being developed to realize
dense WDM (DWDM) optical systems and networks.
[0006] The total gain spectrum for an EDFA is very wide, as shown
in FIG. 2. The usable gain spectrum extends from around 1525 nm to
1565 nm and this is conventionally referred to as the erbium
C-band. With appropriate gain equalization, an approximately 40 nm
bandwidth is provided for DWDM applications. FIG. 2 also shows that
the gain for an EDFA drops sharply in the spectral region below
1525 nm and the spectral region above 1565 nm. Although
conventional gain equalization techniques cannot be practically
implemented to further increase the gain bandwidth of EDFAs, the
demand for higher capacity lightwave systems has renewed the
interest in signal amplification in the longer wavelength range
between about 1565 nm and 1620 nm, commonly referred to as the
L-band or extended band. See, for example, Massicott et al., "Low
noise operation of ER.sup.3+doped silica fiber amplifier around 1.6
micron," Elec. Lett., Volume 26, Number 20, pp 1645-1646, September
1990. In spite of the appreciation of the potential use of the long
wavelength tail of the erbium gain window for optical signal
amplification, little attention is evidenced in the public
literature to the optimization of L-band amplifiers.
[0007] The performance of an L-band amplifier is limited by at
least three inter-related factors. These include a) a reduced gain
coefficient in the band of interest, b) self-saturation by short
wavelength amplified spontaneous emission (ASE), and c) background
loss in the long fiber coils necessary for high gain operation. The
intrinsic reduction in gain/loss ratio for an L-band amplifier over
a C-band amplifier results in reduced power conversion efficiency.
This is further exaggerated by the reduction in average inversion
which accompanies self-saturation and which reduces the already low
gain coefficient even further, resulting in even more length
dependent efficiency reduction. Furthermore, if the first stage of
a multistage amplifier is operated at low inversion, the noise
performance of the amplifier is significantly compromised. However,
operation at high inversion produces C-band ASE which will reduce
the power conversion efficiency of the amplifier.
[0008] Accordingly, the inventors have recognized a need to improve
the performance of an L-band amplifier and more specifically have
targeted the tradeoff between noise figure and power conversion
efficiency to address this.
SUMMARY OF THE INVENTION
[0009] An embodiment of the present invention is directed to an
optical amplifier for amplifying optical signals in a longer
wavelength, tail region of a gain spectrum associated with the
amplifier, including a rare earth-doped gain medium referred to as
a gain stage of the amplifier; a source of pump power connected to
the gain medium; and a filter distributed over the gain stage,
wherein the filter attenuates light associated with amplified
spontaneous emission (ASE) in the amplifier, such that
substantially only the optical signals in the longer wavelength
region of the gain spectrum are amplified.
[0010] Another embodiment of the invention is directed to an
optical amplifier for amplifying optical signals in a longer
wavelength, tail region of a gain spectrum associated with the
amplifier and includes a first rare earth-doped gain medium
referred to as a first gain stage of the amplifier, wherein a
filter is distributed over the first gain medium. The filter
provides an attenuation of light associated with amplified
spontaneous emission. The amplifier further includes a second rare
earth-doped gain medium referred to as a second gain stage of the
amplifier connected to the first gain stage; and a source of pump
power connected to the amplifier for stimulating the rare
earth-doped gain media. In an aspect of this embodiment, the second
gain stage is preferably serially connected to the first gain stage
closer to an output location of the first gain stage than to an
input location in terms of signal propagation direction. In another
aspect of this embodiment, a filter is also distributed over the
second gain stage of the amplifier to further reduce ASE generated
by the amplifier. In another aspect of this embodiment the pump
source is preferably coupled to the first gain stage at a location
closer to an input of the first gain stage than to an output of the
first gain stage.
[0011] In a continuous distributed filtering aspect of both
embodiments described above, the distributed filter is a rare earth
doped, multiple and preferably dual core fiber making up the first
gain stage. One of the cores is pumped to provide gain for the
useful gain spectrum and the other core is unpumped causing it to
absorb the out-of-band (ASE) light.
[0012] In another continuous distributed filtering aspect, the
distributed filter is a non-adiabatically tapered fiber making up
the first gain stage in which mode coupling occurs in the taper
region to provide the filtering effect.
