WO2004104686A1

WO2004104686A1 - Optical filter device with high index switchable grating

Info

Publication number: WO2004104686A1
Application number: PCT/US2004/013699
Authority: WO
Inventors: Keat Leem Chan Edward
Original assignee: Hoya Corporation
Priority date: 2003-05-14
Filing date: 2004-05-01
Publication date: 2004-12-02

Abstract

A waveguide filter device for optical telecommunications network is provided. The device comprises a substrate (1) containing a waveguide (2) disposed close to the surface of said substrate, an overcladding layer (3) overlaid on said substrate and a cover plate (4) overlaid on said overcladding layer. The waveguide has an input port for receiving an input light signal and an output port for outputting the filtered light signal. The overcladding layer and the cover plate lie within the evanescent coupling field of the waveguide. The average refractive index of said overcladding layer is greater than the waveguide core mode index, while the average refractive index of the cover plate is lower than the waveguide core mode index. In a preferred embodiment of the invention the overcladding layer is an Electrically Switchable Bragg Grating.

Description

PLANAR OPTICAL FILTER DEVICE USING HIGH INDEX SWITCHABLE GRATING

This application claims the benefit of a U.S. Provisional Patent Application by inventor: Edward, Keat, Leem CHAN, filed May 14, 2003.

BACKGROUND OF THE INVENTION

This invention relates to optical components for use in fiber optic communications systems. Specifically, the invention relates to a dynamic optical filter device comprised of a planar optical waveguide overlaid by a switchable Bragg grating, wherein the average refractive index of the switchable Bragg grating is higher than the mode index of the waveguide

Fiber optic telecommunications networks incorporate a variety of components to control and switch the optical signals. U.S. Patent 5,937,115 discloses a family of electro- optical components comprised of an optical waveguide fabricated on, or just under, the surface of a substrate, a layer of polymer dispersed liquid crystal material in which an Electrically Switchable Bragg Grating (ESBG) has been formed, and a cover plate. The ESBG is formed by curing a mixture of liquid crystal and monomer materials by means of UV light in the form of an interference fringe pattern. During curing, the liquid crystal material phase separates from the polymerized monomer such that the completed ESBG has fringes in the form of polymer walls separated by liquid-crystal rich regions. The cover plate, substrate, or both must have electrodes for selectively applying an electric field across the ESBG layer. The liquid crystal molecules in the liquid crystal rich regions will rotate to align with the direction of the applied electrical field, thus changing the diffraction efficiency of the Bragg grating and/or the average refractive index of the ESBG layer. Such components are useful as wavelength-selective filters, switches, and attenuators in fiber optic communications systems.

U.S. Patent 5,937, 115 discloses that the ESBG region may be placed in either the guiding region (core) of the waveguide or in the cladding where the ESBG interacts with a portion of the evanescent field of the light guided in the core. This patent also teaches that, if the ESBG region is placed in the cladding adjacent to the guiding region, the ESBG average refractive index must be lower than that of the guiding region. This limitation on the average refractive index of the ESBG limits the applicability of this device construction. In particular, the most common processes used to manufacture planar waveguide devices result in core indices in the range of 1.45 to 1.46, which is very low compared to the refractive index of the preferred polymer dispersed liquid crystal materials. Thus there exists a need for a different device configuration that allows more flexibility in the selection of the ESBG material components.

SUMMARY OF THE INVENTION

The present invention provides an optical telecommunications network component for filtering a guided light beam, comprising: a waveguide having an input port for receiving an input light signal and an output port for outputting the filtered light signal, an overcladding layer in optical contact with said waveguide and a cover plate in optical contact with said overcladding layer. The average refractive index of said overcladding layer is greater than the waveguide core mode index. The average refractive index of the cover plate is lower than the waveguide core mode index. In a preferred embodiment of the invention the overcladding layer is an Electrically Switchable Bragg Grating. BRIEF DESCRIPTION OF THE DRAWINGS

FIG.1A is a schematic front elevation cross-sectional view of a prior art device.

FIG.1 B is a schematic side elevation cross-sectional view of a prior art device.

FIG. 2 is a chart showing the measured wavelength response of a device with an overcladding comprising a layer of Cargille oil.

FIG.3A is a schematic front elevation cross-sectional view of a first embodiment of the invention.

FIG.3B is a schematic side elevation cross-sectional view of a first embodiment of the invention.

FIG. 4 is a chart showing the measured wavelength response of a device according to the principles of the invention with an overcladding containing a Bragg grating. FIG. 5 is a chart showing the simulated attenuation as a function of wavelength for different Bragg grating tilt angles.

