Note: Descriptions are shown in the official language in which they were submitted.
CA 02348995 2001-04-25 PCT/AU99/01000
Received 21 November 2000
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Optical Device and Process
Field of the invention
The present invention relates broadly to an optical
device comprising a waveguide and a process for fabricating
the same.
Background of the invention
In optical waveguides it is often desirable to direct
light around bends, for example to reduce the size of
devices incorporating optical waveguides. An inherent
problem is, however, that due to the refractive index
properties of the waveguide and the material surrounding
the waveguide, it is likely that light will be diffracted
out of bends, in particular tight bends, thereby resulting
in what is commonly referred to as bending losses. Such
losses can limit the performance of the device.
The directing of light signals in different directions
would also be desirable in devices where it is required to
confine light to a predetermined path within the waveguide,
for example in optical filter or optical resonator
structures.
Summary of the Invention
The present invention provides an optical waveguide
structure comprising:
- an optical waveguide having a bend and being
formed of a photosensitive material; and
- a grating structure arranged to guide light of a
predetermined wavelength around the bend in the waveguide,
the grating structure comprising W-induced refractive
index variations in the waveguide.
A substantial reduction in bending loss can be
achieved by guiding light around the bend with the grating
structure.
The present invention allows for angular dispersion to
be added to a propagating light signal which can be
controlled by the properties of the grating structures.
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~~~F_»i::,;.;
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2
For example, this can be utilised for dispersion
compensation, pulse shirping, or pulse compressing. This
is because different wavelengths see a different angular
path with respect to the grating structure.
The device may be utilised in complex light
manipulation circuits both in the spectral and time domain.
The grating structure may comprise a chirped grating.
The grating structure may be disposed to direct the
light in a reflection or in a transmission mode.
Because of an angular dependence of the accepted
wavelength in the grating, the device can depend on angular
sweep to isolate wavelengths or signals.
The grating structure may comprise a continuous
grating. Alternatively, the grating structure may comprise
two gratings which mirror each other.
In one embodiment, the grating structure comprises
regions of constant refractive index which extent in the
propagation direction of the waveguide.
The regions may extend parallel to the propagation
direction.
The regions may extend cylindrically parallel to the
propagation direction.
The regions may extend ellipsoidally parallel to the
propagation direction.
The device may further comprise at least one optical
reflector disposed in a direction transverse to the
propagation direction to aid in confining the light to the
path.
The device may comprise two or more grating structures
angularly disposed with respect to each other to guide
light around the bend.
Accordingly, different confinement conditions may be
realised at different boundaries of the waveguide.
The grating structures may be formed by UV-holography.
The gratings may be chirped gratings.
The gratings may be sampled gratings.
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The device may be a filter, a resonator, or a sensor.
In one embodiment, the device is a sensor further
comprising means for measuring an intensity of the light at
a predetermined point along the waveguide for determining
changes in the intensity due to induced changes in
confinement conditions of the sensor.
The changes may be induced by gas molecules entering
the waveguide.
The present invention may alternatively be defined as
providing a method of adapting a photosensitive waveguide
to guide light of a predetermined wavelength around a bend
in the waveguide, comprising:
- using UV light to induce refractive index variations in
the waveguide such that at least one grating structure is
formed, wherein the grating structure is disposed to guide
the light around the bend.
Preferred forms of the invention will now be
described, by way of example only, with reference to the
accompanying drawings, in which:
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Brief Description of the Drawings
Figure 1 is a schematic drawing of a device embodying
the present invention.
Figure 2 is a schematic drawing of a device embodying
the present invention.
Figure 3 is a schematic drawing of a device embodying
the present invention.
Figure 4 is a schematic drawing of a device embodying
the present invention.
Figure 5 is a schematic drawing of a device embodying
the present invention.
Figure 6 illustrates in an isometric view a method of
fabricating a grating confined waveguide embodying the
present invention.
1~ Figure 7 illustrates in an isometric view another
method of fabricating a grating confined waveguide
embodying the present invention.