[0013] In a further continuous distributed filtering aspect, the
distributed filter is a rare earth doped fiber making up the first
gain stage and having a doped axial core and a doped or undoped
coaxial annular core wherein bend loss provides the filtering
effect over the length of the fiber.
[0014] In an alternative discrete filtering aspect, the distributed
filter is a series of discrete filters such as long period gratings
that are written or spliced into the rare earth doped fiber making
up the first gain stage. In this aspect, it may be desirable to
provide a doped fiber glass host different from a typical
(germano)-alumino-silicate host glass, such as a phospho-silicate
glass, that provides a more efficient medium for writing gratings
therein.
[0015] The invention described herein particularly provides a
device and a method for amplifying light signals in the erbium
L-band having improved performance over L-band amplifiers without
distributed filtering. Distributed filtering according to the
invention substantially eliminates the out-of-band ASE generated in
the amplifier due particularly to hard pumping, which in turn
allows the amplifier to operate at a higher average inversion
without self-saturation by the C-band ASE. Higher average inversion
operation allows for a shorter active coil length for obtaining
target gain values and in addition contributes to improved power
conversion efficiency due to a reduction in background loss.
Amplifier noise figure is also improved by the ability to achieve
the target L-band stage gain at a higher inversion. The invention
thus also provides benefits for amplifier circuit layout and
packaging considerations.
[0016] Although the device according to the invention is
illustratively described as a fiber optical amplifier, it is not so
limited as a planar architecture, for example, can also implement
the invention.
[0017] Additional features and advantages of the invention will be
set forth in the detailed description which follows, and in part
will be readily apparent to those skilled in the art from the
description or recognized by practicing the invention as described
in the written description and claims hereof, as well as the
appended drawings.
[0018] It is to be understood that both the foregoing general
description and the following detailed description are merely
exemplary of the invention, and are intended to provide an overview
or framework to understanding the nature and character of the
invention as it is claimed.
[0019] The accompanying drawings are included to provide a further
understanding of the invention, and are incorporated in and
constitute a part of this specification. The drawings illustrate
embodiments of the invention, and together with the description
serve to explain the principles and operation of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a graphical representation of gain versus
wavelength over the C-band and the L-band for an erbium doped
optical amplifier;
[0021] FIG. 2 is schematic representation of a two stage L-Band
optical amplifier with distributed filtering in the first stage
according to an embodiment of the invention;
[0022] FIG. 3 is a graphical representation of average inversion
values versus active fiber length for various filter bandwidths and
filter depths according to an embodiment of the invention;
[0023] FIG. 4 is a graphical representation of backward ASE power
versus active fiber length for various filter bandwidths and filter
depths according to an embodiment of the invention;
[0024] FIG. 5 is a graphical representation of remnant pump power
versus active fiber lengths for various filter bandwidth values and
filter depths according to an embodiment of the invention;
[0025] FIG. 6 is a graphical representation of amplifier noise
figure versus wavelength in the erbium L-band for various average
inversion values according to an embodiment of the invention;
[0026] FIG. 7 is a schematic diagram of a dual core distributed
filter according to an embodiment of the invention;
[0027] FIG. 8 is a schematic diagram of a discrete distributed
filter amplifier embodiment according to the invention;
[0028] FIG. 9 is a graphical representation of a representative
L-band gain profile provided by a distributed filter amplifier
according to the invention;
[0029] FIG. 10(a) is a schematic representation of a
non-adiabatically tapered fiber distributed filter for an
embodiment of the invention;
[0030] FIG. 10(b) is a schematic representation of loss versus
wavelength provided by the filter of FIG. 10(a); and
[0031] FIG. 11 is a schematical cross sectional view of an
annular/co-axial core filter for an embodiment of the
invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
[0032] To provide the reader with a clearer understanding of the
invention, the term "distributed filter" as used herein refers to
filtering that occurs over some finite physical portion of the
filtered gain stage (i.e., the active fiber), as opposed to at
merely a single or discrete location in the gain stage. Thus the
filter may, for example, be distributed over a length of the gain
stage practically ranging from about 25% of the fiber to
substantially the entire gain stage depending upon how well the
distributed filter provides filtering of the ASE produced by the
amplifier. This will be described in greater detail below in
relation to the amplifier filtering embodiments of the invention;
however, it can be said that the distributed filter may include a
plurality of discrete filters distributed over the physical portion
of the gain medium to provide effective filtering or,
alternatively, a physically continuous filter distributed over a
portion of the gain medium.
