The invention also comprises a method for switching or add/drop multiplexing of wavelength channels in an optical network.

State of the Art A number of different methods are known to further increase the capacity in existing optical networks. One way is to use so called wavelength division multiplexing (WDM) technology to improve the degree of utilization of the available bandwidth on an optical fibre in the optical network. To increase the flexibility of the network, devices that can perform the redirection of traffic in the optical network are needed.

The forementioned devices for the redirection of traffic can also help in using the network as efficiently as possible, and in case of a break in the network.

In "Wavelength Division Multiplexer with Photoinduced Bragg Gratings Fabricated in a Planar Lightwave Circuit Type Asymmetric Mach Zehnder Interferometer on Si", Y. Hibino et. al., an optical element is described in which Bragg grating and phase control elements are used in a Mach Zehnder interferometer. The intended applications are wavelength division multiplexing and wavelength division demultiplexing.

The apparatus mentioned above however cannot be used as a wavelength selective switch. If the above mentioned apparatus is to be used for add/drop of several channels the same number of apparatuses as the number of handled add/drop pairs are needed. This type of apparatus is relatively difficult to reconfigure, that is, it is inflexible.

One problem with wavelength selective switches according to the prior art is that they require very big and complicated structures or several components, which results in large power losses and high costs.

Summary of the Invention A number of known methods can be used to increase the capacity in an optical transmission system. In e.g. wavelength division multiplexing transmission channels are multiplexed and demultiplexed on different carrier wavelengths onto, resp. out from, an information flow. This multiplexing and demultiplexing requires optically wavelength selective devices. It may also be desirable to change the route of transmission through the optical network for certain wavelength channels. For this purpose components with wavelength selective properties are required, for example in the form of a wavelength selective switch.

One problem with known wavelength selective switches is that they contribute to large power losses.

Another problem is that known wavelength selective switches have a relatively complex structure and, in all the cases known to us, a relatively large number of different components.

A further problem is that known wavelength selective switches are relatively expensive to manufacture because of the above mentioned complex structure and the large number of components.

The present invention approaches the above mentioned problems by a wavelength selective switch comprising at least one MMI (Multi Mode Interference) structure, at least one Bragg grating, at least one Mach Zehnder waveguide and at least one phase control element.

The above mentioned MMI structure is used for splitting up light. The distribution of light intensity at one of the inputs of the MMI structure is depicted on all outlets of the MMI structure when its length is selected correctly. A deeper theory behind this is found in L.B. Soldano and E.C.M. Pennings, "Optical Multi Mode Interference Devices Based on Self Imaging: Principles and Application", J. Lightwave Technology. Vol. 13(4), pp 615-627, 1995.

A Bragg grating is used filtering light. Filtering means that certain wavelengths are allowed to pass while other are reflected. The Bragg grating can be said to constitute a kind of wavelength selective mirror. Said reflection of certain wavelengths can be achieved in a number of ways; common to most of these methods is that the reflection is achieved by periodically changing the index of the substance in the wave guide.

The above mentioned phase control element is needed for certain switching functions and for the correction of process imperfections. There are several types of phase control elements; fundamental for all of them however is that the optical wavelength is affected through an applied external signal (voltage, current, light or heat). Usually a so called thermo-optical element is used, that is, the refractive index and thereby the wavelength, is affected by means of heat.

The wavelength selective switch according to the invention may comprise MMI waveguides at both its connection sides. Said MMI waveguides can on their free sides comprise a number of inputs consisting, for example, of so called access waveguides for optical signals. Between said MMI waveguides a number of Mach Zehnder waveguides are arranged. These Mach Zehnder waveguides in turn comprise a number of phase control elements and a number of Bragg gratings. The number of phase control elements may be one more than the number of Bragg gratings in the cases when one only wants to control any transmitted wavelength channels. In the case when only reflected wavelength channels are of interest, the number of phase control elements and Bragg gratings may be the same. The number of Bragg gratings and phase control elements may be equivalent for each Mach Zehnder waveguide. Each Mach Zehnder waveguide comprising said phase control elements and Bragg gratings may be identical.

By adjusting the shape and the dimension of the MMI waveguide and the length of the Mach Zehnder waveguides one can, by means of suitable phase control elements direct an optical signal from an input on the first MMI wave guide to an arbitrary output on the second MMI waveguide.

The object of the present invention is to achieve a wavelength selective switch which by its structure is as simple and as compact as possible so that the manufacturing cost may thus be reduced compared to the prior art technology.

An advantage of the present invention is that the power loss may be kept relatively low.

Another advantage of the present invention is that its performance in other areas such as crosstalk and the like may be improved compared to prior art.

