Note: Descriptions are shown in the official language in which they were submitted.
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GRATING ASSISTED COUPLER DEVICES
Back ound of the Invention
This invention relates to optical wave propagation systems and devices
utilizing electro-optical devices, and more particularly to grating assisted
devices for
filtering, coupling and other functions.
This application is a continuation-in-part of U.S. application entitled
"Wavelength Selective Optical Couplers", filed Aubust 29, /995 , U.S. serial
no.
08rl03,357; is a continuation-in-part of U.S. application entitled "Wavelength
Optical
Devices", filed October 2?, 1995, U.S. serial no. 08/738,068; and claims the
benefit of
U.S. provisional application No. 60/055,157 entitled "Fabrication of Add/Drop
Filters
for Wavelength Division Multiplexing", filed on August 4, 1997, the
disclosures of
which are hereby incorporated herein by reference.
Communication systems now increasingly employ optical waveguides
(optical fibers) which, because of their high speed, low attenuation and wide
bandwidth characteristics, can be used for carrying data, video and voice
signals
concurrently. An important extension of these communication systems is the use
of
wavelength division multiplexing, by which a given wavelength band is
segmented
into separate wavelengths so that multiple traffic can be carried on a single
installed
line. This application requires the use of muldplexers and demultiplexers
which are
capable of dividing the band into given multiples (such as 4, 8, or 16
different
wavelengths) which are separate but closely spaced. Adding individual
wavelengths
to a wideband signal, and extracting a given wavelength from a mufti-
wavelength
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signal require wavelength selective couplers, and this has led to the
development of a
number of add/drop filters, the common terminology now used for devices~of
this
type.
Since wavelength selectivity is inherent in a Bragg grating, workers in
the art have devised a number of grating-assisted devices for adding or
extracting a
given wavelength with respect to a multi-wavelength signal. Typical optical
fibers
propagate waves by the use of the light confining and guiding properties of a
central
core and a surrounding cladding of a lower index of refraction. The wave
energy is
principally propagated in the core, and a number of add/drop filters or
couplers have
been developed using Bragg gratings in the core region of one of a pair of
parallel,
closely adjacent or touching .fibers. The coupling region is commonly termed
"evanescent" in that a signal propagated along one fiber couples over into the
other, as
an inherent function of the design. The wavelength selectivity is established
by the
embedded grating which provides forward or backward transmission of the
selected
wavelength, depending upon chosen grating characteristics. For modern
communication systems, however, this approach has a number of functional and
operative limitations, pertaining to such factors as spectral selectivity,
signal to noise
ratio, grating strength, temperature instability and polarization sensitivity.
The applications referenced above are based upon a novel theoretical
concept and practical implementation. A narrow waist region of two fused
dissimilar
fibers is defined between pairs of tapered coupling sections at each end. At
the waist,
the merged fibers are formed by elongation of an optical fiber precursor of
generally
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conventional size and are so diametrically small that the central core
effectively
vanishes. The wave energy is transferred through the merged fiber region in
two
spatially overlapping, orthogonal modes. Since the propagating energy of the
modes
overlap, the coupling is potentially non-evanescent in the presence of a
coupling
mechanism such as a diffraction grating. For example, a reflective grating
written in
the waist region redirects only a selected wavelength of an input signal at
the input
port to the drop port, while all other wavelengths propagate through the waist
section
without reflection to the throughput port. This reflection grating couples
light
between two optical modes in a non-evanescent manner. Numerous advantages
derive from this concept and configuration, but the realization of its full
potential is
dependent upon other developmental factors.
For example, modern applications require that any add/drop filter
based upon this concept be very efficient at routing channels, having a strong
grating
which can be selectively and precisely placed at or adjusted to a specific
wavelength
and yet have a limited bandwidth, be temperature insensitive, compact, low
cost, and
not subject to spurious reflections or noise in the chosen wavelength band.
Achieving
high drop efficiency and low polarization dependence are particularly
important. The
problems of achieving these operative properties while at the same time
providing a
repeatably producible unit of very small size and high sensitivity have
required much
further innovation.
*rB
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Summary of the Invention
In accordance with the invention, the optical properties and
performance of a grating assisted asymmetric fused coupler are highly
dependent on
the physical characteristics of the coupler waist. Polarization insensitivity
of the drop
wavelength can be achieved, for example, by controlling the shape during
elongation
or by applying a permanent twist to the coupler waist after the grating
exposure.
Furthermore, the small diameter waist renders the coupler sensitive to
diameter non-
uniformities but it is shown that these dimensional variations can be
compensated by
laser trimming or by impressing a compensated index of refraction grating.
Further,
the strength of the grating can be dramatically increased by in-diffusing a
photosensitizing gas during the grating writing process. For improved spectral
characteristics the grating is apodized and unchirped by being written with
concurrent
grating modulated {a.c.) and uniform (d.c.) intensity UV beams. Size and other
characteristics of the waist region are selected such that the drop wavelength
of the
coupler is adequately separated from the backreflection wavelength and the
latter
wavelength lies outside the frequency band of interest.
