Note: Descriptions are shown in the official language in which they were submitted.
~2~
This inven-tion rela-tes -to fiber optic communica-
tions systems, and particularly to a wavelength divi~ion
multiplexer/demultiplexer comprising a novsl thin-film
waveguide lens.
In a f`iber optic communication system, each fiber
typically carries light of a particular wavelength. Existing
multiplexing technology allows hundreds and thousands of
sign~ls to be sent on this wavelength through a single fiber
through time multiplexing. However, when the upper limi-t is
reached a separate fiber has -to be installed in order to
satisPy heavier traf-fic demands. Instead o~ increasing -the
number of fibers, an additional wavelength may be used in one
~iber. Wavelength clivision multiplexers and demull::ipLexers
are the devices which allow this to be achieved. At the
transmission end, signals are fed into one single fiber by
means of multiplexsrs. At the receiver end, the signals are
descrambled into groups, each belonging to a separate
wavelength, by a demultiplexer.
The wa~elengths currently used commonly in optical
communications are in the 800 nm and 1300 nm regions. A
third potential region is the 1500 nm one. Usually
commercial demultiplexers separate only two very different
wavelengths. However, as the technology of light sourcas and
laser ligh-t sources in particular improves, it will become
possible to use many more different wavelengths to incrsase
the signal-carrying capacity of each fiber. Multiplexers and
demultiplexers are needed to effectively handle such
increassd numbers of wavelengths.
51~
An additiona:L technology related to this invention
is integrated optics, i.e. the guicling and manipula-ting of
ligh-t through thin-film optical waveguides. Among -the grow-
ing number of devices being developed for in-tegrated optics,
the op-tical waveguide lens remains the most basic. AD opti-
cal waveguide lens performs a variety of important functions,
including focussing and collimating, Fourier transforma-tion,
imaging, spatial filtering, and the integra-tion of guided-
optical beams. For some types of mul-tiplexers/demulti-
plexers, a lens is a crucial element of the device.
There are many important design criteria -for an
optical waveguide lens. The position of the focal plane,
focal spot size and its in-tensity profile, angular f:ield of
view, the energy in the sidelobes relative to the energy of
-the centraL lobe, and the -throughput losses must all be
considered. ~qua]ly important, the Eabrication techniques
should be simple, inexpensive and compatible with present
technology. Each of these factors mus-t be considered in the
application of the optical waveguide lens in a wavelength
division multiplexing/demultiplexing device.
Presently-marketecl wavelength division multi-
plexers and demultiplexers nearly always use thin-film
filters. When more than two wavelengths, say n, are used,
-the use of thin-film filters is clumsy, because n-1 filters
must be used in a cascaded structure. Such devices are
expensive -to fabricate, and suffer from signal attenuation.
Grating devices can be used as an alterna-tive tv
thin-film filters. The use of a grating to separa-te wave-
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leng-ths is of course well known in spec-troscopy, a~d is also
known in optical signal ~ultiplexing and demul-tiplexing from
Canadian patent no. 1,089,932, described 'below. Another
method of separating wavelengths is in the use of prisms,
which ilowever are not commercially practical.
One specific type of multiplexer/demultiplexer
employing a -thin-film lens is that described in Canadian
patent no. 1,089,932 granted to Northern Telecom Li~ited on
November 18, 1980, which employs a collimating lens to
collimate the light from an optical fiber, a diffraction
/reflection gra-ting, a focussing lens, and an array of ligh-t
de-tectors and sources.
As this device shows, all grating devices require
means to collimate the light rays for arrival at the grating,
and means -to focus the rays subsequently. Thus i-t is
advantageous to simplify the design of the lens element, and
optimize i-ts collimatingr and focussing properties.
