SE520432C2 - Optical wavelength selective device for multiplexing=demultiplexing - Google Patents

Abstract

An optical device includes at least one multi mode interference (MMI) waveguide and at least one Bragg-grating structure [50]. At least one access waveguide [1,2] is arranged on a first side of the MMI-waveguide and at least one access waveguide is arranged on a second side of the MMI waveguide, the first and second sides being the short sides of the MMI-waveguide. The access waveguides are arranged with a so-called taper structure, and the Bragg-grating structure is arranged in the MMI-waveguide. The Bragg-grating may be arranged in the centre of the MMI-waveguide or in an offset relation to the centre of the MMI-waveguide.

Description

25 30 520 432 2 Another problem is to keep a so-called channel crosstalk at an acceptable level.

The present invention addresses the above-mentioned problems by an optical device comprising at least one MMI structure, at least one in which said optical device is further called Bragg reflector arranged with at least two such access waveguides for connection to external optical devices or optical fibers.

The above-mentioned MMI structure (Multi Mode Interference) has the property that the intensity distribution of light at one of the inputs of the MMI structure can be mapped to all the outputs to be used for the MMI-MMI structure. Thus, MMI structures can be fragmented by light. In this invention the length of the waveguide is selected so that 1: 1 imaging takes place, i.e. in the optimal case all incoming light from a first access waveguide arranged g fi x MMI waveguide Lu: is focused on a second access waveguide arranged on opposite side in relation to said first access waveguide . A more basic theory behind MMI structures is discussed in patent specification DE 2506272 and in L.B. Soldano and E.C.M. Pennings, "Optical Multi-Mode Interference Devices Based on Self-Imaging: Principles and Application", J. Lightwave Technol. Vol l3 (4), pp 615-627, 1995.

Bragre reflectors are used for filtering light. The filtering means that certain wavelengths are allowed to pass while others are reflected. Braggle reflectors can be said to constitute some form of a wavelength selective mirror. Said reflection of certain wavelengths can be achieved in a number of different ways, but usually it applies to these methods that the reflection takes place by changing a so-called material index periodically in the waveguide.

The present invention may also comprise a so-called phase control element. The phase control element affects a so-called optical wavelength in a waveguide. This is accomplished by an external signal affecting the guide.

One way of accomplishing said phase control is to subject the guide to an electric field. The electric field changes the effective refractive index in the guide.

Another way of achieving said phase control is to subject the waveguide to thermal changes.

One way to achieve permanent change of the index in the waveguide is to expose it to ultraviolet light, this is usually called because the guide is UV-written. This technique is most often used to periodically vary the refractive index, so-called UV writing. This technique can also be used for adjustment or trimming.

The above-mentioned methods for filtration as well as for phase control in a guide are only a selection and therefore do not exclude that non-mentioned methods can be applied to the invention.

The invention comprises an MMI structure in which a Bragre reflector is arranged. The Bragre reflector is preferably arranged in the center of the MMI structure. The access waveguides are arranged on the MMI structure. The location of these access waveguides on the MMI structure is critical to the operation of the optical device. A number of different designs partly on the MMI structure and partly on the access waveguides together with the Bragre reflector means that the invention solves the above-mentioned problems.

The object of the present invention is thus to obtain an optical device which comprises smaller power losses, smaller channel crosstalk and smaller power variations between different transmission channels compared with known technology.

An advantage of the present invention is that it is more compact compared to the prior art. Another advantage of the present invention is that it is relatively inexpensive to manufacture.

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

DESCRIPTION OF THE FIGURES Figure 1 shows an embodiment of an optical wavelength selective device according to the invention.

Figure 2 shows another embodiment of an optical path length selective device according to the invention.

Figure 3 shows a further embodiment of an optical wavelength selective device according to the invention.

Figure 4 shows another embodiment of an optical wavelength selective device according to the invention.