Figure 8 is a schematic drawing in a cross-sectional
view illustrating a device embodying the present invention.
Figure 9 is shows a plot of resonant angle against
grating period for a grating confined waveguide.
Figure 10 is a schematic drawing in an isometric view
illustrating a device embodying the present invention.
Figure 11 is a schematic drawing in a top view
illustrating a device embodying the present invention.
Figure 12 is a schematic drawing in a cross-sectional
side view illustrating a device embodying the present
invention.
Figure 13 is a schematic drawing in an isometric view
of a resonator structure embodying the present invention.
Figure 14 is a schematic drawing in an isometric view
of a device embodying the present invention.
Detailed Description of the Preferred Embodiments
Turning initially to Fig. l, there is illustrated
3~ schematically a first example embodiment wherein a
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waveguide l, down which light 2 is to be projected,
undergoes a tight bend in the desired path. In the
vicinity of the tight bend, a grating structure 4 is
written. The grating structure 4 effectively has a
photonic band gap preventing the effervescent light 2 from
leaking out and resulting in higher efficiency in the light
coupled to output 5. This results in a substantial
reduction in the bending loss as a result of the
utilization of the defraction grating 4 which in turn
lt~ allows for tighter bends to be formed in the waveguide
structure. The wavelength of the grating 4 can be tuned so
as to match desired frequencies for operation.
Alternatively, as illustrated in Fig. 2, the
grating 6 can be written in a reflection mode so as to
l, provide for reflection of desired frequencies along the
path 7 with losses 8 for those frequencies not having
desired characteristics.
The utilization of the arrangement of Fig. 2 can
be extended so as to provide for wavelength division
2o multiplexing capabilities on a waveguide structure. This
is illustrated in Fig. 3 wherein initial light can be
launched down a waveguide having a number of frequencies
~,1, ~,2, ~.3 coupled out of the waveguide by utilization of
corresponding matched Bragg gratings 12, 13, 14 which
2~ operate so as to filter out the requisite frequencies.
Fig. 9 illustrates a further arrangement whereby
light coupled along waveguide 15 will be coupled to outputs
16, 17 by means of suitably matched Bragg grating 18 having
desired periodic characteristics, matched to the desired
3U frequencies for coupling. The surrounding waveguide
refractive index regions eg. 19 can be tapered to provide
for stronger coupling. Preferably, the splitter
arrangement of Fig. 9 has a Bragg grating coupled such that
50« of the light traverses along each of path 17, 18. This
35 can be achieved for wavelengths twice the Bragg period. Of
course, it is possible to adjust the Bragg period to adjust
the output angle and coupling efficiency.
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Similarly, in Fig. 5 a Bragg grating 20 is
provided for coupling around a bend for light travelling
along the path 21, 22.
In Figure 6, a waveguide 110 in the form of a layer of
photosensitive material has been deposited onto a substrate
112, eg. a silicon wafer having a native oxide layer for
optical isolation of the waveguide material 110.
A UV beam 116 from a UV source 114 is focussed
(through optical elements 118) in the plane of the
waveguide 110. The substrate 112 can be laterally moved as
indicated by arrows 120 and 122 to effect writing of planes
indicated by lines 124 of a first grating I26 of a grating
structure 127, through UV-induced changes of the refractive
I~ index of the waveguide 110.
After completion of the first grating 126, a second
grating 128 of the grating structure 127 is written by
appropriate moving of the substrate 112.
Light of a predetermined wavelength entering the
waveguide 110 at predetermined angles of incidence on the
gratings 126, 128 are confined to a path extending in the
propagation direction 130 in the plane of the waveguide
110. The propagation characteristics of the waveguide 10
will therefore depend on the wavelength of a light signal
131 and an angle 8 under which it enters the waveguide 110.
It is noted here, that in the planar structure
described above the grating confinement is limited to one-
dimension in the plane of the waveguide 110. However, it
will be appreciated that waveguides can be produced in a
photosensitive waveguide material that are grating confined
in two or three dimensions.
