WO2001033263A1

WO2001033263A1 - Optical noise filtering for planar photonic devices

Info

Publication number: WO2001033263A1
Application number: PCT/US2000/028279
Authority: WO
Inventors: Barthelemy Fondeur
Original assignee: Corning Incorporated
Priority date: 1999-11-03
Filing date: 2000-10-12
Publication date: 2001-05-10
Also published as: FR2800475A1; AU1199801A

Abstract

A method of filtering optical noise generated by an optical component (2) in a planar photonic device (1), includes the step consisting in placing refractive index change regions symmetrically on either side of the path of the light downstream from the optical component (2), said regions being adapted to prevent forward propagation of the light outside said path. The refractive index change regions can be constituted by trenches (10) or by regions (20) possessing sawtooth-shaped elements (25). The invention also provides a planar photonic device in which said method is implemented.

Description

OPTICAL NOISE FILTERING FOR PLANAR PHOTONIC DEVICES

Background Of The Invention

1 Field ol the Inv ention

The present invention relates generally to filtering the optical noise that occurs in planar photonic αeviccs and particularly to planar photonic devices including optical noise filters

2 Technical Background

The development of optical systems using the wavelength division multiplexing (WDM) transmission method is leading to an increasing number of complex components being used such as gain flattening filteis, variable attenuation devices multiplexers for adding or dropping wavelengths, etc Typically it is desired to integrate all such components in a planar device since that makes it possible to implement a plurality ot optical tunctions in a single wafer It is also easy to add active devices to the water, thereby creating a hybrid element that is very compact and that supplies a plurality of functions required in optical networks (functions such as switching attenuation control, wavelength division multiplexing and demultiplexing etc )

Clearly, if a plurality of elements are integrated in a single wafer, the performance of the device as a whole will be greatly diminished if the optical noise generated by one of the components interferes ith the other components

In conventional photonic devices there are several elements that constitute sources of optical noise, e g Nx l beam combiners. Mach-Zenders. multiplexers, and switches Figure 1 shows an optical device 1 including one such source of optical noise, in particular a Y-junction (beam) combiner 2, and a tap 3 connected to the waveguide 5 at a position which is downstream from the beam combiner 2 in the propagation path of the light carried by the waveguide 5 The tap 3 serves to measure the level of the optical signal propagating in the waveguide 5

When two light beams reach the combiner 2 with different phases (e g from a Mach-Zender), asymmetric mode is excited in the region of convergence and causes a portion of the light to leave the waveguide Thus, two unguided side lobes are created around the waveguide and diverge into the cladding thereof The light energy present in these side lobes can reach other optical components situated downstream from the combiner 2 on the propagation path of the light, and can give rise to interference

Typically, the Y-j unction combiner 2 in Figure 1 is designed to act as a switch In which case, as shown in Figure 2, the two light beams which reach the phase combiner 2 are at a phase difference of p ( 180°) and asymmetric mode is 100% excited, so there is no longer any light propagating in the waveguide 5 The level of the signal measured by the tap 3 should therefore be equal or close to zero However as can be seen in Figure 2, a portion of the eneigy from the unguided side lobes is detected by the tap 3 In other words, the optical noise generated by the beam combiner 2 causes the measured level of the signal propagating in the waveguide 5 to be too high

An object of the present invention is to provide a method of eliminating or reducing the optical noise that occurs in planar optical devices

Another object of the present invention is to provide optical devices in which optical noise is reduced

More particularly, the present invention provides a method of filtering optical noise generated by an optical component in a planar photonic device, the method being characterized in that it includes the step consisting in placing refractive index change regions symmetrically on either side of the path of the light downstream from the optical component, said regions being adapted to prevent forward propagation of light outside said path

In addition, the present invention also provides a planar photonic device including at least one optical component which constitutes a source of optical noise, the device being characterized in that it includes refractive index change regions disposed symmetrically on either side of the path of light downstream from the optical component, the regions being adapted to prevent forward propagation of light outside said path

Normally the light propagation path is defined by a waveguide connected to the outlet of the component constituting a source of optical noise The refractive index change regions serve to cause the light propagating outside the main waveguide to be absorbed or reflected sideways so that this light does not give rise to interference in other optical components located downstream on the waveguide The fact that these regions are disposed symmetrically on either side of the main waveguide has the consequence that they filter the optical noise effectively, i.e. the light constituting the optical noise is not reinserted into the main waveguide.