Figure 5 shows yet another embodiment of an optical wavelength selective device according to the invention.

Figure 6 shows another embodiment of an optical wavelength selective device according to the invention.

PREFERRED EMBODIMENTS Figure 1 shows an embodiment of an optical wavelength selective device according to the invention. The optical wavelength selective device includes a Bragg reflector 50 and an MMI waveguide. The brag reflector 50 may be arranged in the MMI waveguide in such a way that its center line coincides with the center line of the MMI waveguide.

The Bragg reflector can also, as shown in Figure 1, be arranged at a distance L / 2 + Lphc from one short side of the MMI waveguide, where Lphc denotes said displacement from the center of the MMI waveguide. Lphc can be either positive or negative. The displacement of the Bragg reflector from the center of the MMI waveguide is to compensate for the mode-dependent phase shift, which can otherwise be a threat to the operation of the device. The Bragre reflector has a certain width, which is denoted by LBg. The MMI waveguide has a certain length which in Figure 1 is denoted by L.

The MMI waveguide can be arranged on its short sides with so-called access waveguides 1,2,3,4. In Figure 1, these access waveguides are to the number four, i.e. a pair on each short side. The number of access waveguides may vary from one embodiment to another depending on the application for which the optical wavelength selective device is intended. The figure shows the center lines 10, 20, 30 and 40 of the access waveguides. The distance from a long side of the MMI guide to the center line 10 of the access waveguide 1 is indicated in Figure 1 by a. 2 is denoted in Figure 1 by b. Similarly, the distance from said long side of the MMI guide to the other access guides 3 and 4 is denoted by c and d, respectively. The distances a and c may be equal and the distances b and d may be equal. The distances a, b, c and d depend on the MMI waveguide's effective width We, the number of images and the type of MMI waveguide in question.

A deeper theory behind various MMI waveguides is discussed in Pierre A. Besse et. al, “Optical Bandwidth and Fabrication Tolerances of Multimode Interference Couplers”, J. Lightwave Technology. vol 12 (4), pp 1004-1009, 1994.

The effective width We of the MMI waveguide depends on the wavelength,, the index step of the MMI waveguide, the physical width of the MMI guide, and the polarization of light.

The length of the MMI waveguide depends on the MMI waveguide's effective width We and the effect you are striving for. 10 15 20 25 30 520 432 6 The access waveguides in Figure 1 are wider in connection with the MMI waveguide than they are at their free end. This structure is usually called "tapered" or conical. The effect of this structure is that that field changes optically compared to a straight access waveguide. This means that a greater fault tolerance towards error correction of the access waveguides is obtained. In addition, the effect will to a greater extent be in the lower order mode, which is an advantage since the Bragre reflector will provide a mode-dependent phase shift for the reflected channel.

Phase control elements can also be included in this optical path length selective device. This phase control element can be arranged in a number of different ways. Some possible ways have been discussed under the heading description of the invention and are only already known technology to an average person skilled in the art, so they should not need to be described in more detail.

In Figure 2 we see another embodiment of the optical wavelength selective device according to the invention. This embodiment includes, as previously described, a Bragg reflector 50 and an MMI guide. The width of the Bragre reflector is denoted by LBg. The length of the MMI waveguide is denoted by L just as in the above-mentioned embodiment. What distinguishes this embodiment from the first is the shape of the MMI waveguide. This is similar to the access waveguides 1, 2, 3 and 4 conical. A small section on both sides around the Bragre reflector in the longitudinal direction of the MMI waveguide, the long sides of the MMI waveguide are parallel and orthogonal to an imaginary center line in the MMI width. on the MMI waveguide just with W2. The width of the longitudinal direction of the MMI waveguide. next to the Bragre reflector, the short sides of the waveguide are denoted by W1, where W1 the MMI waveguide may, as shown in Figure 2, comprise an end portion with a length denoted L3. Said length L3 may in another embodiment be equal to zero. Between the widths W1 and W2 in the MMI waveguide, the structure is conical.