For example, as illustrated in Figure 7, holographic
UV grating writing techniques using a phase mask 140 can be
used to produce a waveguide 142 (propagation direction as
indicated by arrow 141) within a block 144 of
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photosensitive waveguide material which is grating confined
in two dimensions through gratings 146, 198 of a first
grating structure 197 and gratings 150, 152 of a second
grating structure 151 respectively.
It is noted that the.one or more of the grating
structures of a device could alternatively comprise a
continuos grating whilst still effecting confinement of
light of a predetermined wavelength entering at a
predetermined angle of incidence on the grating structure.
E.g. the resonator 250 shown in Figure 14 comprises
two continuos grating structures 252, and 259 to effect
channelling of light 256 of a predetermined wavelength
entering the resonator 250 at a predetermined angle of
incidence on the grating structures 252 and 254 around a
t5 ring path 258.
Grating confinement can also be achieved in an optical
fibre, e.g. using a cylindrical grating structure 320
around a guiding core 322 (propagation direction
perpendicular to the drawing plane) of an optical fibre 324
as illustrated in Figure 12. The grating structure 320
effects confinement to a path extending in the propagation
direction of light of a predetermined wavelength entering
at a predetermined angle of incidence on the grating
structure 320.
It will be appreciated by a person skilled in the art
that for a non-cylindrical grating structure confinement
conditions can vary in different radial directions.
The underlying principle of grating confined waveguide
propagation is the Bragg condition. For a ray travelling
in a medium of index n, peak reflectivity occurs when the
wavelength ~, satisfies:
~.=2nABlm (1)
where m is the diffraction order of the grating and 8
is the angle of the ray with respect to a single groove of
the grating. This single equation contains within it the
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_ g _
entire properties of grating confinement such as e.g. so-
called photonic crystal fibres.
Figure 8 shows the plot of resonant angle against
grating period for the wavelength regime 1200-1600 nm for
1st, 2nd and 3rd order grating diffraction. At longer
periods, variations in the resonant angle converge to
within a few degrees, although the effect is largest for
the lst order. The physical interpretation is that for a
large number of wavelengths the incident angle is
approximately the same equating with similar diffraction
properties. Therefore grating confinement will occur over
a large bandwidth for a small input coupling angle at
longer periods under identical launch conditions. Outside
this regime radiation loss will occur.
Other interesting properties are noted. There exist
other regimes of incident angle at which total internal
reflection can occur to enable propagation along the
grating confined waveguide. Light coupled into higher
diffraction orders at much larger incident angles can also
satisfy the Bragg relation, giving rise to higher order
bandgaps. The effective coupling strength is reduced for
higher order mode propagation in these regimes and is
therefore characterised by larger mode areas. Since the
effective index is different, it is possible to have
fundamental-like mode behaviour simultaneously with
different propagation properties. Thus e.g. photonic
fibres have interesting launch regimes which are unlike
conventional effective index fibres. These regimes exist
because there are angular photonic bandgaps at which light
cannot propagate through the surrounding grating cladding.
Further, these bandgaps are robust and do not change much
in angular properties with increasing period and will
therefore be relatively insensitive to bend loss at longer
periods.