The invention will now be described in more detail by means of preferred embodiments and with reference to the enclosed drawings.

Brief Description of the Drawings Figure 1 shows an embodiment of a wavelength selective switch according to the invention.

Figure 2 shows a second embodiment of a wavelength selective switch according to the invention.

Description of Preferred Embodiments In figure 1 a wavelength selective switch according to the invention is shown. The support lines A-K show sections used to describe the invention. In this embodiment the wavelength selective switch can handle four different wavelengths independently of each other. At the first connection end of the wavelength selective switch a first MMI waveguide is arranged and at the other connection end of the wavelength selective switch a second MMI waveguide 20 is arranged. The first MMI waveguide 10 in this embodiment comprises one or more access waveguides 11, 12, 13, 14 and the second MMI waveguide 20 comprises one or more access waveguides 21, 22, 23, 24. Between the first MMI waveguide 10 and the second MMI waveguide 20 four so called Mach Zehnder waveguides 31, 32, 33, 34 are arranged. Each of the above mentioned Mach Zehnder waveguides comprises four phase control elements 51, 53, 55, 57 and three Bragg gratings 62, 64, 66.

The above mentioned wavelength selective switch of course may be upgraded to comprise N wavelength channels instead of the four wavelength channels shown in the above mentioned embodiment. Generally however, in order for the wavelength selective switch to be able to handle N wavelength channels independently of each other, it must comprise Nx(N-1) Bragg gratings, N2 phase control elements and N access waveguides, each arranged in connection to an MMI waveguide.

Suppose that light is excited onto the access waveguide 11 belonging to the MMI waveguide 10 at section A. The length ofthe MMI waveguide 10 is selected so that N images of the light intensity from the access waveguide 11 at section A are achieved in the MMI waveguide 10 along section B. Next suppose that the structure and dimensions of the MMI waveguide 10 have been selected so that four images, that is N=4, of the original distribution in the access waveguide are achieved. If the access waveguides 11, 12, 13, 14 have been arranged at the MMI waveguide 10 in a correct way, that is, if they have been positioned correctly, and if the cross-sectional dimension and positioning of the Mach Zehnder arms 31, 32, 33, 34 have been chosen correctly, a large fraction of the energy in the images will be connected to the Mach Zehnder waveguides 31, 32, 33, 34. The maximum energy of these images is <1/N of the energy along section A if perfect uniformity is achieved, that is, in this case <1/4 of the energy along section A. This intensity distribution will differ very little if light is instead excited from any ofthe access waveguides 12-14 along section A.

In contrast, the phase relationship is strongly dependent on the access waveguide at which light is excited into the MMI waveguide. This input access waveguide dependent phase relationship is the key to the functionality of the component.

Because of reciprocity, light with the corresponding phase relationship at section B travelling in the reverse direction, that is, from the bottom upwards according to figure 1, will be focused on the corresponding access waveguides.

Sections along the support lines D, F and H denote Bragg grating sections. The Bragg gratings along each support line may be identical. If they are identical the grating sections will reflect the wavelength along the corresponding support line for each of the Mach Zehnder waveguides 31, 32, 33, 34. The wavelengths that are reflected return to the MMI 10 with the phase relationship determined by the phase control elements 51, 53, 55.

Suppose, for example, that the Bragg gratings along section D reflect a wavelength X1, and the Bragg gratings along section F reflect a wavelength k2; the phase control elements 51 along section C will then determine which access waveguide 11-14 will be the output for the wavelength B2. The analogous situation is seen along section G, that is, the phase control element 55 along said section G determine which one of the access waveguides 11-14 will be the output for the wavelength R3, which has been reflected by the Bragg gratings along section H.

The phase relationship onto the MMI waveguide 10 in the reverse direction can thus be selected individually for each wavelength, that is, each wavelength channel can be assigned an output independently of other wavelength channels. This of course implies that, for example, the phase control element 53 along section E can compensate for the phase control element 51 along section C and that the phase control element 55 along section G can compensate for the phase control elements 51, 53 along sections C and E respectively, and that the phase control element 57 along section I can compensate for the phase control elements 51, 53, 55 along the sections C, E and G respectively. In general, each phase control element must be able to compensate for the phase control elements arranged before it in the transmission route of the channels along the same Mach Zehnder waveguide.

Of course the phase control element 51 along the support line C also affects the wavelength channels B2, X3 and B4. This compensation can, however, easily be software controlled according to teachings known to a person skilled in the art, and, therefore, will not be described in detail here. There is also the possibility, if said compensation is not desired to be software controlled, to extend the phase control elements 51, 53, 55, 57 successively from section C in the direction towards section I in a suitable manner.