A two-dimensional thin-fil~ optical waveguide lens
can be used as the means for effectin this collimating and
focussing. Three types of -thin-fil~ optical waveguide 1ens
have been proposed and demonstrated for use with guided
optical beams: mode index lenses such as thin-film Luneburg
lenses, geodesic waveguide lenses, and Fresrlel diffraction
lenses. D.B. Anderson et al, "Comparison of Optical
Waveguide I,ens Technologies", J. Quant. Electron., Q.E.-13
275 (1977~. Each of these lenses utiliæes a localized change
in the optical wave guide: an index gradient, a spherical
depression, and a grating respectively, -to alter the
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~625~3~
wave~ ront curvature and ob-tain the desired focussjlng effec-t.
This loca]iæed change in the waveguide structure contributes
to losses by introducing scat-tering and mode conversion at
the ]ens edge.
Amongst these various lens types, the thin film
Luneburg lens is the most practical for multiplexing/
demultiplexing applications with its potential -for reduced
losses and scattering. This lens is fabricated by
depositing the lens material through a circular mask. This
deposition process is the critical step, requiring the
achievement of a par-ticular thickness pro-~ile with radial
symme-try. If the exact thickness profile is achieved, the
circular lens wi:Ll exac-tly focus a collimated beam to a point
on the focal curve, or conversely col]imate a poin-t source
located on thi~ ocal curve. Such a lens can be constructed
with materials having a uniform refrac-tive index. S.K. Yao;
et al., "Guided-wave optical thin-film Luneburg lenses:
Fabrication technique and properties", App. Optics, 18, 4067
~1~79). It can also be constructed with a layer having a
graded index profile, as described by United States pate~t
no. 4,4~,063.
One disadvantage of the Luneburg lens :is it~l
circular shape and the resulting curved image and object
planes. It is di~ficult to match the curved focussing plane
to the endplane of a rectangular planar optical waveguide
without introducing aberrations. The circular shape
necessitates a region surrounding the lens with a constant
thickness, and thus constant re~rac-tive index, to carry -the
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guided ligh-t from the circular lens edge -to the planar
waveguide edge. The transition area from lens to
surrounding planar waveguide is a source of loss, due to
scat-tering a-t the index discontinuity n
This disadvantage is observed in other devices
which utili~e a circular thin-film lens in a rectangular
waveguide. In the multiplexer~demultiplexer previously
described in Canadian patent no. 1,089,932, we observe a
thin-film lens wi-th curved edges focussing onto the straight
endplane of a rectangular waveguide. Similarily, in United
States patent nos. 4,253,060 and 4~348,074, a circular
thin-film lens is utilized to focus a beam to a poin-t on a
straigh-t edge. Thus both the~e devices suf:fer from the
disadvantages mentioned above.
It i8 an object o-~ -the present invention -to
provide a thin-film waveguide lens *or use in a wavelength
division multiplexer/demultiplexer -to overcome some o-f the
problems and o-ffer advantages over the wavelength division
multiplexers an~ demultiplexers and thin-film lenses in the
prior art.
Thus in accordance with one aspect of the present
invention there is provided a rectangular thin-film
waveuide lens for use in a multiplexer/demultiplexer,
comprising a thin-film waveguide having a longitudinal axis
and having` end planes essentially normal to the axis a-t each
end o* the waveguide. A plano-convex overlay layer integral
with the wave~uide extends the length of the waveguide along
the axis, and is symmetric about this axis, the profile of
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the overlay layer being selected so as -to produce a,graded
effective refractive index in the lens in order to collimate
focussed rays and focus collimated rays en-tering at one end
plane for substan-tially collimated or focussed arrival at
-the other end pla-ne.
In accordance with another aspect of the
invention, there is provided a multiplexer/demultiplexer
comprising the above thin-film waveguide lens, in which one
of the end planes constitutes an en-trance/exit plane for
ld receiving optical f'ibers in an abutting connection, and the
other end plane 'bears a diffraction~reflection grating.