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The angular photonic bandgap is described by the
angular reflectivity of the grating. This reflectivity
bandwidth can be extremely small, depending upon the
dimensions of the grating, its coupling coefficient, and
the angle of incidence. For either normal (incident angle,
8 = 90°) or angled incidence, the power reflectivity is
given from coupled mode theory as
K sinh SL z
R=
S cosh SL + i0~sinh SL ( 2 )
where
S = Kz = (~~z
(3)
K is the angle-dependent coupling coefficient for the
grating, L is the length of the grating and 0(3 is the
detuning of the wavevector, defined by
mgr 2~cn
~ sin 8
(4)
Peak reflectivity occurs for 09 = 0 and declines as 06
exceeds the magnitude of K. It is readily shown in grating
confined waveguides that the angular acceptance of the
reflectivity narrows considerably, with deviation away from
near normal incidence (as indicated by the decreasing slope
of Figure 8). Consequently, the higher order photonic
bandgaps will be broader and less spatially selective and
this may have implications for the robustness of singlemode
operation for large input angles. The variation of
detuning 8(03) with angle 8A is easily calculated from
above
S(8B ~ 2~ case
(5}
From this sensitivity to the capture angle it is
possible to vary the angular dispersion significantly by
appropriate selection of the period. Since the angle of
3o incidents are similar at longer periods (Figure 8) the
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propagation constants, and therefore the sensitivity to
capture angle, tend to converge with increasing grating
period - it is therefore possible to achieve a dispersion
flattened profile of the type found numerically.
Note that even for light guided solely under the
effective index picture when the core index is higher than
the surrounding cladding, unless the mode vector has an
angle resonant with that of the grating, light can quickly
couple to radiation modes and leak out. Further, this
intolerance to the mode angle gives rise to the high
spatial selectivity of these angular bandgaps such that
single-moded propagation is robust especially for long
grating periods. The mode profiles that are supported will
therefore resemble the geometric positioning of the
gratings radially around the core region and should differ
from conventional waveguide guidance where such strict
restrictions do not exist.
By recognising the importance of diffraction in a
periodic lattice it is easily shown that grating confined
propagation is readily achieved in so-called photonic
crystal fibres. Further, the associated angular photonic
bandgaps are responsible for a range of phenomena that
distinguish these fibres from conventional effective index
fibres. Extending the applications to resonators made up
of these fibres, very interesting behaviour is predicted to
occur as a result of the strict vector angles of the
propagating modes, including ring-like resonances when the
end reflectors are tilted. The polarisation properties of
such structures may also differ to conventional resonators
and an entire new class of passive and active filters and
resonators are possible.
In Figure 9, a resonator 181 can be utilised for WDM
(wavelength division multiplexing) filtering if the grating
periods (which may be chirped) of gratings 182 and 184 of a
first grating structure 183 and of gratings 186 and 188 of
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a second grating structure 187 a.re carefully selected such
that a ring resonance is different for different
wavelengths and therefore the outputs are spatially at
different points. This is schematically illustrated by
paths 190, 192 and example outputs 199, 196. The grating
structures 183 and/or 187 may be sampled grating
structures.
Complex design with the use of sampled profiles etc.
can be used to achieve WDM operation. In particular the
to angular dependence means that it may be possible to get
much more closely spaced peaks with higher contrast than
conventional normal incidence. It is noted that this is
also applicable to fibre (e. g. photonic crystal fibres)
geometries.
As illustrated in Figure 10, in a resonator laser
design 300 a photonic crystal fibre 302 is located in line
in a ring laser 304 (of any sort) to improve both
linewidth, laser stability and mode selectivity (including
transverse if mufti-mode active fibre is used to increase
power). It is noted that a similar design can be applicable
to linear lasers (of any sort).
As illustrated in Figure 11, in an alternative
embodiment, a helical ring fibre laser 310 comprises an
optical fibre 312 having a grating confined core structure
314 and spaced apart concave reflectars 315, 316 within the
core structure 319. The helical ring fibre laser 310 can
thus provide a circularly birefringent output (as indicated
by arrow 311).
Furthermore, high power fibre lasers may be provided
without using cladding pump configuration. For such lasers,
single mode operation and good stability are possible, as
well as large mode areas. In such embodiments, the modes
are grating diffraction dependent unlike conventional
fibres which are aperture diffraction dependent.
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It will be appreciated by a person skilled in the art
that numerous variations and/or modifications may be made
to the present invention as shown in the specific
embodiments without departing from the spirit or scope of
the invention as broadly described. The present
embodiments are, therefore, to be considered in all
respects to be illustrative and not restrictive.