A small coupler having these properties and wavelength adjustability
as well is enclosed within a prepackage structure which enables optical access
to the
coupler waist for grating writing. An elongated structure consisting of
materials
having different thermal coefficients of expansion is arranged to the
temperature
dependence of the drop wavelength. Moreover, the structure provides fine
tuning so
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that the drop wavelength is precisely adjusted and subsequently maintained
throughout the desired operating temperature range.
Brief Description of the Drawings
A better understanding of the invention arises by reference to the
following description, taken in conjunction with the accompanying drawings, in
which:
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Fig. 1 is a simplified and idealized view of the principal~parts, namely
the tapered coupling branches and the waist region, of a coupler in accordance
with
the invention, useful in describing the optical waveguide modes and
characteristics;
Fig. 2 is an enlarged cross sectional view of the asymmetric waist
section of the coupler of Fig. 1, with the extent of the optical wave energy
propagating
along the waveguide denoted by dotted lines;
Fig. 3 is a pair of graphs (A) and (B) illustrating the relationship
between normalized propagation constants and V number for waveguide
configurations employed in these devices, useful in understanding how lossy
cladding
modes are eliminated, how an adequate separation between drop wavelength and
back-reflection wavelength is achieved, and how diameter uniformity tolerances
relate
to coupler diameter;
Fig. 4 is a simplified and idealized view of a coupler twisted at the
waist region to impart polarization independence;
Fig. 5 is a graphical representation of the drop channel spectral
characteristics; namely, the drop reflectivity versus wavelength, depicting
the effect of
twist on the polarization splitting characteristics of the drop channel;
Fig. 6 is an illustrative graph of (A) local Bragg wavelength variation
along a typical non-uniform diameter coupler waist before correction and (B)
local
Bragg wavelength after correction;
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Fig. 7 is an illustrative graph (not to scale) of UV induced index of
refraction variations in a coupler corresponding to an apodized grating, such
as a
cosine-squared or Gaussian apodization function;
Fig. 8 is a break-away perspective view of an exemplary coupler in
accordance with the invention having a cylindrical housing and an interior
optical
fiber support structure or prepackage;
Fig. 9 is a side sectional view of the coupler of Fig. 8;
Fig. 10 is a fragmentary side sectional view of a fine tuning mechanism
for compensation of wavelength within the coupler of Figs. 8 and 9;
Fig. I 1 is a fragmentary side sectional view of an end portion of the
coupler, showing the manner in which the coupler housing is hermetically
sealed and
the exterior fibers are protected;
Detailed Description of the Invention
An optical fiber wavelength router in accordance with the invention is
exemplified by a wavelength selective filter, here of the type usually
referred to as an
add/drop filter. Such a device, in which multiple channels at different
wavelengths
are applied, redirects in a low loss, highly efficient manner the selected
wavelength
channels into a first optical fiber while transferring the remainder of the
channels to a
second optical fiber. While the concepts employed may be used for other
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applications, such as switches, multiple channel routers, and crossconnects,
the
addldrop filter is perhaps of greatest immediate benefit for multiplexers and
demultiplexers in wavelength division multiplex (WDM) systems.
Figure 1 illustrates the physical structure of this device. The fused
coupler consists of a first fiber 31, 35 and a second fiber 30, 34 dissimilar
in the
vicinity of the coupling region 12 wherein an index of refraction grating 27
has been
impressed. The two fibers may be made dissimilar by locally pretapering one of
them
by 20% in the vicinity of the fused region. Light launched into the single
mode core
of upper fiber 31 evolves adiabatically into an LP1, mode with nominal
propagation
vector iii in the waist, and adiabatically evolves back into the single mode
core of the
output fiber 35. Light launched into the single mode core of the lower fiber
30
evolves adiabatically into the LPo~ mode with propagation vector X32, and
adiabatically
evolves back into the single mode core of the output fiber 34. If an index of
refraction
I5 grating 27 is impressed in the coupler waist 12, and.if the wavelength is
chosen such
that ~3, and (32 satisfy the Bragg law for reflection from an index grating of
period Ag
at a particular wavelength, say a,;:
~a,c~;)~+I~c~;)1=
8
then the optical energy at 7v,; in the single mode core of the first fiber 31
is reflected
non-evanescently by the grating into the single mode core of the second fiber
30. The
spectral response and efficiency of this reflective and mode-converting
coupling
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process is dictated by the non-evanescent coupling strength of the optical
modes with
the grating. If the wavelength of the input mode is detuned, say to ~,~, so
that:
~a~ (~; )~ + (~Z (~~ )~ ~ n 8
then the Bragg law is no longer satisfied and the input mode in the first
fiber 31
travels through the coupler waist 12 and reappears as the output mode of the
first fiber
35, with minimal leakage into the second fiber 34. For these wavelengths the
coupler
is transparent; that is, no coupling occurs, and the two fused fibers remain
optically
independent. Therefore, only a particular wavelength a,; is coupled out of the
first
fiber 3I, 35 as determined by the grating period in the coupling region 12.