Focussed rays entering the lens at the entrance/exi-t plane
are substantially collimated for arrival at the
diffractioll/reflection grating, and rays rerlec-tillg from -the
diffraction/reflection grating are substantially focussed on
returning to the en-trance/exit plane, the locations -for
abuttment oP the optical -fibers being matched to the spot
locations of the focussed rays along the entrance/exit plane.
In accordance wi-th the prePerred embodi~ents of
the above aspec-ts of the invention, the shape of the
plano-csnvex overlay layer is such as to provide an
ef-fective index of re-fraction profile in the waveguide
according to the formula n = nosech(gx), where n is the
efPective refractive index at the dis-tance x from the axis
of the waveguide, no is the eff~ctive refractive index at
the lens axis, sech is the trigonometrical hyperbolic secant
function, and g is a constant (equal to ~/2f, where f is
the focal length of the lens).
.
~12~6J2d5~30
Further *eatures nf -the invention will be
described or will become apparent in the course of the
following detailed description.
In order that the invention may be more clearly
understood, the preferred embodiment thereof will now be
described in detail by way of example, with raference to the
accompanying drawings, in which:
Fig.l is a top view of the preferred embodiment of
the multiplexer/demultiplexer, depicting in representational
-form the case of two wavelengths;
Fig.2 is a view of the en-trance plane A--A' of the
lens of the multiplexer/demultiplexer;
Fig.~ is a cross-sec-tion at the end plane B-~' of
the lens;
Fig.4 is an oblique view of the entrancs pl1}le of
the multiplexer/demultiplexer;
Fig.5 is a graph which shows the thickness profile
of the overlay layer required for a lens with a focal length
of 5.0 mm; and
Fig.6 is a graph which sho~s the expected focal
spot si~e as a function of wavelength for light signals
passing through a multiplexer/demultiplexer with the profile
of Fig.5.
Referring to Figs.l-4, there is illustrated the
multiplexer/demultiplexer I of the preferred embodimen-t.
For convenience, the device will be treated as a demulti-
plexer in this description, although as will hecome
apparent, the device can operate in either roleg depending
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simply on the direction of the signals.
An input fiber Fl feeds signals carried by
different wavelengths in-to the device at an entrance/exit
plane A-A'. Fibers F2 and F3 are representative ou-tput
fibers, each carrying demultiplexed or wavelength divided
signals of a dif-ferent waveleng-th. In practice, there of
course may be more than two wavelengths, and there will thus
be more -than the two representative output fibers F2 aDd F3.
Due to the current limitations of light sources, it is not
likely that more than about ten output -Pibers would be
involved at present, but as laser sources in particular
improve, there could be many more output fibers
corresponding to differen-t transmission wavelengths.
The device consists o~ a uniform flat substrnte 21
of cons-tant refrac-tive index on which is a uniform thin-~ilm
waveguide 3 of constant refractive index, on which is an
overlay layer 4 of constant refractive index and ~ith a
carefully selected relief profile. The relief pro-Pile is
determined, in accordance with the properties of the
thin-film ~aveguide, so as to produce the ef~ective
refractive index distribution n = nosech(gx).
The overlay layer 4 may be of the same material (a
three-layer case, including the overlying medium, usually
air), or of a different material (a four-layer case -Prom -the
material of the wave~uide portion. The optical fibers Fl, F2
and F3 are butt-joined to the uniform thin-film wavegllide
portion 3 o~ the entrance~exit plane A-A' of the device.
In the case of an overlay layer 4 o e the same
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material as the thin-film waveguide portion 3, then instead
of vj.ewing the overlay layer and the Naveguide ~ as beill~
separate, they may be properly viewed as being and the same.
For convenience of description, however, they will be
referred to as separate elements -throughout -this
specification. A dotted line is used in Figs.2 and 3 to
indicate the plane of division between -these elements,
whether real or notional.
At the end plane B-B' of the device remote from
the entrance/exit plane A-A' is an integral dif-fraetion/
ref:Lection gra-ting ~. The grating 5 is scribed or embossed
or otherwise prepared on the end plane B-B', and a suitable
thin film is deposited on the grating ~ to turn i-t into a
reflecti.on grating with high efficiency and low losses.
