METHOD AND WAVELENGTH SELECTIVE SWITCHING FOR SWITCHING OPTICAL WAVELENGTHS

Abstract: The invention relates to a device and a method for switching optical wavelength channels. Said optical wavelength channels are introduced into at least one access waveguide provided on a first side of a first multi-mode waveguide (10). Subsequently, the wavelength channels are transmitted through said multi-mode waveguide (10) and projected on at least two connection waveguides provided on the opposite side. Subsequently, the optical wavelength channels are transmitted through the connection waveguides. For each wavelength selective cross-connection structure (2, 4, 6, 8) the phase is changed for a reflecting wavelength of two phase control elements (C1, C2, D1, D2, E1, E2, F1, F2) arranged in a first and a second connection waveguide on a first side of said wavelength selective cross-connection structure (2, 4, 6, 8), simultaneously as at a second side of said wavelength selective cross-connection structure (2, 4, 6, 8) said reflecting wavelength phase remains relatively unchanged. For each wavelength selective cross-connection structure (2, 4, 6, 8) the phase is changed for transmitting wavelengths once in a first and a second direction per wavelength selective cross-connection structure (2, 4, 6, 8). The phase difference between the optical signal in each access waveguide provided on the first side of the second multi-mode waveguide (20) determines where the optical signal is focused on the opposite side.

Description: METHOD AND WAVELENGTH SELECTIVE SWITCHING FOR SWITCHING OPTICAL WAVELENGTHS Technical Field The invention relates to an optically wavelength selective device, in more particular to a wavelength selective switch for re-routing wavelength channels in an optical network, in which each wavelength channel can be re-routed separately and independently on the other wavelength channels. The invention also relates to a method for re-routing wavelength channels in an optical network. State of the art To be able to further increase the capacity on already existing optical networks, there are a number of different known methods. One way is to use so-called wavelength division multiplex (WDM) technology for improving the degree of operation of available bandwidth connection an optical fibre in the optical network. To be able to increase the flexibility of the network, devices which can provide re-routing of traffic in the optical network are necessary. Said devices for re-routing traffic are also suitable for employing the network in the most effective way as possible and at interruption in the network. To be able to provide re-routing for each wavelength channel individually, wavelength selective switches are required. 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, IEEE Photonics Technology Letters, Vol. 8, No. 1, January 1996, 99 84- 86 an optical element is disclosed in which Bragg-gratings and phase control elements are used in a Mach Zehnder interferometer. The proposed applications are wavelength division multiplexing and wavelength division de-multiplexing. However, above disclosed switch cannot be used as a wavelength selective switch. If above disclosed device is to used for add drop of more channels a number of devices are required equal to the number of handled add/drop pairs. This type of device is relatively hard to re-configure, i. e. inflexible. The problems with above described technology as regards wavelength selective switches is that these require very large complicated structures or several components, which results in large power losses and a high cost. Disclosure of the invention To be able to increase the capacity in an optical transfer system, a plurality of different methods may be employed. At for instance wavelength division multiplexing, transmission channels are multiplexed and de-multiplexed on different carrier wavelengths to and from an information flow. Said multiplexing and de-multiplexing require optically wavelength selective devices. It can also be desirable to determine way of transmission through the optical network for each wavelength channel individually. For that purpose switching components are required with wavelength selective properties. One problem with known wavelength selective switches is that these contribute to large power losses. Another problem with known wavelength selective switches is that known wavelength selective switches have a relatively complicated structure and in all known cases a relatively large number of different elements. Yet another problem is that known wavelength selective switches are relatively expensive to manufacture based on above disclosed complicated structure and the number of comprised components. The present invention tackle the problem by means of a wavelength selective switch comprising at least two multi-mode waveguides, at least one wavelength selective cross-connection structure, at least two phase control elements and at least four connection waveguides. The wavelength selective cross-connection structure is arranged between a first and a second multi-mode waveguide. Said first multi-mode waveguide is provided with at least one access waveguide on a first side and at least two access waveguides on a second side and said second multi-mode waveguide is provided with at least two access waveguides on a first side and at least one access waveguide on a second side. Said wavelength