In addition to backwards coupling of light into the adjacent fiber, the
grating typically reflects some light back into the original fibers at
different
wavelengths given by 2I~3, (~ ), = kg and 2I~ (~ )I = kg. To ensure that ~,2
and ~,3 are
outside the wavelength operating range of interest, the difference between (3~
and ~3~ is
made sufficiently large. The difference increases as the waveguides become
more
strongly merged or as the fused coupler waist decreases in dimension. This
general
trend is depicted in Fig. 3, whereby the vertical axis separation between
adjacent
characteristic curves for eigenmodes of the waveguide generally increases for
smaller
diameters (smaller V's). This difference is maximized for small coupler
waists, where
Vii, and ~i2 correspond substantially to the LPo, and LP" modes of an air-
glass optical
waveguide. The LPo, mode is a common representation of the HE;, , HE;, modes,
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and the LP1, mode is a common representation of the HEz, , HE2, , EHo, , and
EHo,
modes, illustrated in Fig. 3. It is common in the art to speak in terms of
these LP
modes in waveguide structures such as coupler waists that do not exhibit
circular
symmetry.
Furthermore, the tilt angle of the transversely asymmetric grating can
be selected to reduce the coupling strength for backreflection of the LPo~
into LPo,
modes and the LPI, into LP" modes. The other consideration in selecting angle
is to
maximize the mode conversion efficiency of the LP,o into LP1~ and LP, i into
LPo,
modes. The typical angles to minimize backreflection coupling are in the range
of 3
to 5 degrees and the angle increases as the coupler waist diameter decreases.
This
angle is slightly different than the angle to maximize the drop efficiency.
To form this fiber optic coupler, two locally dissimilar fibers are fused
to a narrow waist typically 1 to 50 microns in diameter, forming a waveguide
in the
fused region which supports at least two supermodes or eigenmodes of the
composite
waveguide. The number of supermodes supported by this composite waveguide
structure is determined by the index profile and dimensions of the structure.
When
this waveguide structure is significantly reduced in diameter, the waveguiding
characteristics resemble that of an air-glass waveguide. The mode propagation
behavior of this simplified step index waveguide is then partially described
by a
parameter defined as the V number, which decreases as the radius a of the
waveguide
core is decreased, and depends on the optical wavelength ~ of the mode, the
core
index more and the cladding index n~,~a:
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2,?C!1 2 _ 2
ncore nclud '
0
For an air-glass waveguide n~o,~ = 1.45 and n~,~ = 1Ø For an elliptical
cross section waveguide, the first or lowest order mode is nominally LPoI and
the
second mode is nominally LPI,. Typically, higher order modes exist within the
coupler waist, as the total number of modes supported by such a waveguide is
N ~ V Z I 2 , which is 8 - 9 for a 4 micron diameter waist at 1550 nm.
However, the
two lowest order modes are principally important in the add/drop operation. In
general, a lossy peak appears for each higher order mode greater than two.
Because
the two waveguides are sufficiently dissimilar and the tapered transition
region is
sufficiently long, the input optical modes traveling along the single mode
cores of the
original fibers adiabatically evolve into the superrnodes of the coupling
region. Upon
exiting the coupling region, the supermodes will evolve adiabatically back
into the
original optical modes as the waveguide splits into the two original fibers.
Thus, the
optical energy passes from the input to the output without being disturbed. A
typical
fiber asymmetry of ( ~(31I - ~(32I ) ~ ( ~~il + I~zl ) = 0.005 and a taper
angle of 0.01
radians results in less than 1 % in undesired leakage of optical energy from
one fiber to
the other. To achieve the asymmetry, a pair of identical fibers can be made
dissimilar
by stretching (adiabatically pretapering) one fiber in a central region. The
two fibers
are then merged or joined into one waveguide in the coupling region, yet the
two
fibers behave as if they were optically independent. A grating is next
impressed in the
coupling region to redirect light at a particular wavelength from one fiber to
another.
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For example, a 125 micron diameter fiber is pretapered by 25%, then elongated
and
fused to another 125 micron diameter fiber to form a 4.5 micron diameter, 2 cm
long
waist region with taper lengths of 2 cm. The resulting taper angle is
sufficient to
produce a low loss, adiabatic taper. For a UV impressed grating period of
0.540
micron, the wavelength of the drop channel of representative devices is in the
1550
nm range.
A suitable starting fiber from which such a coupler may be fabricated is
characterized in part by a photosensitive cladding which may be manufactured
using
known fabrication procedures by doping the cladding region at least partially
with a
photosensitive species while maintaining the waveguiding properties (i.e., the
N.A.) of
a standard single mode core fiber. The goal of the deposition processes for
use in the
present invention is to dope a significant volume fraction of the cladding.
The farther
the dopant (e.g., Ge) extends out along the radius of the fiber, the more
photosensitive
the resulting coupler waist will be after the fusion and elongation stages. It
is also
important that the fiber be doped in a manner that minimizes thermal stress
and
material property mismatch within the doped cladding.