The profi:Le of the overlay :Layer 4 is se:lected so
as to produce a graded effective re:fractive index in -the lens
in order to collima-te -the rays -for arrival at the dif-fraction
/reflection grating 5 and so as to focus the wavelength-
divided light rays leaving the grating at spots along the
entrance/exit plane A-A', corresponding to the locations of
the output fibers F2, F3, etc.. This results in a
plano-convex shape for the overlay layer 4, aligned alon~ the
longitudinal axis of the thin-fi:Lm waveguide 3, as
illustrated in the accompanying drawings, and extending from
the entrance/exit plane A-A' all the way back to the end
plane B-B' with the grating 5. The grating 5 is of course
slightly angled :~rom the longitudinal axis so that the
diffracted/re-flected signals are directed slightly away -from
lQ
2~
the inpu-t fiber Fl, towards represen-tative output -~ibers F2
and F3.
The position of each ou-tpu-t fiber is of cours.e
dependent on the wavelength to be picked up by that fiber.
The ceparation be-tween fibers thus dependci on the separation
between wavelengths. A certain physical separation, as for
exa~ple at least one fiber diame-ter be-tween cen-ters, is
desirable to avoid undesirable cross-talk in the eveDt of
inaccuracies in signal focussing, which are to a certain
extent unavoidable. Ideally, each spot is focussed
essentially entirely within the circum-ference o-f the
corresponding ~iber connection point, so -that there is
essentially no crosstalk, and -the focussing propertles of the
present invention are such tha-t this should be essentially
achievable.
To improve signal focussing, the entrance/exi-t
plane A-A' is ang].ed slightly -from the plane normal to the
axis of the th:in--fi.lm waveguide 3, as can be seen in
exaggerated fashion from Fig.l, based on calculations from
ray-trace data, to provide a flat-plane approxima-tion of the
optimum ~ocal plane for the different -focal points o-f the
~arious wavelengths. Thus ~oth -the end plane B-B' with the
grating 5 and the entrance/exit plane A--A' are preferably
sligh-tly angled. The location of the optimum -focal plane is
naturally dependent on the angle selected for the diffraction
grating 5.
The details of the lens element of the inven-tion
are now discussed. It is known that a GRIN rod lens with an
11
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index gradient according to -the fvrmula n = Dosech(gx)
focusses all meridional rays e~actly. In -the preferred
embodimen-t o* the present invention -this :index distribll-tion
is adapted to the two dimensions of -the thin-film waveguide
3, wi-th the plane of the -thin-film wave guide 3 coincident
with a meridional plane of the GRIN rod lens, the resul-t
being a thin-fi]m lens which focusses all incident rays
parallel to its axis. While other suitable profiles may be
developed, it has been de-termined by the inventors that this
particular profile is quite suitable as a profile for the
overlay layer 4 in the present invention.
To fabricate the index distribution given by the
above-men-tioned equation in a planar waveguide, -the concept
of an effective index is used. Although the bulk refractive
index in a waveguide is cons-tantl changing the thin-film
thickness alters the v010ci ty of a guided optical wave. The
result is that the propagating phase -~ront has a velocity
that is equivalent to bulk propagation in a material having
-the appropriate effective index. In a waveguide constructed
with the appropriate bulk materials and with the properly
shaped pro-~ile or appropriate overlay layer 4, the required
effective distribution can be obtained. This effective index
gradient is equivalen-t to a physically produced gradient
fabricated by varying the bulk index of refraction by such
means as dif-fusion or doping.
To determine ~hether there exist prac-tical
materials for the realiza-tion o~ this lens, the dispersion
curves for propagation in thin-film waveguides were
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examined. For both the three-layer and four-layer cases
there exist Q ~ini~um and m~ximum waveguide thickness
allowed for optimum lens performançe. It was found that for
the three-layer ca6e with an SiO2 substrate a variety of
focal lengths could be constructed with most waveguide
materials. The only restriction occur~ with short focal
lengths ~hen the bulk waveguide refractive index is close to
that of the substrate. Similar construction flexibility is
found for the four-layer cas~; aDd with different substr~te
material~.
