Arrayed waveguide grating

Details Arrayed waveguide grating Arrayed waveguide grating

2002-09-10

2004-07-27

Ullah, Akm Enayet (Department: 2874)

Optical waveguides

With optical coupler

Input/output coupler

C385S024000

Reexamination Certificate

active

06768842

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to arrayed waveguide gratings (AWGs). In particular, though not exclusively, the invention concerns passband flattening in AWGs and an improvement for increasing passband uniformity in AWGs having flattened passbands.
BACKGROUND TO THE INVENTION
AWGs, sometimes also known as “phasars” or “phased arrays”, are now well-known components in the optical communications network industry. An AWG is a planar structure comprising an array of waveguides arranged side-by-side which together act like a diffraction grating in a spectrometer. AWGs can be used as multiplexers and as demultiplexers, and a single AWG design can commonly be used both as a multiplexer and demultiplexer. The construction and operation of such AWGs is well known in the art. See for example, “PHASAR-based WDM-Devices: Principles, Design and Applications”, M K Smit, IEEE Journal of Selected Topics in Quantum Electronics Vol.2, No.2, June 1996, and U.S. Pat. No. 5,002,350 and WO97/23969.
A typical AWG mux/demux
1
is illustrated in FIG.
1
and comprises a substrate or “die”
1
having provided thereon an arrayed waveguide grating
5
consisting of an array of channel waveguides
8
, only some of which are shown, which are optically coupled between two free space regions
3
,
4
each in the form of a slab waveguide. At least one substantially single mode input waveguide
2
is optically coupled to an input face
9
of the first slab waveguide
3
for inputting a multiplexed input signal thereto, and a plurality of substantially single mode output waveguides
10
(only some shown) are optically coupled to an output face
20
of the second slab waveguide
4
for outputting respective wavelength channel outputs therefrom to the edge
12
of the die
1
. The array waveguides all have an aiming point C
1
which is located on the output face
20
of the second slab waveguide
4
and which is the centre of curvature of the input face
15
of the second slab waveguide which input face lies on an arc of radius R. This arc of radius R is also sometimes referred to as the “grating line”. The output face of the second slab waveguide lies on an arc, sometimes referred to as the “focal line”, having a radius R/2 and a centre of curvature C
2
. The centres of curvature C
1
, C
2
of the input and output faces
15
,
20
of the second slab
4
lie on a straight line X. The array waveguides
8
are all equally angularly spaced, there being a fixed angle &Dgr;&agr; between neighboring array waveguides, with respect to the centre of curvature C
1
of the input face
15
of the slab waveguide
4
. There is also a fixed lateral spacing d
a
between neighboring array waveguides
8
, at their interface with the slab waveguide
4
. The input and output faces of the first slab waveguide have a similar (but inverted) arrangement, as indicated in FIG.
1
.
In generally known manner, there is a constant predetermined optical path length difference between the lengths of adjacent channel waveguides
8
in the array (typically the physical length of the waveguides increases incrementally by the same amount from one waveguide to the next) which determines the position of the different wavelength output channels on the output face of the second slab coupler
4
. Typically, the physical length of the waveguides increases incrementally by the same amount, &Dgr;L, from one waveguide to the next, where
&Dgr;
L=m&lgr;
c


c
where &lgr;
c
is the central wavelength of the grating, n
c
is the effective refractive index of the array waveguides, and m is an integer number. In known manner, the transmission waveguides and slab waveguides are typically formed (e.g. using standard photolithographic techniques) as “cores” on a silicon substrate (an oxide layer and/or cladding layer may be provided on the substrate prior to depositing the waveguide cores) and are covered in a cladding material, this being done for example by Flame Hydrolysis Deposition (FHD) or Chemical Vapour Deposition (CVD) fabrication processes.
In such an AWG, the passband (i.e. shape of the transmission spectrum T(&lgr;), which is a plot of dB Insertion Loss against Wavelength) for each output channel generally corresponds to the coupling of a Gaussian beam into a Gaussian waveguide, and is therefore itself Gaussian-shaped. In many situations it would be more desirable for the AWG to have a flat passband. This is generally because a Gaussian passband requires accurate control over emitted wavelengths, thus making it difficult to use in a system. Various ways of achieving a flat passband have been proposed, one way being to use “near field shaping”. This involves creating a double-peaked mode field from the (single peak) input mode field. When this double-peaked field is convoluted with the single mode output waveguide, the resulting passband takes the form of a single, generally flat peak. One way of creating the necessary double-peaked field is to use an MMI (Multi-Mode Interferometer) on the end of the input waveguide (or each input waveguide, where there is more than one), adjacent the first slab coupler, as shown in FIG.
3
(
a
). The MMI creates higher order modes from the single mode input signal and these multiple modes give rise to a double-peaked field at the output of the MMI.
U.S. Pat. No. 5,629,992 (Amersfoort) describes this passband flattening technique in detail. An alternative technique is to use a parabolic-shaped taper or “horn” on the end of the input waveguide, as shown in FIG.
3
(
b
). This is described in JP 9297228A. The parabolic taper gives rise to continuous mode expansion (by excitation of higher order modes) of the input signal along the length of the taper, until both the fundamental and second order modes are present, thus forming a double-peaked field at the output end of the taper. Other non-adiabatic multimode waveguide taper shapes can alternatively be used to achieve the desired passband flattening effect, for example a curvilinear taper shape based on a cosine curve, as described in our pending UK Patent application No. 0114608.3 the entire contents of which are incorporated herein by reference.
Near-field shaping to produce the desired multiple peak field at the input to the first slab waveguide can also be achieved using other techniques such as a Y-branch coupler, as described in U.S. Pat. No. 5,412,744 and illustrated in FIG.
3
(
c
), which splits the input single mode field into two peaks. Another technique is the adiabatic mode shaper structure described in our pending UK patent application No. 0114494.8 which uses an extra tapered waveguide disposed adjacent the or each input waveguide to convert the single peak field of the input waveguide to a double peak field.
However, when any of the above-described features (namely the MMI, parabolic horn, multimode waveguide, Y-branch coupler or other passband flattening structure) are employed for the purpose of passband flattening, it is found that AWGs fabricated according to these designs in practice suffer from asymmetry in the passbands of different wavelength output channels. This is illustrated in
FIG. 4
which shows that although the passbands P
1
,P
2
,P
3
,P
4
(plotted as the Transmission, in dB, against wavelength, &lgr;) of the central four output channels of the AWG are generally symmetrical as expected, the passbands of the other channels become asymmetrical to the right and left of these central four, the asymmetry in the channels to the right being generally inverse to the asymmetry in the channels to the left. This asymmetry is believed to be caused by off-axis aberrations (sometimes referred to as “COMA”) in the second slab waveguide
4
. The further the optical signal condenses away from the aiming point C
1
of the array waveguides on the output face
20
of the slab waveguide
4
, the greater the asymmetry in the passband becomes. One undesirable effect of this asymmetry is that it causes undesirable fluctuation in insertion loss with variation in wavelength of the input optical signal.
This asymmetry effect in the flat

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