WDM systems enable multiple wideband signals to be transmitted on a
single optical fiber, provided that individual wavelengths can be precisely
centered at
given values and have narrow bandwidths with high signal-to-noise ratios.
These
properties must be provided by the add/drop filters, and the concept as
disclosed and
claimed in the above mentioned applications have special advantages in these
respects. However, the technical requirements are so critical, as is described
hereafter,
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that production of units in quantity at low cost without the need for
instrumentation,
testing and burning-in at each stage, presents formidable challenges.
As described in the previous applications and seen in Fig. 1, the
add/drop filter, also referred to as a coupler 10, has a narrow waist 12
formed by
elongation from optical fiber precursors. The waist 12, which is in the range
of 2-3
cm long, comprises a pair of locally dissimilar, longitudinally merged fibers
14, 15
forming a merged region typically less than 10 microns in cross sectional
dimension.
Specifically in this example, the waist region 12 is a hybrid dumbbell-
ellipsoid in
cross-section, here having a major dimension (A) of 10 microns or less and
with a
minor (B) dimension that provides a 0.$2 ratio between the axes. The hybrid
dumbbell-ellipsoid (Fig. 2) is a shape having characteristics resembling a
cross
between a dumbbell shape and an ellipsoid. This shape also has a transverse
asymmetry best characterized as a "peanut" or "pear" shape. The asymmetry is
the
result of the initial pretaper. The smaller fiber 15 in the waist 12 is
prestretched
before elongation and merging so that it is about 20% smaller (in this
example),
although the relative size can vary within a range of 10-30% or more. Where
the
facing sides of the fibers 14, 15 are fused and merged they introduce
irregularity into
the ellipsoid and retain the asymmetry of the original fibers. The waist
region 12 is
preferably elongated without twist to prevent the loss of the reference plane
defining
the centers of the original cores, now only vestigial in character.
Maintaining this
reference plane in the prepackage before exposure is essential to producing
the proper
grating tilt asymmetry.
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At each end of the waist 12 the fibers extend outwardly in a divergent
taper 2-3 cm long along separate tapered coupling branches 20, 21 and 24, 25.
This
taper is adiabatic and transitions from the small diameter waist region 12 to
the much
larger single mode optical fibers (not shown) which have diameters of the
order of 90-
125 microns. These fibers have metallized outer surfaces (not shown) suitable
for
soldering the coupler to the prepackage and precisely and stably maintaining
coupler
tension after final packaging. Within the waist region 12, a Bragg grating 27
is
recorded that is of selected periodicity suitable for the chosen drop
wavelength, and
the grating planes are tilted (typically 3°-5°) with respect to
the larger of the transverse
dimensions of the waist 12. A mufti-wavelength input propagating along one
branch,
e.g., the first tapered coupling branch 21 into the waist region 12 is
selectively filtered
by the Bragg grating 27, which couples only the drop wavelength into the
second
tapered coupling branch 20 and the other fiber 30.
In accordance with the invention, the modal relationships, dimensions
and properties of the coupler are selected and modified such that a number of
advantageous properties are concurrently achieved. Referring now to Fig. 2,
the
reduced diameter waist sections 14, 15, derived from precursor fibers are
doped to be
photosensitive (8 mol % germanium is suitable) in the original cladding region
surFOUnding the small higher index of refraction core and have only minute
vestigial
cores after elongation. Energy is thus confined and propagated in what may be
called
an air-glass waveguide, the term "air" here meaning the surrounding
environment
about the physical fiber, whether air or some other medium. Some
characteristics of
such an air-glass waveguide include a large numerical aperture and multimode
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waveguiding properties. The radial extent of the field outside the fiber is
represented
by the dashed line 17.
Within the air-glass waveguiding region of the waist (Fig. 2), the
orthogonal optical modes completely occupy and overlap the internal volume of
the
adjacent fiber 14 or 15, regardless of whether the light originated in fiber
3I or 30.
Because of this complete mode overlap, when a grating is impressed within the
waist
region, the coupling is "non-evanescent", since the modes completely overlap
with the
grating. Note that the optical mode originally associated with a particular
fiber is not
localized within that original fiber region in the coupler waist. The modes in
the waist
are no longer waveguiding in their original fiber material alone.
The air-glass waveguiding property of the coupler waist leads to unique
optical characteristics. First, all lossy cladding modes are eliminated.
Unlike the
precursor optical fiber, whose cladding-air interface also serves as a
waveguide, the
coupler waist has a new uniform cladding (air) that does not support secondary
guiding. The waist supports multiple optical modes, but their number decreases
as the
diameter decreases. However, a very small waist diameter reduces the number of
higher order modes that degrade the short wavelength transmission of this
device.
These modes are guided in the waist region, yet they escape from the fiber in
the
adiabatic transition regions of the taper sections and contribute only to
background
loss at particular wavelengths. In addition, by proper tilt asymmetry of the
grating, the
coupling strength to these higher order modes can be dramatically reduced or
suppressed entirely.
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These characteristics become clear upon analyzing the mode diagrams
of elliptical cylinders representative of coupler waists, depicted in Fig. 3.