The wavelength channel or channels not reflected by a Bragg grating will reach the MMI 20 and the phase relationship along support line J will determine to which output along support line K the respective wavelength channel is excited. If length of the Mach Zehnder waveguides 31, 32, 33, 34 are equivalent the wave length channels that reach the MMI 20 will be focused on the same output. The difference in length between the Mach Zehnder waveguides 31, 32, 33, 34 can also be selected so that different wavelengths that reach the MMI 10 are focused on different access waveguides 21, 22, 23, 24 along the support line K.

In figure 2 another embodiment of a wavelength selective switch according to the invention is shown. The support lines A-H show sections used to describe the invention. This embodiment comprises an MMI waveguide 10 and four Mach Zehnder waveguides 31, 32, 33, 34. On one side of the MMI waveguide 10 four access waveguides 11, 12, 13, 14 are arranged. On the opposite side relative to said access waveguides, said Mach Zehnder waveguides 31, 32, 33, 34 are arranged. On each of these Mach Zehnder waveguides 31, 32, 33, 34 three Bragg gratings 62, 64, 66 and three phase control elements 51, 53, 55 are arranged.

Suppose that a wavelength channel is sent to an access waveguide 11 arranged on the MMI waveguide 10. This wavelength channel passes through the MMI waveguide. The length and the structure of the MMI waveguide are selected so that N images of the light intensity from the access waveguide 11 at section A are achieved in the MMI waveguide along section B. In this case, suppose that the length and the structure have been selected so that four images are achieved. If the access waveguides 11, 12, 13, 14 have been arranged at the MMI waveguide 10 in a correct way, that is, if they have been positioned correctly, and if the cross-sectional dimension and positioning of the Mach Zehnder waveguides 31, 32, 33, 34 have been chosen correctly, a large fraction of the energy in the images will be connected to the Mach Zehnder waveguides 31, 32, 33, 34. The maximum energy for these images is <1/N of the energy along section A if perfect uniformity is achieved, that is, in this case <1/4 of the energy along section A. This intensity distribution will differ very little if light is instead excited from any of the access waveguides 12-14 along section A.

The phase relationships on the other hand depend strongly on at which of the access waveguides light is excited onto the MMI waveguide. This input access waveguide dependent phase relation is the key to the functionality of the component. Because of reciprocity, light with the corresponding phase relationship at section B travelling in the reverse direction, that is from the bottom upwards according to figure 2, will be focused on the corresponding access waveguide.

Sections along the support lines D, F and H denote Bragg grating sections. The Bragg gratings along each support line may be identical. If they are identical the grating sections will reflect the wavelength along the corresponding support line for each of the Mach Zehnder waveguides 31, 32, 33, 34. The wavelengths which are reflected return to the MMI 10 with the phase relationship determined by the phase control elements 51, 53, 55.

Suppose, for example, that the Bragg gratings along section D reflect the wavelength X1, and that the Bragg gratings along section F reflect the wavelength B2; then the phase control elements 51 along section C will determine which access waveguide 11-14 will be the output of, for example, a wavelength Xl, the phase control elements 53 along section E will determine which access waveguide 11-14 will be the output of, for example, a wavelength B2. The analogous situation is seen along section G, that is, the relevant phase control element 55 along said section will determine which one of the access waveguides 11-14 will be the output of the wavelength X3, which has been reflected by the Bragg gratings along section H.

The phase relationship onto the MMI waveguide 10 in the reverse direction may thus be selected individually for each wavelength, that is, each wavelength channel may be assigned an output independently of other wavelength channels. This of course implies that, for example, the phase control element 53 along section E can compensate for the phase control element 51 along section C and that the phase control element 53 along section G can compensate for the phase control elements 51, 53 along sections C and E respectively. In general each phase control element is to be able to compensate for the phase control elements arranged before it in the transmission path of the channels along the same Mach Zehnder waveguide.

It is of course also the case that the phase control element 51 along the support line C also affects the wavelength channels X2, S3 and X4. This compensation can, however, easily be software controlled according to theories well known to the person skilled in the art, and thus will not need to be described in more detail here.

There is also the possibility, if said compensation is not desired to be software controlled, to extend the phase control elements 51, 53, 55 from section C in the direction towards section G successively in a suitable manner.

The wavelength channel or channels which are not reflected by any Bragg grating will be excited from the relevant Mach Zehnder waveguide 31, 32, 33, 34.

The materials that may be suitable for manufacturing of the present invention are, for example, quartz (SiO2), polymers, lithium niobate (LiNbO3) or a semiconductor system.

The invention is of course not limited to the embodiments described above and shown in the drawings, but may be modified within the scope of the appended patent claims.