selective cross-connection structure is provided with at least two access waveguides on a first and a second side. At a wavelength selective cross-connection structure a first access waveguide on the second side of the first multi-mode waveguide is arranged to a first access waveguide on the first side of the wavelength selective cross-connection structure via a first connection waveguide, a second access waveguide on the second side of the first multi-mode waveguide arranged to a first access waveguide on the second side of the wavelength selective cross-connection structure via a second connection waveguide, and a second access waveguide on the first side of the wavelength selective cross- connection structure is arranged to a first access waveguide on the first side of the second multi-mode waveguide via a third connection waveguide and a second access waveguide on the second side of the wavelength selective cross-connection structure is arranged to a second access waveguide on the first side of the second multi-mode waveguide via a fourth connection waveguide. The phase control elements are arranged in the connection waveguides on one of the sides of the wavelength selective cross-connection structure. At a cross-connection structure there are two different so-called Mach-Zehnder paths for each particular wavelength in the optical signal. A first Mach-Zehnder path goes from a first access waveguide on the second side of the first multi-mode waveguide to the second access waveguide on the first side of the second multi-mode waveguide via the first and the fourth connection waveguide and via the wavelength selective cross- connection structure for wavelengths which are transmitted through the cross- connection structure. A second Mach-Zehnder path goes from a second access waveguide on the second side of the first multi-mode waveguide to the first access waveguide on the first side of the second multi-mode waveguide via the second and the third connection waveguide and via the wavelength selective cross-connection structure for wavelengths which are transmitted through the cross-connection structure. A third Mach-Zehnder path goes from a first access waveguide on the second side of the first multi-mode waveguide to a first access waveguide on the first side of the second multi-mode waveguide via the first and the third connection waveguide and via the wavelength selective cross-connection structure for wavelengths which are reflected by the cross-connection structure. A fourth Mach-Zehnder path goes from a second access waveguide on the second side of a first multi-mode waveguide to the second access waveguide on the first side of the second multi-mode waveguide via the second and the fourth connection waveguide and via the wavelength selective cross-connection structure for wavelengths which are reflected by the cross-connection structure. Only wavelengths which are reflected in any cross-connection structure can be controlled actively, i. e. the wavelength which goes via the third or fourth Mach- Zehnder path as disclosed above. Said Mach-Zehnder paths are preferably equal in length in the wavelength selective switch element according to the invention. Thus, for a wavelength selective cross-connection structure there are two different Mach-Zehnder paths for wavelengths which are reflected by the cross-connection structure and two different Mach-Zehnder paths for wavelengths which are transmitted by the cross-connection structure. Thus, for two different Mach-Zehnder paths for two different wavelengths and for N wavelength selective cross-connection structures there are 2XN different Mach-Zehnder paths for N wavelengths. The first and the second multi-mode waveguide preferably have the same length-width ratio when they are of the same type, i. e. for instance of 2X2 type. In an embodiment according to the invention the multi-mode waveguide can comprise a MMI waveguide. At two or more wavelength selective cross-connection structures for each wavelength selective cross-connection structure two phase control elements and two connection waveguides are added. Each wavelength selective cross-connection structure is provided with two phase control elements on the opposite side in relation to a nearest arranged wavelength selective cross-connection structure. The wavelength selective cross-connection structures are mutual arranged to each other via connection waveguides from an access waveguide on a first wavelength selective cross-connection structure to an other access waveguide on an adjacent wavelength selective cross- connection structure. Said access waveguide are selected to being nearest to each other and on the same side. According to a method of the invention for re-routing optical wavelength channels in an optical network, wavelength channels are exited in at least one access waveguide provided on a first side of a first multi-channel waveguide. The optical wavelength channels are then transmitted through said first multi-mode-waveguide and projected on at least two connection waveguides provided on opposite side in relation to said access waveguide. Subsequently, the optical wavelength channels are transmitted through the connection waveguides. For each wavelength selective cross-connection structure, the phase is changed for a reflecting wavelength two times by two phase control elements arranged in a first and a second connection waveguide on a