[0033] A helpful distinction is also attempted for the reader
between a single stage amplifier and a multi-stage amplifier as
those terms are herein used. A single stage amplifier refers to a
single section of gain medium pumped by a source of pump light. A
multi-stage amplifier refers to at least two sections of gain media
that are physically separated or connected via a discrete
component, and in which each section provides more than nominal
gain to a signal. Therefore, as these terms are herein used, a
single gain stage can include a series of discrete filters along
its length and remain a single stage. We believe that this will be
clearly understood by a person skilled in the art. Accordingly, the
amplifier shown in FIG. 2 is a multi-stage amplifier having a first
stage 2 (with discrete distributed filters 14 in an exemplary
embodiment of the invention) and a second stage 4 connected at
component 12.
[0034] It will also be appreciated by those persons skilled in the
art that a multi-stage rare earth-doped L-band optical amplifier
may provide advantages over a single stage amplifier in terms of
design, control, and management of system architecture and
performance. For example, gain equalization, dispersion
compensation, wavelength routing, and other functions are
conveniently performed between optical amplifier stages.
Accordingly, for illustrative purposes, the invention will be
described in detail with reference to a multi-stage amplifier;
however this in no way limits the invention nor precludes an
amplifier that embodies the invention from being a single stage
L-band amplifier.
[0035] Reference will now be made in detail to the present
preferred embodiments of the invention, examples of which are
illustrated in the accompanying drawings. Wherever possible, the
same reference numbers will be used throughout the drawings to
refer to the same or like parts.
[0036] FIG. 1 shows a representative gain spectrum 20 for a
conventional (germano)alumino-silicate erbium doped fiber covering
the spectral region from approximately 1520 nm to 1620 nm. This
spectral region is hereinafter referred to as the known gain
bandwidth 22. A spectral region that is typically referred to as
the C-band wavelength range for a EDFA is shown as, and is
hereinafter referred to as, the first gain spectrum 24. It occupies
the shorter wavelength region of the known gain bandwidth 22
extending approximately from 1520 nm to 1560 nm. According to an
embodiment of the invention, the first gain spectrum 24 is
primarily associated with a first gain stage 2 of amplifier 10 of
FIG. 1. The wavelength region generally referred to as the L-band
(or extended band) of an EDFA occupies the longer wavelength (tail)
region of the known gain bandwidth 22 and extends from about 1560
nm to 1620 nm. It is hereinafter referred to as the useful gain
spectrum 26 and is the emission spectrum primarily associated with
a second gain stage 4 of amplifier 10. In operation, an optical
amplifier emits a broad based noise spectrum known as amplified
spontaneous emission (ASE) in both the forward and backward
directions when the gain medium of the amplifier is pumped. The ASE
will hereinafter be referred to as the out-of-band light emission
28 and, with respect to the instant invention, occupies a spectral
region that is substantially coincident with the first gain
spectrum 22. The attributes of ASE in rare earth doped optical
amplifiers are believed to be well understood by those persons
skilled in the art and require no further discussion for an
understanding of the invention.
[0037] FIG. 2 illustrates an exemplary embodiment of an amplified
spontaneous emission (ASE) managed L-band erbium doped fiber
amplifier (EDFA) 10 according to the invention. A first stage
erbium doped fiber gain medium 2 is serially coupled to a second
stage erbium doped fiber gain medium 4 at coupling point 12.
Although both stages 2, 4 of the L-band EDFA are shown coupled
immediately adjacent to one another, it will be appreciated by
those skilled in the art that various components such as optical
isolators, filters, or other signal management components may be
inserted between the two stages. A source of pump power 8 is
appropriately coupled to the amplifier, preferably to the first
stage at or near an input location thereof to provide pumping
energy in a forward (i.e., co-directional with the signal)
direction. It is believed that this pumping architecture gives
amplifier operation with a lowered noise figure and increased power
conversion efficiency. This is typically accomplished by relatively
hard pumping at the input of the first stage of the amplifier where
a high inversion occurs. However, as further appreciated by those
skilled in the art, different pumping schemes are possible; for
example, either or both stages of the EDFA may be forward and/or
backward pumped, typically in the 980 nm pump band and/or the 1480
nm pump band, or other pump bands depending upon the amplifier
design and architecture, performance requirements, gain stage
compositions, and other considerations. An input optical signal
.lambda..sub.in, 6, lying within the useful gain spectrum 26 of
FIG. 1 is input to the amplifier 10 at an input end of the first
gain stage 2 (as shown traveling from the left to the right in FIG.