The exact waveguide profile re~uired to obtain the
required effective index distribution iB obtained in
accordance with the properties oE the thin-film waveguide.
Sun, M.J. and Muller M.W., "Measuremen-ts of four-layer
i~otropic waveguides", Appl. Optics, 16, 814 ~1977). The
profile is such a~ to produce the effective refractive index
distribution n = nosech(gx). Accordingly, the relief
profile has a thickness profile given by:
T(x) ~ r t -1~ n21x~ - nl2 ~1~2 ta~ (n32-~2~ /2
2~ ~n4 -n21xJJ~ ~ ~ n2-n21xJ J L~ ''A J
~ (n32 _ n2~A3 ~ tan ~ ( n2~r~ - n2~
where:
n(x) = n~sech(gx);
x is the distance from the lens axi~;
13
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ni is -the re-frac-ti-ve index of -the ith layer;
d is the thickness of layer 3i and
g~is the design wavelength
This equa-tion yields the local thickness of the
overlay layer 4, given the distance -from the axis of -the
lens, the wavelength to be focussed and the bulk refractive
indices of -the materials to be used~ In Fig.~ the -thickness
profile T(x) is plotted ~or a typical lens design, whose
parameters are described in Table 1.
TABLE 1
_____ __________________________________________________ ____
Focal Length f ~.Omm
Lens Wid-th 2.Omm
F-number 7.8
nl (air~ 1.00
nz (S iO2 ) 1 . 47
n~ (GeO2) 1.61
n4 (~eO2) 1.61
Wavelength 1.3um
____ _ ________ ____ __~________________________ ________
By ray-tracing through the refractive index distribution
produced by such a profile, it can be shown that
1~
cliffract:ion-limited focussing can be obta:ined for many
focussing and collimating requirements. Ray trace results
for -the lens described above and wavelengths in -the 1.3
mi.cron range are presented in Fig.6, which show that
di.-ffrac-tion limited focussing can be obtained over a
suitable range about the design wavelength. From the ray
trace da-ta, the siæes and positions of the focal spots are
determined. This permits a proper choice of the output
-fiber diameters and the position of the focal plane.
The complete design of the present device can be
accomplished with the abo~e procedures. When -the device
parameters have been determined, as in Table ]., Eor example,
the required profile of the over:Lay :Layer is determined with
the above equation. This completes the lens component. Ray
tracing is -then used to determine such things as optical
fiber placement on the endplanes, angle of the diffraction/
re-.Flection grating and angle of the entrance/exit plane.
Such steps are considered to be routine and within
the ordinary knowledge of those knowledgeable in the field.
This design enables all the wavelengths, which may
be close -together, to be separated with only one lens
element and grating of integral construction. The integral
construction of -the device is particularly advantageou~ since
there are no discontinuities in the path of -the signal, the
same element being used for both collimating and focussing.
The lens structure uses the entire thin-film waveguide to
obtain the collimating and focussing effect~, by virtue of
the effecti~e index gradient which extends from the
,,,,, :L~
en!;rance~exi-t p]ane h--A' to -the diffrac-tion/reflec-tion
grating ~. This struc-ture, in contrast to existing devices
such as those in the above-men-tioned Canadian patent no.
1,089,93?, eliminates two dielec-tric boundaries, be-tween
planar waveguide and lens, and between lens and planar
waveguide. Such prior art devices used isola-ted thickness
varying regions. This resulted in a localized ]ens element
bounded by a planar ~aveguide. Eliminating these boundaries
provides the potential -to minimize mode-conversion and
scattering losses.