first side of said wavelength selective cross-connection structure, simultaneously as at a second side of the wavelength selective cross-connection structure, the phase of the reflective wavelength remains relatively unchanged. For each wavelength selective cross-connection structure, the phase for transmitting wavelengths is changed once. The optical wavelength channels are excited in via at least two connection waveguides in a second multi-mode waveguide. The phase difference of each wavelength will be decisive for how much power of said wavelength and to which of the output waveguides of the multi-mode waveguide each wavelength is transmitted. The phase control elements belonging to a particular wavelength selective cross- connection structure operate synchronously and change phase to the same extent. According to an embodiment of the invention, the wavelength selective directional switching structures are of MMI-Bragg-grating type. The object of the present invention is to obtain a wavelength selective switch, in which each wavelength can be switched independently of the rest of the wavelengths and to all possible selections of outputs, which by its structure is so simple and compact as possible, so that the manufacturing cost thereby can be reduced compared to known techniques. An advantage of the present invention is that the power loss can be kept relatively low. .Another advantage of the present invention is that its further performance, such as cross-talk etc can be improved compared to prior art. The invention will now be described in more detail with reference to preferred embodiments and to the appended drawings. Description of the drawings Figure 1 shows an embodiment of a wavelength selective switch according to the invention. Figure 2 shows an example of a symmetrical add/drop-structure based on a so-called MMIBg- principle. Preferred embodiments Figure 1 shows an embodiment of a wavelength selective switch 1 according to the invention. The wavelength selective switch 1 comprises two multi-mode waveguides 10, 20, four wavelength selective cross-connection structures 2, 4, 6, 8, eight phase control elements Cl, C2, Dl, D2, El, E2, FI, F2, ten connection waveguides and access waveguides provided on both of the multi-mode waveguides 10, 20 and on the cross-connection structures 2, 4, 6, 8. An optical signal comprising a number of different wavelengths is excited into the access wave guide provided on a first side of a first multi-mode waveguide 10. The optical signal is transmitted through the multi-mode waveguide 10 and projected on the access waveguides provided on an opposite side compared to where the optical signal was excited in. The signal is projected depending on length-width ratio on the multi- mode waveguide 10 and the arrangement of the access waveguides. Assume that length- width ratio is like that an optical signal which is excited in at a first side of the multi-mode waveguide 10 is projected in the same way in both of the access waveguides on the opposite side, i. e. that the optical signal has been divided into half the intensity in both of the access waveguides. To the access waveguides on the second side of the first multi-mode waveguide 10 there are provided connection waveguides. A first connection waveguide is connected to a first access waveguide on a first side of a first wavelength selective cross-connection structure 2. A second connection waveguide is connected to a first access waveguide provided on a second side of the first wavelength selective cross-connection structure 2. One of these connection waveguides comprises a phase control element Cl. In the embodiment illustrated in figure 1, said phase control element is comprised in said first connection waveguide. A second wavelength selective cross-connection structure 4 is arranged to the first wavelength selective cross-connection structure via two connection waveguides. A second access waveguide provided on the first side of the first wavelength selective cross-connection structure 2 is arranged to a first access waveguide on the first side of the second wavelength selective cross-connection structure 4 via a third connection waveguide. A second access waveguide on the second side of the first wavelength selective cross-connection structure 2 is arranged to a first access waveguide on a second side of the second wavelength selective cross-connection structure 4 via a fourth connection waveguide. In figure 1 it is illustrated that four wavelength selective cross-connection structures 2, 4, 6, 8 are arranged after each other. The wavelength selective cross-connection structures 2, 4, 6, 8 are mutual arranged to each other via connection waveguides from an access waveguide on a first wavelength selective cross-connection structure to an other access waveguide on an adjacent wavelength selective cross-connection structure, in which said access waveguide is selected so that they are nearest to each other on the same side. A last wavelength selective cross-connection structure 8 is arranged to a second multi- mode waveguide 20 via connection waveguides. A second access waveguide on a first side of the last wavelength selective cross-connections structure 8 is arranged to a first access waveguide on a first side of the second multi-mode waveguide 20 via a connection waveguide. A second access waveguide on a second side of the last wavelength selective cross-connection structure 