Each curve
represents one particular mode supported by the waveguide. Figure 3 [taken
from
Lewis, J.E. and G. Deshpande, "Modes on elliptical cross-section dielectric
tube
waveguides", Microwaves, Optics, and Acoustics, Vol. 3, 1979, pp. 147-155]
illustrates the normalized propagation constants for modes of a coupler waist
with an
ellipticity of 0.9 (i.e. greater than the present coupler example of 0.82).
The top figure
(A) illustrates the odd modes, and the bottom figure (B) illustrates the even
modes.
The horizontal axis corresponds to the V number of the waveguide, and the
vertical
axis corresponds to (3/(30 =n~ff, equivalent to the modal index of refraction
of the
individual optical modes of the waveguide.
The waveguide characteristics may be expressed in terms of different
mode expressions. For example, the LPo, (linearly polarized) mode is
equivalent to a
linear combination of the even and odd HE" modes, and the LP" mode is
equivalent
to a linear combination of the even and odd EHoi and HE2, modes. LP mode
descriptions assist in the analysis of polarization behavior. The mode
evolution
properties of elliptical waveguides are more amenable to an LP mode
description.
The slope of these characteristic curves is a measure of the effective
mode index sensitivity to diameter variations. The greater the sensitivity,
the greater
the chirping or broadening of the Bragg grating due to a given magnitude of
diameter
non-uniformity. For smaller diameter couplers (smaller V's) the slope
increases and
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the diameter sensitivity increases. That is, smaller diameter couplers have
more
challenging diameter uniformity requirements to achieve a narrow spectral
bandwidth
grating. In addition, the separation between effective index for the LPo, and
LP, I
modes increases, corresponding to a larger wavelength separation between the
drop
and backreflection wavelengths (which can be important, as noted below). The
separation between these modes and all the additional higher order modes also
increases, ensuring that the lossy peaks associated with coupling to higher
order
modes are pushed out of the spectral region of interest (e.g., the 1530-1560
erbium
doped fiber amplifier (EDFA) window).
Unlike fiber gratings, there are no Iossy cladding modes which
contribute to losses in grating assisted mode couplers, because the actual
cladding
material of the coupler (typically air) does not have a secondary waveguide
structure
which supports additional optical modes. Only the doped silica coupler waist
supports optical propagation.
The grating assisted mode coupler reflects light at a particular
wavelength from one fiber back into the same fiber (the backreflection), and
reflects
light at a different wavelength from one~fiber into the other (the add/drop).
The
addklrop response leads to the desired wavelength routing of light from one
fiber to
another, while the backreflection response is usually undesirable. Therefore,
the
wavelength at which the backreflection occurs should lie outside the operating
wavelength region of the add/drop filter. For example, for dense WDM
applications
in the 1530 to 1565 wavelength range, the backreflection wavelength should be
either
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below 1530 nm or above 1565 nm, or lie at a wavelength between the active
wavelength channels. The backreflection/drop wavelength splitting should be 18
nm
or more.
This wavelength splitting is readily achieved by making the waist of
the add/drop coupler sufficiently narrow (< 7 microns) such that the
wavelength of the
backreflection is far from the drop wavelength. By fabricating fused couplers
with
small waists, the difference between the modal propagation constants of the
LPoI and
LPG, modes increases. Therefore, the wavelength splitting of the drop and
backreflection also increases. This wavelength splitting is in excess of 15 nm
for an
elliptical cross section waist with a major axis of approximately 3.5 microns
using a
specialty doped starting fiber. Further reduction in the coupler waist
diameter leads to
a further increase in wavelength splitting. The exact relationship between
waist
diameter and wavelength splitting depends strongly on the physical shape and
the
exact index of refraction profile of the coupler waist. A general rule would
be to
make the waist smaller than 5 microns. However, the required uniformity of the
coupler waist diameter becomes increasingly stringent as the waist diameter
decreases; therefore, the waist diameter is usually selected to be that
diameter which
gives a backreflection/drop wavelength splitting slightly larger than 15 nm. A
given
add~drop filter has a backreflection peak on either the short (for pretapered
fiber input)
or long (for non-pretapered fiber input} wavelength side of the drop peak.
For many telecom applications of add/drops, such as
multiplexers/demultiplexers, this drop/backreflection wavelength splitting
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requirement is substantially relaxed to a splitting on the order of a WDlVi
channel
spacing (0.8 nm or 1.6 nm, for example). Therefore, larger splittings are not
be
necessary if the wavelengths are demultiplexed from the fiber in a sequential
manner
(shorter wavelengths to longer wavelengths, for example). Even though the
longer
wavelength devices have short wavelength backreflections, those channels at
these
wavelengths are already extracted from the fiber by the previous add/drops.
Thus for
these units, this waist diameter may be larger, reducing the diameter
uniformity
tolerance of the coupler.