The present invention applies not only to reducing the optical noise caused by elements such as Y-junction combiners, Mach-Zenders, and switches in photonic devices, but also to reducing the optical noise generated by other components which give rise to unwanted side lobes, such as phasars for example.

In a first embodiment of the invention, the refractive index change regions consist in a trench or a series of trenches one after another etched in the photonic device and disposed symmetrically on either side of the light propagation path (usually on either side of the main waveguide).

The trenches used in the first embodiment of the invention absorb the light of the unwanted side lobes propagating outside the main waveguide Alternatively, the walls of the trenches can be coated, or the trenches are filled, with a reflecting material so as to reflect the light of the side lobes

In a second embodiment of the invention, the refractive index change zones consist in sawtooth-shaped regions created in the core layer of the medium constituting the photonic device. Each of these sawtooth-shaped regions possesses a plane surface placed substantially perpendicularly to the light propagation direction in the main waveguide.

The sawtooth-shaped regions used in the second embodiment of the invention reflect sideways the light which constitutes the optical noise. It can thus happen that this light gives rise to interference if other optical devices are located on the sides of the main waveguide However, in the second embodiment of the invention it is possible to fabricate the noise-filtering element during one of the normal steps of etching the core layer (typically during formation of the waveguide). Thus, the second embodiment of the invention does not require an additional step in the fabrication of the photonic device.

Brief Description Of The Drawings

Other characteristics and advantages of the present invention appear from the following description of representative embodiments of the invention given as examples for the purpose of better understanding how to practice and utilize the present invention, and illustrated in the accompanying drawings, in which: Figure 1 is a diagram showing a planar photonic device including a source of optical noise; Figure 2 is a diagram showing energy distribution in the Figure 1 device when asymmetric mode is 100% excited;

Figure 3 is a diagram showing the Figure 1 photonic device modified by adding filter means constituting a first embodiment of the invention; Figure 4 is a diagram showing energy distribution in the Figure 3 device when asymmetric mode is 100% excited;

Figure 5 is a diagram showing energy distribution in the Figure 3 device when symmetric mode is excited for the purpose of conveying energy along the main waveguide; Figure 6 is a diagram showing the photonic device of Figure 1 modified by adding filter means constituting a second embodiment of the invention;

Figure 7 is a diagram showing the effect of the filter means of the second embodiment of the invention on the light of the side lobes;

Figure 8 is a diagram showing energy distribution in the Figure 6 device when asymmetric mode is 100% excited; and

Figure 9 is a diagram showing energy distribution in the Figure 6 device when symmetric mode is excited in order to convey energy along the main waveguide.

Detailed Description Of The Preferred Embodiments

The present invention is described below with reference to a planar photonic device of the same kind as that shown in Figure 1 , i.e. a device in which the source of optical noise is a Y-junction combiner. However, it should be understood that the present invention applies more generally to filtering the optical noise that occurs in any device including optical elements which create undesired side lobes (Mach-Zenders, phasars, etc. ...).

Figures 3 to 5 show the planar photonic device of Figure 1 in which filter means constituting a first embodiment of the invention are disposed downstream from the Y- junction combiner 2 that is a source of optical noise. As can be seen in Figure 3, in the first embodiment of the invention, the filter means consist in two symmetrical rows of trenches 10 etched in the wafer. The trenches 10 are substantially perpendicular to the propagation direction of the light beam and they are disposed on either side of the main waveguide 5. It is possible for the trenches 10 to be left empty, in which case they absorb part of the energy. Alternatively, it is possible to fill the trenches 10 with a material that is reflective or absorbent (or else the walls of the trenches can be lined with such material) so that they reflect or absorb energy. The function of these trenches 10 on light power is comparable to that of a series of irises When the side lobe reaches the first ins (i.e. the first pair of trenches 10), the peripheral portion of the beam is absorbed or reflected, but the central portion of the beam passes through. The energy of this central portion of the beam is diffracted so that a diverging beam is produced The next trench absorbs or reflects the peripheral portion of this diverging beam, allowing its central portion to pass through, and so on. Thus, less and less side lobe energy propagates forwards.

The effect of these rows of trenches 10 in the first embodiment of the invention is shown in Figures 4 and 5 As can be seen in Figure 4. the unwanted side lobe energy of asymmetrical mode is filtered so hardly any of the energy reaches the tap 3 As can be seen in Figure 5 (which shows symmetrical mode), the presence of the trenches 10 gives rise to little attenuation of the energy since most of it is confined in the main waveguide

The second embodiment of the invention is described below with reference to Figures 6 to 9 which show the planar photonic device of Figure 1 in which the filter means of the second embodiment of the invention are disposed downstream from the Y-junction combiner 2 acting as a source of optical noise.