8 is arranged to a second access waveguide on the first side of the second multi-mode waveguide 20 via a connection waveguide. On a second side of the second multi-mode waveguide 20 illustrated in the embodiment in figure 1 there are provided two waveguides. The optical signal which was fed to the access waveguide along section A on the first side of the first multi-mode waveguide 10 is transmitted as described above first through said first multi-mode waveguide 10 and projected along section B on both connection waveguides arranged on the opposite side in relation to said access waveguide. Subsequently, the optical signal is further transmitted in both of the connection waveguides. In one of these connection waveguides, a phase control element Cl is arranged. Said phase control element Cl influences all wavelengths passing therethrough. The optical signal is then transmitted into the first wavelength selective cross-connection structure 2 at section C. Each one of these wavelength selective cross-connection structures 2, 4, 6, 8 at section C. D, E and F are arranged to reflect a wavelength and transmit the remaining. Assume that the first wavelength selective cross-connection structure 2 at section C is arranged to reflect a wavelength λl, that the second wavelength selective cross-connection structure 4 at section A is arranged to reflect a wavelength λ2, that the third wavelength selective cross- connection structure 6 at section E is arranged to reflect a wavelength λ3 and that a fourth wavelength selective cross-connection structure 8 at section F is arranged to reflect a wavelength λ4. For the wavelengths λl, λ2, λ3, λ4 it is stated that each one of these wavelengths are reflected by one and only one wavelength selective cross-connection structure 2. 4, 6, 8 and transmitted through the rest. According to the embodiment shown in figure 1, each wavelength channel, which is reflected by any one of the wavelength selective cross-connection structures, which are transmitted out of one of the connection waveguides along section B will be influenced by two phase control elements compared to the wavelength channels which are reflected by any of the wavelength selective cross-connection structures, which are transmitted out of the second connection waveguide along section B. When for instance wavelength λl reaches the first side of the second multi-mode waveguide 20 along section G. the phase will be different in the first and the second access waveguide depending on, as described above, that said wavelength has been influenced by a different number of phase control elements Cl, C2. Dl, D2. El. E2, FI, F2. The phase for each of the different reflecting wavelengths in the first and the second access waveguide along section G will differ from each other. In one of these access waveguides along section G in figure 1, for a given reflected wavelength, the phase has changed twice compared to the phase in the second access waveguide. Each reflected wavelength has a unique path, Mach-Zehnder path, through the structure. In that way a wavelength can be influenced independently on the rest and by help of a suitable embodiment as regards length, width and arrangement of access waveguides on the second multi-mode waveguide 20, then each of the reflecting wavelengths can be directed to any output along section H. For a given length- width ratio of the second multi-mode waveguide, the phase difference between the optical signals in the access waveguides along section G determines where the optical signal will be focused along section H. The phase control elements operate preferably synchronously and with the same phase change. In figure 2, it is illustrated an example of a wavelength selective cross-connection structure 100, which by advantage can be employed in the wavelength selective switch according to the invention. In principle the wavelength selective cross-connection structure 100 in figure 2 is an add/drop mux element. The specific device in figure 2 is called a symmetrical MMI-Bragg-grating structure. Any add/drop element with functionality like the cross-connection structure in figure 2 is possible to use, but of most importance for the realisation of the component, is that this element does not have to be trimmed. When trimming is necessary the wavelength selective switch 1 can be impossible to operate if quite a few wavelengths shall be operated, i. e. when many cross-connection structures 2. 4, 6, 8 are required. An optical signal is excited for instance in at an access waveguide 40 in the cross- connection structure in figure 2. One channel is reflected to an access waveguide 30 and the rest are transmitted to an access waveguide 50. Due to reciprocity and the symmetry of the device, an optical signal which is excited in at an access waveguide 50 will either be reflected to the access waveguide 50 or transmitted to the access waveguide 30. The cross-connection structure is preferably symmetrical, i. e. if it is cut in the middle, both halves should be its own mirror-image. The present wavelength selective switch 1 can be made of any monolithic semiconducting system or dielectric waveguide system of Si0 2 /Si type. The wavelength selective switch can be arranged with all connection waveguides having the same length. The invention is not limited to the above described and in the drawings illustrated embodiments, but can be modified within the appended claims.