For most telecom applications the polarization properties of the coupler
are important. Optical fields are vectorial in nature; that is, they have
direction. This
direction is quantified by the state of polarization of the optical field. The
polarization
of an optical signal may be linear, circular, elliptical, or unpolarized. Two
linearly
polarized optical signals are othogonally polarized if the electric field
vectors lie
perpendicular to one another. For example, the LPoi and LPt, modes can be
substantially polarized along the x and y directions, where x and y are the
major and
minor axes of an ellipse.
The grating assisted mode coupler can readily exhibit a polarization
dependence of the wavelength of light coupled from the input fiber to the
drop. This
polarization dependence is due to the form birefringence of the coupler waist
in the
region of the Bragg grating. In general, the modal propagation constants (3
within the
waists of fused couplers, for light in the two orthogonal polarization states,
are not
equal. Referring to Fig. 4, it can be seen, by referring to the two dotted
line peaks,
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that a wavelength separation exists between the two orthogonally polarized
modes
under these conditions. However, for certain cross sectional shapes and index
of
refraction profiles of the waist, the polarization dependence vanishes (i.e.,
I(3LPo~,XI +
I~iLP",xl = I(3LPo,,yl + I~LPI,,yI). Note that the left and right sides of
this equation are
equal, even though individually I~iLPo,,xl is not equal to i(3LPot,yl and
I(3LP> >,XI is not
equal to I(3LPl,,yl. In fact, counter-intuitively, the polarization dependence
of the drop
wavelength of a coupler waist of circular cross section does not vanish,
because the
polarization degeneracy of the LPl ~ mode does not vanish for a circular
waveguide
(that is, I(3LP, l,xl ~ 1~3LPi i,yl for a circular waveguide), while the
polarization
dependence of the LPo~ modes does vanish (I(3LPo,,xl = I[3LPoi,yl).
One waist cross sectional shape for which the polarization splitting
does vanish at the drop channel is the hybrid dumbbell-ellipsoid with a ratio
of minor
to major axes of about 0.8. Alternate descriptors include "pear" or "peanut"
shaped.
Such a waist cross section is achieved when elongating a fused coupler under
tension
by heating it with a highly controlled and repeatable heat source that is
varied in
temperature and exposure time to achieve the desired cross section until the
monitored
polarization characteristics disappear. Examples of suitable heat sources are
well
known in the art and include C02 lasers, gas flames and resistive heaters.
Alternate
waist cross sections have also been designed to eliminate polarization
dependence but
the hybrid dumbbell-ellipsoid is more readily fabricated. It has been
demonstrated
that polarization dependence can be reduced to « 0.1 nm by manufacturing the
coupler so that its waist has a precise amount of shape asymmetry, as with the
preferred elliptical shape. The present add/drop filter has been fabricated in
a manner
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that ensures that the polarization splitting of the add/drop wavelength is
less than 0.05
nm. The operation of such a device is then essentially polarization
insensitive for
gratings of FWHM bandwidth a few times the polarization splitting, or about
0.2 nm.
The optical transmission spectra are then independent of the polarization of
the input
signal. Under such conditions the orthogonally polarized modes merge into the
single
drop wavelength, as shown in Fig. 5.
Alternately, if any polarization dependence of the fused coupler
remains, it can be dramatically reduced by twisting the fused coupler waist in
the
region containing the grating, as shown in Fig. 4. As another alternative, for
suitable
UV grating writing conditions (e.g., polarization and intensity), polarization
dependence can be trimmed out by the UV exposure. The UV exposure process
produces material birefringence within the glass that can compensate for the
form
birefringence of the coupler waist.
The response of a reflection grating in a coupler waist is often
undesirably "chirped" or spectrally distorted if the diameter of the waist is
non-
uniform, because the local propagation constants and drop wavelengths vary
with
changing diameter. Therefore, a grating of constant period within a non-
uniform
waist will have a broader spectral width than a grating of constant period
within a
uniform waist. A grating with a 1 Angstrom spectral width requires that the
variation
in diameter be less than 0.01 microns for a 5 micron cross sectional coupler
waist over
that region of the waist containing the grating. Similarly, the shape of the
coupler
should be constant over this region containing the grating to prevent
additional chirp
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and polarization dependence. A highly uniform heat source such as a
reciprocating
C02 laser or flame can be applied to give highly uniform coupler waists.
Alternately,
grating chirp due to small variations in the diameter of the coupler waist on
the grating,
response can be substantially reduced or effectively eliminated, in accordance
with the
invention, by local C02 laser heating to correct diameter variations, by
locally varying
the grating period impressed in the coupler waist to maintain a constant Bragg
wavelength, or by laser trimming of the background d.c. index of refraction
along the
grating. These techniques effectively reduce spatial variations in the local
Bragg
wavelength of the grating along the coupler waist, as illustrated in Fig. 6.
This
confronts one of the key issues in the manufacture of narrow bandwidth
add/drop
devices, such as those required for 100 and 50 GHz WDM systems based on
grating
assisted mode couplers. To determine the non-uniformities of diameter in a
manner
that can be scaled up to a manufacturable process, they can be directly
measured by
scanning electron microscopy, atomic force microscopy, by reconstructing the
index
profile from the complex reflectivity profile, by measuring the local amount
of UV
induced fluorescence, by examining the position and wavelength dependence of
reflectivity, or by analyzing the spectral and spatial characteristics of
light scattered
transversely off the coupler waist..
