Arrayed waveguide grating

Details Arrayed waveguide grating Arrayed waveguide grating

2001-11-06

2003-02-18

Ullah, Akm E. (Department: 2874)

Optical waveguides

With optical coupler

Input/output coupler

Reexamination Certificate

active

06522811

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an arrayed waveguide grating (AWG), and the manufacture and use of such a grating. In particular, the present invention relates to the geometry of an AWG that is suitable for, but not exclusively for providing a thermal operation.
BACKGROUND OF THE INVENTION
Optical systems increasingly use wavelength division multiplexing (WDM), in which a number of distinct optical signals are transmitted at different wavelengths, generally down an optical fibre. For example, optical communication in the so called “C” band may use 40 channels, all frequencies spaced apart by regular intervals. One optical signal can be transmitted at each frequency down a single optical fibre. There are the possibilities, for example, that 56 channels may be used in the “L” band.
A key component in WDM systems is the demultiplexer for separating the optical signals at a plurality of wavelengths into the individual channels at individual wavelengths. This may be done by using a splitter and a number of different filters tuned to the individual frequencies, by components that demultiplex the light directly, or by a combination of these components.
One approach to filtering and demultiplexing is an arrayed waveguide grating (AWG), also known as a phased-array device. The operation and design of AWGs is described, for example, in “PHASAR-based WDM-devices; principles, design and applications”, Meint K. Smit, IEEE Journal of Selected Topics in Quantum Electronics Vol. 2, June 1996.
FIG. 1
illustrates a conventional AWG device. The device includes an array
11
of waveguides
3
arranged side by side on a substrate
1
and extending between an input star coupler
13
and an output star coupler
15
. The input and output star couplers
13
,
15
may be defined by a wide core region in which light can travel freely in the two-dimensional plane of the substrate. This region is known as the free propagation region. Input
17
and output
19
optical waveguides are divided to input light into the array
11
of waveguides and to output light respectively. There may in particular be a plurality of input waveguides
17
and/or output waveguides
19
.
As an example,
FIG. 2
illustrates the output star coupler of a system with a plurality of output waveguides. The ends
21
of the array waveguides
11
are usually on a geometric circle
23
of radius r whose centre is at centre
25
of an image plane
27
. The output waveguides
19
are arranged on the image plane, which also constitutes a circle. Note that the centres of the circles are not coincident, and need not have equal radii.
The lengths of the individual waveguides
3
of the array
11
differ (see FIG.
1
), and the shapes of the star couplers
13
,
15
are chosen so that light input on the input optical waveguides
17
passes through the array of waveguides and creates a diffraction pattern on the output waveguide or waveguides, such that light of a predetermined central wavelength creates a central interference peak at the centre
25
of the image plane. Light with frequencies slightly higher or lower than the predetermined central frequency is imaged with a central interference peak slightly above or below the centre of the image plane.
In order to achieve this result the optical path length difference between adjacent waveguides of the array is chosen so that it is an integral multiple of the central wavelength. Thus light at the central wavelengths which enters the array of waveguides in phase will also leave in phase and will create the central diffraction spot at the centre of the image plane. Light with a slightly different frequency will arrive at the output star coupler with slight phase differences across the array, which will cause the light to be imaged to a spot on the image plane a little further away from the central spot.
Thus the plurality of output waveguides arranged on the output plane receive light of slightly different frequencies. Equally spaced output waveguides correspond to equally spaced frequencies, to a first order of approximation.
FIG. 2
shows the use of one or more output waveguides connected to the output star coupler
15
. It is alternatively or additionally possible to arrange a plurality of input waveguides on the input star coupler with the same effect. Equally, it will be appreciated that the terms input and output are merely used for convenience, as such a device is in fact bi-directional and reciprocal in its performance.
It will be appreciated that the lengths of the individual waveguides in the array
11
are critical to the performance of the AWG. If the difference in length between the optical paths change, then so will the transmission wavelengths of the AWG filter. The optical path length is the physical path length multiplied by the refractive index of the material.
AWG filters are hence inherently temperature sensitive on account of the temperature sensitivity of the refractive index of the waveguide material and, to a lesser extent, due to the expansion coefficient of the material. For instance, in a silica-based planar waveguide device, the wavelength at the centre of the filter passband typically increases by about 0.01 nm (nanometer) per degree Celsius. In other words, for each 10 degree Celsius rise in temperature, the centre wavelength will increase by 0.1 nanometers.
Due to the decreasing wavelength spacing between adjacent optical channels (as WDM systems have developed into DWDM systems), it is desirable to have a filter which is not sensitive to changes in temperature.
Various solutions have been proposed to prevent such AWGs being affected by variations in ambient temperature.
For instance, the use of a heater or a Peltier element attached to the AWG to ensure that the AWG temperature remains at a predetermined, preferred value. Such an arrangement requires control circuitry, as well as consumes power.
It is therefore preferable that the operation of the AWG is itself largely independent of temperature i.e. the AWG operation is athermal.
“Optical Phased Array Filter Module With Passively Compensated Temperature Dependence” by G Heise et al, ECOC 98, describes one manner of stabilising such filters passively against thermal drifts. The proposed solution is to utilise a modified input coupling section whereby the input waveguide
17
is attached to a compensating rod. As the temperature changes, the thermal expansion of the compensating rod shifts the physical position of the input
17
so as to retune the filter to compensate for the temperature drift of the phased array chip. This mechanical movement of the position of the input
17
will be subject to effects such as vibration and fatigue.
European Patent application no. EP 0 919 840 A1 by Inoue et al describes an alternative athermal optical waveguide device, in which a groove is formed across the array
11
of waveguides. The groove is then filled with a material having a temperature co-efficient of refractive index of different sign from that of the temperature co-efficient of refractive index of the waveguide. For example, the groove is typically filled with silicone, which has a refractive index temperature co-efficient of −30 x that of silica (the typical material used to form the waveguides). Thus by using silicone filled slots which vary in length between the array guides by about {fraction (1/30)}
th
of the path length difference between the individual array guides, the AWG filter becomes temperature insensitive. The centre frequencies of the filter pass bands are therefore also temperature insensitive.
However, the main problem with this approach is that the diffraction loss across these slots is relatively significant. This is due to the fact that the optical energy is not guided by a waveguide across the slot.
“Athermal and Center Wavelength Adjustable Arrayed-Waveguide Grating” by K Maru et al, OFC 2000, describes an alternative manner of providing an athermal AWG. The proposed AWG has several trenches with a crescent shape formed in the input star coupler
13
. Such trenche

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