The invention concerns an integrated optical multi-beam interferometer with an arrayed waveguide grating.
Conventional optical switching equipment and optical multiplexers and demultiplexers comprise a connecting device with one or more input waveguides which are arranged close to one another and which are connected to the input of a free space region (star coupler). The output of the free space region is connected to an arrayed waveguide grating (AWG) which comprises a series of optical waveguides. The number of these waveguides can be more than 50. This is then called a dense wavelength division multiplexer (DWDM). The arrayed waveguide grating is connected to the entrance of a second free space region, the outputs of which are the outputs of the relaying device or the switch or the multiplexer or demultiplexer (see for example U.S. Pat. No. 5,136,671).
The waveguides of the arrayed waveguide grating differ in length in comparison to their closest neighbour by a predetermined fixed amount in each case. This enables several separate optically diverse wavelengths which are supplied to the device in different and separate input ports to be combined with one another by interference at a predetermined output port. In this manner the device is used as a multiplexer. The same device can also operate as a demultiplexer. In this case, several input wavelengths are present at a predetermined input port of the device. Optionally a single input port may be available. The input wavelengths are separated from one another and applied to a predetermined output port of the device. If several input ports are available a suitable selection of the input wavelengths also permits a switching between any selected input port and any selected output port. Optical frequency multiplexers/demultiplexers of this type can be produced in SiO2/Si technology for example in the form of so-called optical phase arrayed (phasar). Here a layer (buffer layer) of for example 15 xcexcm of SiO2 is grown on a silicon substrate by oxidation under high pressure steam. It serves to insulate the silicon substrate which has a very high refractive index. A second layer (core layer) of doped glass, doped for example with phosphorus or germanium, is laid over the oxide. The multi-beam interferometer is structured into this last layer by dry etching i.e. the free beam regions and all waveguides of the arrayed waveguide grating, and then covered with a layer of glass doped with phosphorus or boron which is several micrometres thick. The typical width of the core of a waveguide of the phase grating is in the range of 4.5 to 6.5 xcexcm. The typical decoupling distance of adjacent waveguides of the arrayed waveguide grating is 30 xcexcm from centre to centre of the respective waveguides. In addition the waveguides of the arrayed waveguide grating exhibit a circular arc-shaped form with, for example, a minimum radius of curvature of approximately 15 mm in the plane into which they are structured.
The loss on combining (multiplexer) or separating (demultiplexer) light signals which have wavelengths in the range of 1,200 nm up to 1,600 nm and differ from one another by up to only 0.4 nm or 0.2 nm for example is greatly dependent on the quality of the interference pattern produced from the light signals in the free space region radiating from the waveguides of the arrayed waveguide grating. Even with the most modern technology a certain aberration of the interference pattern in the free space region during production of the arrayed waveguide grating from a large number of waveguides with strictly defined optical lengths can only be avoided with difficulty.
A tuning device for an integrated optical multi-beam interferometer for correcting the aberration of the interference pattern in the free space region is known from EP 756 184 A2. The tuning device is based on the exploitation of thermo-optical effects which influence the core refractive index of the waveguides. The tuning device consists of an electrode structure which is applied over a region of the arrayed waveguide grating. The production of a certain amount of heat by this electrode structure causes a change in the refractive index of the core of the respective waveguides which are covered by it. Their optically effective length can be trimmed by this, which leads to the shift in the interference pattern of the light signals in the free beam region.
EP 662 621 B1 discloses a controllable phase shift device of the different waveguides of an arrayed waveguide grating. With the aid of this controllable phase shift device a phase shift of an optical wave guided in a specific waveguide can be produced in a targeted manner. A phase shift can be produced which differs greatly from waveguide to waveguide.
Controllable tuning devices of this kind have the great disadvantage that the arrayed waveguide grating must be provided with additional electronics in order to operate this tuning device. This creates increased costs and holds the danger that these electronics might fail which would lead to an unusable tuning device. An alternative is known from the essay xe2x80x9cAn all-fibre dense-wavelength-division multiplexer/demultiplexer using photoimprinted Bragg gratingsxe2x80x9d by F. Bilodeau et al in IEEE Photonics Technology Letters, 7 (1995), page 388f. It is based on the trimming of the core refractive index of a glass fibre by irradiation after it has been previously doped with H2 molecules in order to increase the efficiency of this irradiation. A change in the refractive index of at least 10xe2x88x923 can only be achieved with the aid of these H2 molecules used as a catalyst. Here these molecules diffuse in time out of the doped sample of their own accord. This behaviour is even assisted in that the sample is usually annealed subsequently by heating. This leads however to a change in the Bragg wavelength which in the case of planar arrayed waveguide gratings are called central wavelengths. Therefore it is impossible to predict in which region exactly the sample handled according to this process will operate. This makes such a type of trimming very problematic and unreliable.
The object of the invention is to further develop an area waveguide grating of an integrated optical multi-beam interferometer in such a way that the aberrations in the interference pattern in the multi-beam interferometer are minimised and that this is done in a controlled manner.
The object is achieved by an integrated optical multi-beam interferometer with an arrayed waveguide grating comprising a large number of N waveguides (i), wherein at at least one (j) of the N waveguides (i) has a selective region (Bj) along its optical path length (Lj) for trimming of the multi-beam interferometer which is treated only by an irradiation which changes its core refractive index (nj).
By the targeted irradiation with UV of regions of some of the waveguides of the arrayed waveguide grating alone the core refractive index is lowered in these regions. The aberration of the interference pattern in the free space region can be minimised by this. As individual regions of the waveguide can be treated by focusing the UV beam it is possible to carry out the irradiation on the connected integrated optical multi-beam interferometers selectively as a function of the measured values of the combined or separated light signals. Therefore the arrayed waveguide grating can easily be trimmed in order, for example, to minimise the losses which occur in the transition from the arrayed waveguide grating to the respective waveguides in the free space region at certain wavelengths.
It has been found that this treatment according to the invention of the arrayed waveguide grating with UV radiation without doping these regions beforehand with H2 molecules permits a controlled change (decrease or increase) in the core refractive index of these regions. In this way such integrated optical multi-beam interferometers can be trimmed very accurately and therefore have a distinctly improved mode of operation as a result of low loss rates and mixing rates of the processed optical signals.
Advantageous developments of the invention emerge from the dependent claims, the following description and the drawings.