2) and exits an output of the second gain stage 4 as amplified
signal .lambda..sub.out, 18. A filter 14 is distributed along the
first gain stage 2 to provide filtering of the ASE 28 as shown in
FIG. 1, as will be described in greater detail below. Optionally, a
filter 16 is distributed along the second gain stage 4 to further
reduce ASE in the second gain stage.
[0038] The invention addresses at least three factors that limit
the performance of an L-band amplifier. The first is a reduced gain
coefficient in the L-band compared to the C-band gain at particular
amplifier inversion values as illustrated by gain curve 20 in FIG.
1 over the useful gain spectrum 26. The second factor is
self-saturation by broadband ASE generated in the first gain stage
2 which depletes the upper energy level population and reduces the
average inversion of the amplifier, thus reducing signal gain. The
third factor relates to background loss in the long lengths of
active fiber that are typical for high gain operation in the
L-band. The intrinsic reduction in the gain to loss ratio for an
L-band amplifier over a C-band amplifier results in reduced power
conversion efficiency. Moreover, when the first stage 2 of the EDFA
10 is operated at a low average inversion value, the noise figure
(NF), a primary figure of merit for optical amplifier performance,
is significantly compromised, as shown by the curves 1-7 in FIG. 6,
respectively, for increasing inversion values. However, when the
first stage 2 is pumped relatively hard so as to provide a high
average inversion value in the first stage 2, the ASE 28 emitted in
the C-band (i.e., the out-of-band light emission), will
significantly compromise the power conversion efficiency of the
amplifier. To address these issues, a preferred amplifier
embodiment of the invention as shown in FIG. 2 includes a filter 14
distributed over the first gain stage 2. The effective
characteristics of filter 14 are its bandwidth and depth. Filter
bandwidth is conventionally defined as the spectral region over
which filtering is provided to a certain degree; while filter depth
is defined herein as the magnitude of the filtering over a
particular bandwidth. Preferably the filter 14 will have a depth
that is approximately equal to or greater than a peak gain
coefficient of the know gain bandwidth 22, typically occurring at
or near 1530 nm.
[0039] According to an embodiment of the invention, the filter 14
exhibits a bandwidth and depth sufficient to attenuate and,
preferably, to effectively eliminate most, if not all, of the
out-of-band light emission 28, which in turn allows the amplifier
of the invention to operate at a relatively higher average
inversion without self-saturation by the C-band ASE. This in turn
leads to the ability to use a shorter active coil length for a
particular target gain, also reducing background loss and improving
the power conversion efficiency of the amplifier which is already
improved by the C-band ASE suppression. In addition, the noise
figure of the amplifier is improved by the ability to achieve the
target L-band stage gain (second gain stage 4) at a higher average
inversion value. Distributed filtering according to the invention
thus provides a peak gain in the useful gain spectrum 26 of FIG. 1
that is also the peak gain over the known gain bandwidth 22. FIG. 9
shows a representative gain profile 90 for the distributed filter
L-band amplifier of the invention. Such a gain profile may still
require gain flattening or gain equalization. The distributed
filter 14 in the first gain stage 2 of FIG. 2 is intended to
eliminate C-band emission from about 1520 nm to 1565 nm and allow
the amplifier to provide L-band gain from about 1565 nm to 1620
nm.
[0040] In an alternative embodiment of the invention, a filter 16
is additionally distributed over the second gain stage 4 as
illustrated by the dashed line in FIG. 2. Distributed filtering
over the second gain stage 4 reduces local inversion variance due
to the concentrated pumping at the second stage input from first
stage remnant pump power, reduces competition between backward ASE
and forward traveling signals, and further contributes to
increasing the power conversion efficiency of the amplifier.
[0041] According to alternate aspects of the invention, the filter
14 (and optional filter 16) may take the form of a continuous
filter extending over a finite physical portion of the first gain
stage, ranging from about 25% of the length to substantially the
entire length; or the filter may constitute a number of discrete
filters that extend likewise over the gain stage. As will be
appreciated, the distributed filtering according to the invention
is intended to attenuate and, preferably eliminate, a broadband
spectrum of light coincident with the erbium C-band. The ability of
the filter to achieve this will determine the extent of its
distribution over the gain stage.
