Optical arrayed waveguide grating devices

Details Optical arrayed waveguide grating devices Optical arrayed waveguide grating devices

1999-12-23

2002-04-16

Spyrou, Cassandra (Department: 2872)

Optical waveguides

With optical coupler

Input/output coupler

C385S024000, C385S027000, C385S140000

Reexamination Certificate

active

06374013

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to optical arrayed waveguide grating (AWG) devices, particularly such devices suitable for use as optical wavelength multiplexers, demultiplexers, or filters. Such devices find particular, but not necessarily exclusive, application in wavelength division multiplexed (WDM) optical transmission systems.
BACKGROUND TO THE INVENTION
WDM optical transmission systems ideally require passive optical wavelength multiplexers, demultiplexers and filters which ideally should have isolated pass-bands which are flat-topped so as to allow a measure of tolerance in the spectral positioning of the individual signals of the WDM system within these pass-bands. One method of multiplexing, demultiplexing or filtering channels in an optical WDM system relies upon the use of multilayer dielectric interference filters. Another relies upon Bragg reflection effects created in optical fibers. A third method, the method with which the present invention is particularly concerned, relies upon diffraction grating effects.
The particular format of optical waveguide diffraction grating with which the present invention is concerned is derived from the format that includes a set of optical waveguides in side-by-side array, each extending from one end of the array to the other, and being of uniformly incrementally greater optical path length from the shortest at one side of the array to the longest at the other. Such an optical grating, sometimes known as an arrayed waveguide grating (AWG), constitutes a component of the multiplexer described by C Dragone et al., ‘Integrated Optics N×N Multiplexer on Silicon’, IEEE Photonics Technology Letters, Vol. 3, No. 10, October 1991, pages 896-9. Referring to accompanying
FIG. 1
, the basic components of a 4×4 version of such a multiplexer comprise an optical waveguide grating array, indicated generally at
10
, whose two ends are optically coupled by radiative stars, indicated schematically at
11
and
12
, respectively with input and output sets of waveguides
13
and
14
. Monochromatic light launched into one of the waveguides of set
13
spreads out in radiative star
11
to illuminate the input ends of all the waveguides of the grating
10
. At the far end of the grating
10
the field components of the emergent light interfere coherently in the far-field to produce a single bright spot at the far side of the radiative star
12
. Increasing the wavelength of the light causes a slip in the phase relationship of these field components, with the result that the bright spot traverses the inboard ends of the output set of waveguides
14
as depicted at
15
. If the mode size of the waveguides
14
is well matched with the size of the bright spot, then efficient coupling occurs at each of the wavelengths at which the bright spot precisely registers with one of those waveguides
14
. The propagation distances across the radiative stars themselves contribute to the FSR of any particular combination of waveguide
13
and waveguide
14
, and so not all the FSRs are precisely identical.
The difference in optical path length between the inboard end of a waveguide
13
and that of a waveguide
14
via adjacent waveguides in the array
10
(the optical path length of a waveguide being the product of its physical length with its effective refractive index) determines the value of the Free Spectral Range (FSR) of the grating for this particular pair of waveguides, being the frequency range over which this difference in optical path length produces a phase difference whose value ranges over 2&pgr; radians. Accordingly the single bright spot is produced in the same position each time the optical frequency of the light is changed by an amount corresponding to a frequency difference that is an integral number of FSRs. It can thus be seen that, for optical transmission from any particular one of the set of waveguides
13
to any particular one of the set of waveguides
14
, the device of
FIG. 1
operates as a comb filter whose teeth are spaced in frequency by the FSR of its grating
10
. The propagation distances across the radiative stars themselves contribute to the FSR of any particular combination of waveguide
13
and waveguide
14
, and so not all the FSRs are precisely identical.
The movement of the bright spot across the end of the particular waveguide
14
that occurs in consequence of a change of wavelength, results in an approximately Gaussian transmission pass-band for each channel of the multiplexer/demultiplexer. For operation in a practical WDM transmission system a more nearly flat-topped transmission pass-band is generally a requirement in order to avoid excessive uncertainties in the value of insertion loss that the device is liable to provide as the result of tolerances allowed for in the emission wavelengths of the optical sources employed in that transmission system, and to allow for the modulation bandwidth of the signals transmitted in the individual WDM channels. In this context, it may be noted that the drive to narrower channel spacings will typically aggravate this problem because, in general, the tolerances imposed upon the precision of source wavelengths are not tightened in proportion to the narrowing of the channel spacings, and/or the modulation bandwidth tends to constitute a greater proportion of the channel spacing.
In U.S. Pat. No. 5,629,992 there is described a method of providing a measure of flattening of the transmission pass-band of an AWG this method involving the interposing of a length of wider waveguide between the input waveguide
13
and the first star coupler
11
. This wider waveguide (also known as a multimode interference (MMI), or mixer, waveguide section) is capable of guiding, not only the zeroth order mode, but also the second order mode, both of which are excited by the launch of zeroth order mode power into it from the waveguide
13
because the transition between the waveguide
13
and its MMI section is abrupt, i.e. is non-adiabatic. These two modes propagate with slightly different velocities, and the length of the wider waveguide is chosen to be of a value which causes &pgr; radians of phase slippage between them. Under these conditions, the field distribution that emerges into the star coupler
11
from the end of the wider waveguide is double peaked. The image of this field distribution is formed at the end of star coupler
12
that is abutted by the waveguides
14
. The overlap integral between this image and the field distribution of the zeroth order mode of any one of the waveguides
14
then determines the transmission spectrum afforded by the device in respect of the coupling to that waveguide. The amount of band-pass flattening thereby occasioned can be expressed in terms of an increase in the value of a Figure of Merit (FoM) parameter arbitrarily defined as the ratio of the −0.5 dB pass-band width to the −30.0 dB pass-band width. A significant drawback of the mixer section approach to pass-band flattening is that the insertion loss is intrinsically increased consequent upon the mismatch between the size of the flattened field distribution that is incident upon the inboard end of the output waveguide
14
and that of the field distribution of the zeroth order mode that is guided by that waveguide
14
. By way of example, a mixer section supporting the zeroth and second order modes can be employed to increase the FoM of an AWG from about 0.14 to about 0.30, but this improvement in FoM is achieved at the expense of increasing the insertion loss of the device by approximately 2 dB. Further flattening can be obtained by widening still further the width of the mixer section to enable it to guide a larger number of even order modes, but this introduces yet higher increases in insertion loss. For instance, if the FoM is increased in this way to about 0.45, this is achieved at the expense of an excess insertion loss of approximately 4 dB. (No explicit mention has been made concerning the propagation of modes of odd order number in the MMI section. This

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