The structure may be either linear, parabolic or some other form. The task of the structure in this case is to reduce the difference between the propagation modes and. at the so-called depth of penetration into the lattice of the reflected modern. way to reduce the difference in the so effective On the short sides of the MMI waveguide, access waveguides 1, 2,3 and 4 are arranged. In Figure 2, these are two in number on each short side. In the same way as in the previous embodiment, center lines 10, 20, 30 and 40 for the respective access waveguides 1, 2, 3 and 4 have been marked in the figure. The distance from one end of the short side to said center line 10 belonging to access waveguide 1 is denoted by a. The distance from to associated access waveguide 2 is denoted by b. In the same way the same said end of the short side centerline 20 is denoted d. The distances a and c can be equal and the distances b and d can be equal. As mentioned in the previous embodiment, either the Bragre reflector can. be arranged in the center of the MMI waveguide or offset a little to the center has just distance from this. The reason is the displacement of the Bragre reflector from the same cause as mentioned in the previous embodiment, i.e. to compensate for any mode-dependent phase shift.

Figure 3 shows a further embodiment of an optical wavelength selective device according to the invention. The only thing that distinguishes this embodiment from that shown in Figure 2 is that the optical so-called path length has been changed for a number of access waveguides. In Figure 3, the optical path length has been extended for access waveguides 2 and 3 by arranging these on a projecting part of the MMI waveguide. The width of the part has in Distances e and f can be equal or different mentioned protruding figure 3 is denoted by e and f respectively. Effect one strives for. Of course, it is possible that any, one or more, of the access waveguides provided on the MMI waveguide are arranged on some form of means for changing the optical wavelength. The purpose of changing the path length for some access path guides is to compensate for mode-dependent phase shifts.

If we assume that the length L of the MMI guide corresponds to a so-called cross position, a so-called bar position can be obtained by increasing the length of the MMI guide to 2L. By cross mode is meant that the word indicates that at least one path length channel coming from one side of the MMI guide is transmitted through the MMI guide, the other side of the MMI guide where said access guide on the other side of the MMI guide is laterally offset from said access guide from which the signal was exited. .

An example of cross mode is when a path length channel is transmitted from access path guide 10 and focused on access path guide 40. By bar position is meant that the path length channel is transmitted from an access path guide on one side of the MMI guide and is focused on the corresponding access path guide arranged on the other side. An example of bar mode is when a wavelength channel is transmitted from access waveguide 10 and focused on access waveguide 30.

In Figure 4 we see a further embodiment of the optical wavelength selective device according to the invention. In this embodiment, two MMI guides are arranged one after the other. These have been joined together via either a guide or an optical fiber. The two MMI guides have a structure substantially similar to that shown in Figure 2 except at the ends to which they are joined together. In figure 4 we see that said ends only comprise an access guide. In addition, a portion p, q of this short side is not orthogonal to the center line of the access guide.

The reason for this is that light that is not desirable in the MMI waveguide must be able to be refracted at this part and disappear out of it. The effect that is achieved by cascading two MMI guides one after the other is that the crosstalk is also thought to be reduced. Embodiment may comprise a phase control element of the type mentioned in the description of the invention. The number of access waveguides arranged on the two MMI waveguides can be selected as required, preferably two on one side and two on the opposite other side. As shown in the figure, the Bragre reflector may be offset from the center of the MMI guide or be arranged in the center thereof.

In Figure 5 we see another embodiment of an optical wavelength selective device according to the invention. In this embodiment, the two MMI waveguides are combined directly with each other.

