Guided wave spatial filter

• US 20040131310A1
• Filed: 09/11/2003
• Published: 07/08/2004
• Est. Priority Date: 10/09/2000
• Status: Active Grant

• Current Assignee: Lumentum Technology UK Limited (Lumentum Holdings Inc.)
• Original Assignee: Bookham Technology PLC (Lumentum Holdings Inc.)
• Inventors: Walker, Robert Graham
• Granted Patent: US 7,174,080 B2
• Time in Patent Office: Days
• US Class Current: 385/29
• CPC Class Codes: G02B 2006/12097 Ridge, rib or the like
• CPC Class Codes: G02B 2006/12109 Filter
• CPC Class Codes: G02B 2006/12173 Masking
• CPC Class Codes: G02B 2006/12178 Epitaxial growth
• CPC Class Codes: G02B 6/125 Bends, branchings or intersections
• CPC Class Codes: G02B 6/136 by etching
• CPC Class Codes: G02B 6/14 Mode converters
• CPC Class Codes: G02B 6/2813 based on multimode interference effect, i.e. self-imaging

Abstract

The invention concerns a guided wave spatial filter (300) for receiving input radiation and outputting corresponding filtered output radiation, the filter comprising first and second waveguide sections (30, L3, L5) connected in series, the sections: (a) mutually matched for transmitting fundamental mode radiation components present in the input radiation therethrough to provide the output radiation; and (b) mutually mismatched for hindering higher-order mode radiation components present in the input radiation from propagating therethrough and contributing to the output radiation. The spatial filter (300) is implemented using rib waveguides (30) for the sections with associated relatively deeply and relatively shallowly etched structures (20, 310, 320) for imparting to the sections their radiation mode filtration characteristics. The spatial filter according to the invention can be incorporated into optical splitters and optical modulators to enhance their performance and desensitise them to higher-order mode radiation injected thereinto.

Claims

1. A guided wave spatial filter for receiving input radiation and outputting corresponding filtered output radiation, the filter comprising first and second waveguide sections connected in series, the sections:

(a) mutually matched for transmitting fundamental mode radiation components present in the input radiation therethrough to provide the output radiation; and

(b) mutually mismatched for hindering higher-order mode radiation components present in the input radiation from propagating therethrough and contributing to the output radiation.

2. A spatial filter according to claim 1 wherein the waveguide sections comprise rib waveguides.

3. A spatial filter according to claim 2 wherein the first section is flanked by first regions and the second region is flanked by second regions, the first regions being more shallowly etched relative to the second regions.

4. A spatial filter according to claim 3 wherein the second regions are etched to a sufficient depth so that radiation coupled from the waveguide sections is substantially unable to propagate in the second regions.

5. A spatial filter according to claim 3 or 4 wherein the second regions are spatially isolated from the second section by the first regions.

6. A spatial filter according to claim 5 wherein the second regions are tapered towards the second section to provide a pin-hole type filter.

7. A spatial filter according to claim 3 or 4 wherein the second regions extend to abut onto the second section.

8. A spatial filter according to claim 7 wherein the second section includes outwardly inclined walls to its rib waveguide, the walls mutually inclined to hinder lateral mode radiation propagation along the second section.

9. A spatial filter according to claim 3 or 4 wherein the second regions have boundary edges which are disposed at non-orthogonal angles relative to an elongate axis of the second section, the edges operable to deflect bound higher-order mode radiation received thereat away from a radiation propagation path along the second section.

10. A spatial filter according to any one of claims 2 to 8 wherein the rib waveguide of the second section is laterally broader than the rib waveguide of the first section.

11. A spatial filter according to any preceding claim fabricated using III-V compound materials.

12. A spatial filter according to any preceding claim wherein the second section is provided with two first sections configured to precede and follow the second section, the sections being connected in series.

13. A radiation splitter comprising a radiation mixing region for supporting multimode radiation propagation therein, an input waveguide for injecting radiation into the mixing region, a plurality of output waveguides for receiving radiation output from the mixing region, and a spatial filter according to any preceding claim included between the input waveguide and the mixing region for transmitting fundamental mode radiation from the input waveguide into the mixing region and for hindering higher-order mode radiation from being injected from the input waveguide into the mixing region.

14. A splitter according to claim 13 wherein at least one of the output waveguides is connected to the mixing region through a spatial filter according to any one of claims I to 11, the filter operable to hinder coupling of higher-order mode radiation from the mixing region into said at least one of the output waveguides.

15. An optical modulator incorporating modulating means for selectively diverting radiation input to the means between a plurality of output paths in response to a control signal applied to the means, at least one of the output paths including a filter according to any one of claims 1 to 11 for transmitting fundamental mode radiation output from the means to said at least one of the output paths and for hindering propagation of higher-order mode radiation from the means to said at least one of the output paths.

16. A modulator according to claim 15 wherein the modulating means and output paths are implemented in rib-waveguide form, the modulating means and the filter being flanked by regions which are sufficiently deeply etched to render them substantially unsusceptible to supporting the propagation of higher-order mode radiation therethrough.

17. A modulator according to claim 16 wherein at least one of the output paths includes a faceted rib waveguide boundary where the path adjoins onto the modulating means, inclusion of the boundary enabling the path to project directly away from the modulating means at a greater angle than possible without the faceted boundary.

18. A modulator according to any one of claims 14 to 16 comprising an input path for injecting radiation into the modulating means, the input path connected through a filter according to any one of claims 1 to 11, the filter operable to transmit fundamental mode radiation from the input path to the modulating means and to hinder higher-order mode radiation being injected from the input path to the modulating means.

19. A modulator according to claim 14, 15, 16 or 17 wherein the modulating means comprises a structure susceptible to electric field induced changes of refractive index for selectively diverting the radiation input to the modulating means to the output paths.

20. A method of fabricating a guided wave spatial filter for receiving input radiation and outputting corresponding filtered output radiation, the filter comprising first and second rib-waveguide sections connected in series, the sections mutually matched for transmitting fundamental mode radiation components present in the input radiation therethrough to provide the output radiation, and mutually mismatched for hindering higher-order mode radiation components present in the input radiation from propagating therethrough and contributing to the output radiation, the method of fabrication including the steps of:

(a) providing a substrate onto which the filter is to be formed;

(b) defining and forming the rib-waveguide sections; and

(c) defining and forming structures to impart to the sections their radiation mode filtering characteristics, the rib-waveguide sections being used in the method to define boundaries of the structures and thereby render the method a self-aligning process.

21. A guided wave spatial filter substantially as hereinbefore described with reference to one or more of FIGS. 3 to 9.

22. A radiation splitter substantially as hereinbefore described with reference to one or more of FIGS. 3 to 9.

23. An optical modulator substantially as hereinbefore described with reference to one or more of FIGS. 3 to 9.

24. A method of fabricating a guided wave spatial filter substantially as hereinbefore described with reference to one or more of FIGS. 3 to 9.