~ In addition to producing uniform gratings within fused couplers,
precisely apodized gratings are necessary to reduce grating sidelobes and
eliminate
adjacent channel crosstalk. Apodized gratings are key to meeting the
performance
requirements of WDM systems. Apodization is understood to have been achieved
by
*rB
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several methods, including variable speed scanning, dithering of the phase
mask and
apodized phase masks.
An apodized grating can be written by spatially varying the modulation
amplitude or a.c. component of the index of refraction in the longitudinal
direction
along the grating. At the same time, however, the d.c. or background index of
refraction must remain extremely uniform (variations less than 0.0001) to
prevent
undesirable chirp or broadening of the grating. Raised cosine (cos2(z)),
sinc2(z), and
Gaussian (exp-z2/2az) apodization functions are all effective in reducing the
grating
sidelobes to below -30 dB.
An apodized grating exhibits a longitudinally varying index of
refraction modulation amplitude as well as a uniform period pattern along the
waveguide. That is, the grating is gradually (over a large number of grating
periods
>1000) turned on and then off along the light propagation direction. This
smoothly
varying window function reduces the spectral ringing or sidebands resulting
from
gratings with a sharp, rectangle window function. In general, the frequency
spectrum
of the filter is the Fourier transform of the spatial window function of the
filter. A
superior method to achieve apodization is to use a scanning exposure, in which
the
contrast of the optical interference pattern is varied as the grating is
recorded while the
total incident intensity is contrast. To achieve this, the waist region is
simultaneously
exposed with a d.c. beam counter-propagating with the modulated a.c. beam
while the
interference pattern is imprinted. The sum of the intensities of the
interference pattern
and the uniform beam are kept constant, eliminating undesirable chirp arising
from
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variations in the background index of refraction. As seen in Fig. 7, the index
of
refraction function for the cost apodized grating is thus an apodized periodic
wave
varying in bipolar fashion about a center line over a grating length L and is
given hy:
~)t(Z) _ ~11~ (Z)Cl + Sln k8 Z COSZ ~~
The intensity of the a.c. beam is:
1 (z) = l o (sin k8 z + I) cost nz l L
and the intensity of the d.c. beam is:
I(z) =1" sine ~cz l L .
Important advantages of the invention also reside in the features included in
the
example of Figs. 7-10, to which reference is now made. The add/drop coupler
device
50 comprises a cylindrical housing 52 of stainless steel tubing that has a
0.270" OD
and a length of 3.67" and what may be termed a "prepackage" or support
structure 54
internal to the housing 52. The prepackage structure 54 is inside the housing
52 after
assembly but used as a preliminary retainer to hold in the optical fiber
coupler 53
precisely during processing steps in which the grating is written and
adjustments are
made. The prepackage structure 54 extends longitudinally along and within the
*rB
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housing 52, and centrally supports and retains the optical fiber coupler 53,
in position
along the approximate central axis.
In an initial assembly the opposite ends of the optical fiber coupler 50
are fixed to spaced apart brass end hubs 58, 59 on a pair of parallel invar
rods 62,63
that extend along the majority of the inside length of the housing 52. For
solderability
and freedom from contamination, the hubs 58, 59 and rods 62, 63 are nickel
plated,
preferably by an electroless process, as are the other elements within the
housing 52.
The prepackage structure is completed by interposition between the
end hubs 58, 59 of a pair of spaced apart base hubs 66, 67, one of which is
proximate
to the end hub 59 and is soldered or welded to the first invar rod 62. The
other base
hub 66, on the second rod 63, is adjacent a reference hub 69 on the first end
hub 58
side, also on the second rod 63.
In the prepackage assembly and adjustment phase, there are two
subassemblies of rods and hubs, longitudinally slideable relative to each
other. One
subassembly comprises the first invar rod 62, the first end hub 58 and the
second base
hub 67, each hub being fixed in position on the rod 62. The other assembly
comprises
the second invar rod 63, the reference hub 69, the first base hub 66 and the
second end
hub 59. Engaged in this way, the whole assembly may be mounted in a fitted jig
or
tray (not shown) with the waist region of the optical coupler 53 being open,
for
writing of a Bragg grating, to an optical system mounted on the side.
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The optical fibers in the coupler 53 diverge from the waist region at
each end to where they are fusion spliced to metaIlized optical fibers of
standard
dimension. The entire optical coupler 53 extends along the approximate central
axis
of the housing 52, passing through radial slots 70 provided in each of the
hubs 58, 59,
66, 67 and 69. When the prepackage structure 54 is held rigidly in place in
its
positioning tray, the coupler 53 can then be soldered at its end regions to
the central
regions of the spaced apart end hubs 58, 59. The waist region is thus stably
configured for photosensitization and grating writing steps.