As can be seen in Figure 6, in the second embodiment of the invention, the filter means consist in two regions 20 of relatively high refractive index disposed on either side of the main waveguide, each of said regions 20 having an edge 25 that is sawtooth- shaped disposed facing the main waveguide. These regions 20 are etched in the same core layer as the main waveguide 5. Implementing this embodiment therefore does not require a new step in the process of fabricating the photonic device (since the regions 20 are etched simultaneously with the core of the waveguide) Figure 7 shows how the saw teeth 25 deflect the energy from the unwanted side lobes generated by the combiner 2. Each tooth 25 has a surface 25a disposed substantially perpendicularly to the propagation direction of the side lobe light, and a sloping surface 25b The side lobe light enters the region 20 of high refractive index by passing through the surface 25a of a tooth 25 When the energy reaches the sloping surface 25b, it is reflected by internal reflection so as to go away from the main waveguide.

As can be seen in Figure 8, which shows asymmetric mode light, the energy of the unwanted side lobes is deflected so that it leaves the wafer instead of remaining in the central portion. This eliminates interference with other optical devices (such as the tap 3) to be found downstream on the path of the light conveyed by the waveguide. The last saw tooth 25 performs greater filtering on the power of the side lobe since the energy is now confined in the region of high refractive index 20. As in the first embodiment, the filter means of this second embodiment of the invention give rise to minor attenuation only of the wanted energy, i.e. the (symmetrical mode) energy propagating in the main waveguide 5 (see Figure 9). However the evanescent portion of symmetrical mode propagating in the waveguide 5 is disturbed to a small extent because of the presence of the regions 20, as with a "directional coupler".

Unlike such a coupler, the regions 20 can be placed further away from the waveguide than would be required for a rectangular region. It is thus easy to adapt the parameters of the device so as to minimize losses in the transmitted symmetrical mode. Although the above description of the invention relates to particular embodiments thereof, the invention is not limited to the specific characteristics of those embodiments. On the contrary, numerous modifications and adaptations can be made to the embodiments described herein while still remaining within the scope of the accompanying claims.

Claims

What is claimed is:

1. A method for filtering optical noise generated by an optical component in a planar photonic device having defining a path for light, the method comprising the step of: placing refractive index change regions symmetrically on either side of the path of the light downstream from the optical component, said refractive index change regions being adapted to prevent forward propagation of light outside the path.

2. The method of claim 1 wherein the optical component has an optical medium that the step of placing refractive index change regions comprises the step of: creating one or more trenches on either side of the path, the one or more trenches interrupting the optical medium constituting the photonic device.

3. The method of claim 2 wherein the one or more trenches have walls, and the method further comprises the step of: lining the walls of the one or more trenches with a material that either reflects or absorbs light.

4. The method of claim 2 further comprising the step of: filling the one or more trenches with a material that either reflects or absorbs light.

5. The method of claim 1 wherein the photonic device has a core layer formed from a medium, and wherein the step of placing refractive index change regions comprises the step of: creating regions possessing elements in the form of saw teeth in the core layer of the medium constituting the photonic device, each of the elements possessing a plane surface disposed substantially perpendicularly to the path along which light propagates.

6. The method of claim 5wherein the step of creating regions possessing elements in the form of saw teeth forms a portion of the step in the method of fabricating the core layer of the medium constituting the photonic device.

7. A planar photonic device for propagating light along a path, said planar photonic device comprising: at least one optical component including a source of optical noise and a path for propagating light, a refractive index change region disposed symmetrically on either side of the path of the propagating light downstream from the optical component, the refractive index change regions being adapted so as to prevent forward propagation of light outside the path.

8. The photonic device of claim 7 wherein the refractive index change region comprises: one or more interrupting the optical medium constituting the photonic device and disposed on either side of the path of the propagating light.

9. The photonic device of claim 8 wherein the one or more trenches are filled with a material that reflects or adsorbs light.

10. The photonic device of claim 8 wherein the one or more trenches define walls, and the walls are lined with a material that reflects or adsorbs light.

11. The photonic device of claim 7 wherein photonic device has a core layer formed from a medium, and further wherein the refractive index change region comprises: one or more regions possessing sawtooth-shaped elements defined in the core layer of the medium, each of the sawtooth-shaped elements possessing a plane surface disposed substantially perpendicularly to the path of the propagating light.