It will be readily appreciated from -the above
description that the device can operate either as a mul-ti-
plexer or as a demu:ltiplexer with equal -fac-ility, depending
mere~ly on the direction of the signals. When operating as a
rnul-tiplexer, the signals fro!n the fibers E'2, F3, e-tc. are
combined at the diffraction/reflection grating 5, producing
an output signal which is focussed at the fiber F1.
The lens can be manufactured using existing
techniques used for Luneburg lens construction, using a wide
variety of materials from low loss optical glasses to active
materials such as LiNbO3.
Two methods could conceivably be used to -fabrica-te
such a lens. First, sputtering or evaporation could be used
with appropriate masks. Alternatively, a lens with a
sui-table pro-file could be emboæsed on a dip-coated de-formable
gel film on a subs-tra-te. Subsequen-t heat treatment would
transform the molded gel into an inorganic hard oxide
ma-terial. Each o-f -these techniques requires computer aided
16
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design aDcl a high precision mask or mold. However, once the
mask or mold is made, it can be used to fabricate many
lenses.
It will be extremely important -to keep the
~abricati~n process wi-thin tolerances, since any error in
th~ wavegu:ide thickness will increase aberrations and result
in a corresponding error in the lens focal length. ~owever,
small shif-ts in the focal plane position may be tolera-ted
since the optimum focal plane can be reached through grinding
and polishing of the waveguide edge.
The principal technologies involved are listed
below with a brief` description of their applica-tio~s to the
fabrication of -the different parts oP the device.
1. Fiber cut-ting - I:he eods of each fiber m-lst
be creaved with flat surfaces perpendicular to the f'iber
axis.
2. Polishing - -the two ends oP the integrated
thin-film lens (surfaces AA' and BB' in Figs.2 and 3) and
also the fiber ends mus-t be polished.
ZO 3. Butt-joining - -Pibers have to be very
accurately butt-joined to -the end of -the in-tegrated thin-
film waveguide 3 by a suitable method, possibly by using a
~uitable epoxy or by fusion.
4. Thin-film deposition - the film with an
appropriate thickness must be deposited on -the flat surface
of a substrate. Also, the end sur~ace BB' (Fig.3) must be
provided with an appropriate ~ilm which is used to fabricate
a grating ~. h dipping and baking technique is proposed.
17
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5. ~mbossing - this technique will be used -to
shape the curved surface with a pre-determined rellef
profile on top of the integrated thin-film waveguide 3. I-t
will also be used to produce the grating ~.
6. Conven-tional thin-film deposition - a -thick
film has to be deposited on the grating ~ to turn it into a
re~lection grating with very high e*ficieDcy and very low
losses. A sputtering or a high-vacuum evaporation system may
be used.
This new optical waveguide lens has been analyzed
and its fabrication and focussing properties have been
theoretically examined. It has been found to have
reasonable fabrication tolerances compatible with existing
shadow loasking sputtering techniques and a new emboss:ing
technique. Considerable design flexibility allows lts
construction with a varie-ty of materials. Ray tracing
reveals that di-ffraction limited focussing should be
possible. Also, low f-numbers can be obtained, making the
de~slgn promising for use in miniaturized integrated optic
circuits. The embossing technique will likely be suitable
for mass production, resulting in lower costs when compared
with other existing methods.
I-t should be noted that althou~h in Figs. 2 and 3
the plane of divi~ion between the thin-film waveguide
portion 3 and the o~erlay layer 4 lS shown as be1ng such
tha-t the overlay layer has zero thickness at -the edges of
the waveguide, the plane could be positioned so tha-t the
overlay layer do~es have a de~finite thickness at the wavé
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guide edge. This of course is only relevent irl thelfour-
layer case, since in the three-layer case the division is
notional rather -than real.
It will be appreciated that the above description
relates to the preferred embodiment by way of example only.
Many variations on the invention will be obvious to those
knowledgeable in the field, and such obvious varia-tions are
~ithin the scope of the inven-tion as described and claimed,
whether or not expressly described.
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