When a grating is written in the waist region with a selected periodicity
its drop wavelength must be adjusted to sub-nanometer precision and this
wavelength
should be essentially constant over the required operating temperature range,
normally
-35°C to 85°C. Although invar has a very low temperature
coefficient, it alone cannot
meet the athermal. The prepackage requirements are that the separation between
the
end hubs 58, 59 decreases as temperature is increased. This decreases the
tension and
resulting strain ~ within the coupler waist with increasing temperature T is a
manner
that satisfies the following equation:
as _ng ~e~ ang
aT - _ n~~ aT _ aT
For Ge-doped silica glasses, the primary contribution to the temperature
dependence
arises from the first term on the right of the above equation; that is, the
temperature
dependence of the effective index of refraction n~~ . The second term on the
right, the
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thermal expansion contribution to the change in grating period, is typically
an order of
magnitude smaller than the first term.
Note that the base hubs 66, 67 include aligned longitudinal grooves 72,
73 respectively, in their peripheries, and that an adjustment screw 75 extends
through
the reference hub 69 and the first base hub 66. Consequently, after first
adjusting the
end hubs 58, 59 to tune the grating, the wavelength is locked in by soldering
a
stainless steel rod to grooves 72, 73 in the base hubs 66, 67. This inner
structure
within the prepackage establishes an interior length which has a different
thermal
coefficient of expansion than the invar rods 62, 63 which are seated at each
end but
not otherwise spatially defined except through the interior stainless steel
connection.
Each base hub 66, 67 is coupled to a different invar rod 62 or 63 respectively
but has a
different spacing along that rod from the end hub 58 or 59 which determines
the
grating periodicity.
With the stainless steel rod 80 in place, the unit can be inserted into a
temperature controlled chamber and cycled through the required temperature
range
while reading the drop wavelength with optical spectrum analyzer
instrumentation.
To adjust the drop wavelength so that it is the same at 25 °C as 85
°C, the adjustment
screw 75 is threaded inwardly or outwardly relative to the reference hub 69.
As best
seen in the fragmentary view of Fig. 14, the screw has a first short thread 76
mating in
the reference hub 69, and a terminal second thread 77 mating in the first base
hub 66.
By turning the screw 75 at the screw head 78, the screw engagement point with
reference hub 69 can be shifted longitudinally, increasing or decreasing the
length of
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one invar segment and increasing or decreasing the length of the stainless
steel
segment.
When the prepackage structure 54 including the optical coupler 53 is
adjusted, it is removed from the holder or tray and inserted into the
cylindrical
housing 52. The prepackage 54 is fixed in position relative to the housing 52
simply
by crimping the housing 52 (see Fig. 7), onto the second end hub 59. End caps
82, 83
with central bores 83 providing openings for the fibers are engaged into the
housing
52 open ends, and soldered or welded into place. The outwardly extending
fibers at
the exit points are soldered to the end cages 82, 83 to produce hermetic
seals.
Preferably, for longer life, the housing 52 is filled with an inert gas before
the housing
52 is sealed. The outwardly extending fibers are protected against kinking and
strain
by shrink fit tubes 87, 88 of a suitable length.
To reduce the propagation of cracks within the coupler waist and
failure of the coupler after packaging, the coupler should be hermetically
packaged.
The presence of water within the package can lead to coupler failure. This
problem is
exacerbated when the coupler is packaged under tension, which is necessary to
provide a temperature insensitive mount, for example. Preferably, the coupler
is
packaged either in argon, helium, nitrogen gas or a mixture thereof, or in
vacuum.
A device similarly packaged can be rendered tunable by the addition of
a active tuning mechanism. We have disclosed in US 08/703,357 the design of a
low
loss, all-fiber optical switch using two addldrop filters whose add and drop
ports are
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joined to one another. If the drop wavelength of the two filters coincide,
light from a
first fiber will be routed to a secondlfiber. If the drop wavelength of one of
the filters
is de-tuned so that the two drop peaks no longer coincide, then the light will
return the
fast fiber. Thus, a light signal can be switched from one fiber to another.
One
mechanism to instantaneously de-tune an add/drop filter is to launch a high
intensity
switching beam down the coupler waist. If the beam is of sufficiently high
intensity,
the index of refraction of the waist can change almost instantaneously (on the
order of
fsec) through the Kerr effect. By modulated the switching beam, light can be
rapidly
switched from one fiber to another. This technique does not required precise
control
of the intensity of the switching beam. It only requires that the intensity be
sufficient
to de-tune the two filters. Since silica glass used in the manufacture of
optical fibers
does not display large optical nonlinearities, relatively high optical
intensities are
required. However, other optical materials, such as specially doped silica,
electrically
poled silica, crystalline or polymer materials are promising candidates for
this
application. Another mechanism to switch an add/drop filter is to surround the
coupler waist by a gas, liquid, or other substance whose optical properties
can be
changed by the application of an electrical, magnetic, or optical control
signal. One
example is a liquid crystal, whose index of refraction varies upon application
of an
electric field.
Although there have been described above various forms and
modifications it will be appreciated that the invention encompasses all
variations and
expedients within the scope of the appended claims.