Claims: Claims 1. A wavelength selective switch (1) for switching optical wavelengths comprising at least two multi-mode waveguides (10, 20), at least one wavelength selective cross- connection structure (2, 4, 6, 8), at least two phase control elements (Cl, C2, Dl, D2, El, E2, FI. F2), at least four connection waveguides, said wavelength selective cross-connection structure being arranged between a first (10) and a second (20) multi-mode waveguide, said first multi-mode waveguide (10) being provided with at least one access waveguide on a first side and at least two access waveguides on a second side, said second multi-mode waveguide (20) being provided with at least two access waveguides on a first side and at least one access waveguide on a second side, said wavelength selective cross-connection structure being provided with at least two access waveguides on a first and a second side, wherein at a wavelength selective cross-connection structure, a first access waveguide on the second side of the first multi-mode waveguide is arranged to a first access waveguide on the first side of the wavelength selective cross-connection structure via a first connection waveguide, a second access waveguide on the second side of the first multi-mode waveguide arranged to a first access waveguide on the second side of the wave length selective cross-connection structure via a second connection waveguide, a second access waveguide on the first side of the wavelength selective cross- connection structure is arranged to a first access waveguide on the first side of the second multi-mode waveguide via a third connection waveguide, and a second access waveguide on the second side of the wavelength selective cross-connection structure is arranged to a second access waveguide on the first side of the second multi-mode waveguide via a fourth connection waveguide, in which the phase control elements are arranged in the connection waveguides on one of the sides of the wavelength selective cross-connection structure, wherein at two or more wavelength selective cross connection structures for each wavelength selective cross-connection structure two phase control elements and two connection waveguides are added, each wavelength selective cross-connection structure being provided with two phase control elements on opposite side in relation to a nearest adjacent wavelength selective cross-connection structure and said wavelength selective cross-connection structures being mutual arranged to each other via connection waveguides from an access waveguide on a first wavelength selective cross-connection structure to another access waveguide on an adjacent wavelength selective cross-connection structure, in which said access waveguide are selected to being nearest to each other and on the same side. 2. A wavelength selective switch (1) according to claim 1, characterized in that the multi-mode waveguide is a MMI -waveguide. 3. A wavelength selective switch (1) according to claim 2, characterized in that the wavelength selective cross-connection structure is symmetrical. 4. A wavelength selective switch (1) according to claim 2, characterized in that the wavelength selective cross-connection structure (2, 4, 6, 8) is a MMI-Bragg grating structure (100). 5. A wavelength selective switch (1) according to claim 3 or 4, characterized in that the phase control elements (Cl, C2, Dl, D2, El, E2, FI, F2) belonging to each wavelength selective cross-connection structure changes the phase to the same extent. 6. A wavelength selective switch (1) according to claim 5, characterized in that all connection waveguides are of the same length. 7. A wavelength selective switch (1) according to claim 6, characterized in that for N wavelength selective cross-connection structures there are 2XN different Mach- Zehnder paths for N different wavelengths. 8. A method for switching optical wavelength channels in an optical network, characterized in -that the optical wavelength channels are connected in at least one access waveguide provided on a first side of a first multi-mode waveguide, -that the optical wavelength channels are transmitted through said first multi-mode waveguide and projected on at least two connection waveguides provided on the opposite side in relation to said access waveguide, -that the optical wavelength channels are transmitted through the connection waveguides, -that for each wavelength selective cross-connection structure, the phase is changed for a reflecting wavelength twice by two phase control elements arranged in a first and a second connection waveguide on a first side of said wavelength selective cross- connection structure, simultaneously as at a second side of said wavelength selective cross-connection structure, said reflecting wavelength phase remains relatively unchanged. -that for each wavelength selective cross-connection structure the phase is changed for transmitting wavelengths once in a first and a second direction per wavelength selective cross-connection structure, -that the optical wavelength channels are connected via at least two connection waveguides in a second multi-mode waveguide and -that the phase difference between the optical signal in each access waveguide provided on the first side of the second multi-mode waveguide determines where the optical signal is focused at the second side of the second multi-mode waveguide for a given length- witdh ratio of said multi-mode waveguide. . A method according to claim 7, characterized in that the multi-mode waveguide is a MMI- waveguide . 10. A method according to claim 9, characterized in that all connection waveguides are of the same length. 11. A method according to claim 10, characterized in that for N wavelength selective cross-connection structures there are 2XN different mach-Zehnder paths for N different wavelengths .

Inventors: AUGUSTSSON TORSTEN, AUGUSTSSON, TORSTEN
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET LM ERICSSON (PUBL)
Application year: 1999
Main classification: H04J14
IPC codes: G02B6, G02F1, H04B10, H04Q3, H04Q11