In Figure 5 we see that the MMI waveguide is only conical on the side at which the access waveguides are arranged. Between the two, the MMI waveguides' respective long sides MMI-Bragre reflectors are parallel to each other. The centerline of one waveguide is laterally parallel shifted relative to To eliminate the centerline of the other MMI waveguide. unwanted light reflections in the MMI waveguides, the portions p and q, respectively, which were left, so to speak, at the above-mentioned lateral parallel displacement, have been angled on each MMI waveguide. This embodiment can also be provided with a phase control element of the type mentioned under the heading description of the invention. The number of access waveguides in the free ends of the MMI waveguides can be freely selected as needed, where the dimensions of the MMI waveguide set the practical limit for said number.

Instead of arranging the access waveguides on the above-mentioned part, can be changed in connection with suitable access waveguides so that projecting refractive indices in the MMI waveguide obtain the same effect, i.e. change the optical wavelength inside the MMI waveguide in order to compensate for mode-dependent phase shifts. This is shown in Figure 6. Here, the refractive index i. Has the MMI guide. raised in a rectangular area 60 just adjacent to a pair of access waveguides with the longitudinal centerline of the rectangle 10 520 432 10 'coinciding with the centerline of the respective access waveguides. Said refractive index change can be obtained by transforming existing material in the MMI waveguide by, for example, UV writing. The shape and dimension of said refractive index change is decisive for its effect.

The materials which may be suitable in the manufacture of the present invention are, for example, quartz (SiO2), polymeric materials or semiconductor systems or Lithiumniobate (LiNbO2).

Quartz is preferably used.

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

Claims (14)

lO 15 20 25 30 520 432 ll PATENT REQUIREMENTS An optical device comprising at least one MMI guide and at least one Bragg reflector structure, characterized in that at least one so-called access guide is arranged. on at least one of the MMI first side of the MMI guide and that the access waveguide is arranged on a second side of the guide, said first and second side being made up of the short sides of the MMI guide, that said access waveguide is arranged with a so-called structure and is arranged in the MMI guide. Optical characteristic of the Bragre reflector being arranged in a device according to claim 1, the center of the MMI waveguide. 3. An optical device according to claim 1, characterized in that the Bragre reflector is arranged offset relative to the center of the MMI waveguide. 4. An optical device according to claim 2 or 3, characterized in that it comprises a thermal, optical or electrically acting phase control element. 5. An optical device according to claim 4, characterized in that the MMI waveguide is arranged with a structure on each side of the Bragre reflector structure. Optical device according to claim 5, characterized in that said structure on the MMI waveguide is linear. Optical device according to claim 5, characterized in that said structure on the MMI guide is parabolic. 8. An optical device according to any one of claims 6 or 7, characterized in that at least one access waveguide on one side of the MMI guide is arranged thereon in such a way that the wavelength differs from the the other access guides. Optical device according to one of Claims 6 or 7, characterized in that at least one access waveguide on the first and the second side of the MMI waveguide is arranged on them in such a way that the path length differs from the other access guides. Optical device according to claim 8 or 9, characterized in that an access waveguide on a first MMI guide is connected to an access waveguide on a second MMI waveguide. 11. ll. Optical device according to claim 10, characterized in that a portion adjacent to at least one arranged with an access waveguide in the MMI waveguide is a refractive index that differs in relation to the refractive index in other parts of the MMI waveguide. 12. An optical device according to claim 11, characterized in that in the case of at least two MMI waveguides and at least two Bragre reflectors where at least one so-called access waveguide is arranged on a first side of a first MMI waveguide and at least one access waveguide is arranged on a second side of a second MMI guide where said first and second sides are constituted by the short sides of the MMI guide, that a second short side of the first MMI guide and a first side of the second said MMI guide are interconnected, that the access guide is arranged with a so-called structure and that the Bragre reflector structures are arranged in the MMI waveguide. 13. An optical device according to claim 12, characterized in that the second side of the first MMI waveguide and the first side of the second MI waveguide are laterally offset from each other. 520 432 13 14. An optical device according to claim ~ 13, characterized in that the MMI guides are arranged with a structure on each side of the Bragre reflector structures.