[0042] In a preferred embodiment of the invention, the filter 14 is
a continuously distributed filter having appropriate depth and
bandwidth to substantially eliminate the out-of-band light emission
generated in the first gain stage. One example of such a continuous
filter is shown schematically in FIG. 7(a) as a fiber 70 having two
erbium doped cores 72, 74, one of which (72) is optically pumped to
provide gain for the L-band signals and the other of which (74) is
unpumped to absorb the out-of-band light and thus provide the loss
for the C-band ASE that couples over to the unpumped core (74).
This fiber performs the desired function if the waveguide
dispersion of the two cores is such that coupling occurs only for
C-band wavelengths. In such an embodiment the doping concentration
of the unpumped core (74) may be much higher than that of the
amplifying core (72) to enhance the filtering characteristics. It
may even be deliberately clustered to prevent absorption
saturation. Two aspects of this embodiment include a continuous
unpumped core 74 as shown in FIG. 7(a), and a segmented unpumped
core 74' as shown in FIG. 7(b). Fibers of these types are generally
known in the art and require no further description for the purpose
of understanding the invention described herein. See, e.g., U.S.
Pat. Nos. 5,087,108 and 5,218,665, the disclosures of which are
herein incorporated by reference.
[0043] Another aspect of the distributed filter amplifier according
to the invention relies on the gain fiber having a non-adiabatic
taper; that is, a fiber taper construction which induces mode
coupling between the LP.sub.01 and LP.sub.02 modes. Such a
distributed filter gain fiber is shown schematically in FIG. 10(a)
where the fiber 102 includes a series of non-adiabatic tapers 104.
FIG. 10(b) shows a representative loss spectrum profile 106 for the
non-adiabatic fiber taper filter 102. The loss spectrum essentially
mimics the gain curve of an erbium doped fiber at a particular
inversion value. The use of such a filter in the amplifier
according to the invention ideally produces an L-band gain as
representatively shown in FIG. 9. Non-adiabatic taper based fibers
are preferably constructed from fibers with a secondary co-axial
annular core or a depressed cladding which improve the ease of
manufacture. See, e.g., U.S. Pat. No. 4,877,300, which is
incorporated herein by reference as though fully set forth in its
entirety, for a more detailed explanation of construction of
non-adiabatic taper based fibers.
[0044] In another embodiment illustrated in part by FIG. 11, a
hybrid distributed filter L-band amplifier combines the mechanisms
of coaxial coupling with dual core coupling and includes a gain
fiber 110 having a conventional axial doped core 112 and a coaxial
annular core 114 that can be doped or undoped depending upon design
and performance considerations. In distinction to the non-adiabatic
fiber taper filtered amplifier embodiment described above in which
coupling only occurs where the fiber is tapered, in the instant
embodiment power is continuously exchanged between the inner and
outer cores 112, 114, respectively. Experimental results indicate
that such fibers do not exhibit the expected sinusoidal length
dependence to the transverse power distribution associated with
mode beating. Rather the loss of the designed coupling bandwidth
increases monotonically with fiber length. It is believed that this
due to an increased bend sensitivity of the ring waveguide 114 with
respect to the axial core waveguide 112. Hence the device will
operate similar to the doped dual core fiber except that the lossy
core 114 relies on microbend or macrobend induced leakage rather
than absorption.
[0045] An example of a discrete type distributed filter for an
amplifier embodiment according to the invention is shown
schematically in FIG. 8 and comprises an active fiber 80 having a
plurality of long period gratings (LPGs) 82 written over some
finite physical length of the fiber. Long period gratings, as used
herein, include fiber gratings having a period which causes light
of a given wavelength to couple into the cladding of the fiber and
radiate out. Due to the fact that conventional alumino-silicate
erbium doped fibers are only weakly photosensitive, it may be
desirable to splice the LPGs into the amplifier fiber. However,
since this may add excess loss and production costs, it may be more
desirable to use a modified gain fiber composition and/or fiber
structure to effectively write the LPGs in the gain fiber. For
example, a phospho-silicate host having gratings written with a 193
nm writing wavelength would address the aforementioned
difficulties. Long period gratings and their production in fibers
is well known in the art and requires no further discussion for an
understanding of the invention.
[0046] In the discrete distributed filter aspect of the amplifier
invention, a preferable inter-filter spacing in a single stage of
the amplifier would be less than or equal to about a 20 dB gain
length at a peak gain wavelength of the amplifier. In this aspect
of the invention at least some of the filters may have filter
bandwidths that overlap. Furthermore, at least some of the filters
will have a filter bandwidth that substantially extends to cover
the out-of-band light emission 28.
[0047] Presented below in conjunction with FIGS. 3-6 are the
results of modeling the effect of distributed filtering on L-band
amplifier performance for a typical (germano)alumino-silicate
erbium doped fiber. The amplifier pump power was 140 mW at 980 nm,
and 40 signal channels of -21 dBm input power were lumped, for
simplicity, at 1590 nm.
[0048] FIG. 3 shows plots of average inversion versus first stage
active fiber length for the amplifier. The plot labeled 1 is the
result for no (distributed) filtering. Plots 2 and 3 result from a
constant filter bandwidth from 1525 nm to 1545 nm with the
difference being a filter depth of 5 dB/m for plot 2 and a filter
depth of 20 dB/m for plot 3. Plots 4 and 5 result from a filter
bandwidth of 1525 nm to 1560 nm with plot 4 representing a filter
depth of 5 dB/m and plot 5 representing a filter depth of 20 dB/m.
Comparing plots 4 and 5 with plots 2 and 3 of FIG. 3 indicates that
increasing the filter bandwidth in the C-band reduces inversion
reduction due to ASE self-saturation. However, increasing the
filter depth much above the peak gain coefficient (i.e., the
maximum gain at 1530 nm at full inversion) has a negligible affect.
It is thus evident that the target gain for the 1590 nm signal can
be achieved in a shorter length of fiber operating at a relatively
higher average inversion. This provides additional benefits for
improved packaging and amplifier layout.
[0049] FIG. 4 shows plots of backward traveling ASE versus first
gain stage fiber length where plots 1-5 have the same filter
bandwidth and filter depth characteristics as plots 1-5,
respectively, in FIG. 3. Comparison of plots 2 and 3 with plots 4
and 5 indicate that the filter depth, once the threshold filter
depth has been reached, has little effect on backward ASE; however,
progressive reduction in the back traveling ASE results from
increasing the filter bandwidth.
[0050] The benefit of the reduction in ASE described above is
accompanied by an increase in the remnant pump power which confirms
the anticipated improvement in power conversion efficiency obtained
with distributed filtering according to the invention. This is
indicated with reference to FIG. 5 which is a plot of remnant pump
power in milliwatts versus first gain stage fiber length. The plots
labeled 1-5 have identical filter bandwidth and filter depth
characteristics as those associated with plots 1-5 of FIG. 3. As
shown, filter depth is an inferior control compared to filter
bandwidth which can be observed by comparing plots 2 and 3 with
plots 4 and 5. More importantly, distributed filtering with the
appropriate filter depth and bandwidth shortens the necessary fiber
length and increases the amount of remnant pump in the first stage
for pumping the second stage, thus improving the power conversion
efficiency of the amplifier.
[0051] In addition to the improved power conversion efficiency
obtained with the instant invention, noise figure improvement is
also achieved as shown by plots 1-7 in FIG. 6. FIG. 6 is a graph of
noise figure versus signal wavelength for average inversion values
ranging from 0.4 to 1.0 as shown by plots 1-7 respectively.
Therefore the higher average inversion obtainable by distributed
filtering according to the invention results in lower noise figure
values.
[0052] Based upon the information provided in FIGS. 3-6, for an
exemplary amplifier having a typical first stage target gain of 10
dB, distributed filtering from 1525 nm to 1560nm according to the
invention would enable a reduction in length of the first gain
stage active fiber from 30 meters (without a filter) to
approximately 20 meters. Moreover, 20 mW of pump power would be
saved and the average inversion would be increased from 0.6 to
0.85. This increase in the average inversion value would
significantly improve the noise figure of the amplifier especially
at wavelengths on the shorter side of the L-band where sensitivity
to inversion is highest.
[0053] It will be apparent to those skilled in the art that various
modifications and variations can be made to the present invention
without departing from the spirit or scope of the invention. Thus,
it is intended that the present invention cover the modifications
and variations of this invention provided they come within the
scope of the appended claims and their equivalents.
* * * * *