FIG.6 is a flow diagram of a method of fabricating a waveguide filter device in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be further described byway of example only with reference to the accompanying drawings. FIG.1 A and FIG.I B are two cross-sectional views of a prior art device. The device is comprised of a substrate 1 , an optical waveguide core 2 fabricated on, or just under, the surface of said substrate, an overcladding layer 3, and a cover plate 4. The overcladding layer 3 can be a layer of polymer dispersed liquid crystal material in which an Electrically Switchable Bragg Grating (ESBG) has been formed. The device further comprises patterned switching electrodes, which are not shown. The direction of propagation of the guided wave is generally indicated by 100 in FIG.1 A and by 200 in FIG.1 B. As the index modulation of the ESBG changes under the action of an electric field, the interaction of the mode field with the grating changes thus varying the amount of light at particular wavelengths that is coupled out of the forward propagating waveguide core mode.

Although a buried waveguide of circular cross section disposed just under the surface of the substrate is shown in FIG.1 , other configurations capable of providing the required optical coupling with the overcladding could be used. For example, the waveguide core may be in contact with the overcladding layer. The waveguide may be part of a coupler half. The waveguide may have a rectangular cross section.

U.S. Patent 5,937,115 teaches that if the ESBG region is placed in the cladding adjacent to the guiding region, the ESBG average refractive index must be lower than that of the guiding region for low insertion losses. This limitation on the average refractive index of the ESBG limits the applicability of this device construction shown in FIG.1. In particular, the most common processes used to manufacture planar waveguide devices result in core indices in the range of 1.45 to 1.46 (for light with wavelengths around 1550nm), which is very low compared to the refractive index of the preferred polymer dispersed liquid crystal materials. Thus there exists a need for a different device configuration that allows more flexibility in the selection of the ESBG material components.

The present invention uses overcladding layers with higher refractive index than the waveguide core mode yet has low insertion loss within the wavelength window of interest. In order to achieve the low insertion loss, the thickness of the overcladding must be chosen carefully and the overcladding must be capped by a cover with lower refractive index than the waveguide mode.

Higher-index over claddings sandwiched between materials of lower refractive indices have one or more well-defined modes with well-defined mode indices. By designing the overcladding modes to have indices sufficiently different from the effective index of the waveguide within the wavelength span of interest, coupling into these overcladding modes from the waveguide core mode is minimized. Hence, the loss of power is minimized. The overcladding mode indices are primarily a function of the overcladding index, the overcladding thickness and the wavelength. The overcladding mode indices can be computed from the theory of waveguides as discussed in a paper by D.G. Moodie and W. Johnstone entitled "Wavelength tunability of components based on the evanescent coupling from a side-polished fiber to a high-index-overlay waveguide," Optics Letter, Vol. 18, No. 12, June 15, 1993, pages 1025-1027.

FIG.2 shows the throughput of a device comprising a polished fiber block (half-coupler) with an overcladding of Cargille Oil. In a similar fashion to the construction in FIG.1 , the polished fiber block functions as the waveguide core and substrate whereas the Cargille Oil is the overcladding and fused silica glass is the cover. The Cargille Oil overcladding layer is 4um thick and has refractive index of 1.4996 for light with wavelengths around 1550nm. This index is higher than the mode index of the polished fiber block of about 1.447. The fused silica cover has refractive index of about 1.444.

As shown in FIG. 2, even though the refractive index of the Cargille Oil is higher than the waveguide mode index, the insertion loss is low for the wavelength span 1470nm to 1700nm. In this particular device, the thickness of the oil was chosen such that the mode indices of the overcladding would match that of the waveguide core at a wavelength of about 1420nm leading to the attenuation peak in FIG. 2. Thus, the device has low insertion loss in the C-band telecommunications window from 1520nm to 1570nm.

Typically, the overcladding thickness is chosen to be as small as possible while still allowing good evanescent coupling and manufacturability. This is chosen so that the mode indices of the various overcladding modes will be widely separated leading to large wavelength spans of low insertion losses.

FIG. 3 shows a cross-sectional view of a first practical embodiment of this invention. The device is comprised of a substrate 10, an optical waveguide core 20 fabricated on, or just under, the surface of said substrate, an overcladding layer 30, and a cover plate 50. The overcladding layer 30 is a layer of polymer dispersed liquid crystal material in which an ESBG has been formed. The waveguide core has mode index of about 1.447 which is the most commonly available. The cover is fused silica, which has an index of about 1.444. The ESBG overcladding has an average index of about 1.470 when unswitched (no applied electric field) and 1.500 when switched. The ESBG layer is about 4um thick. The thickness was chosen to ensure that the mode indices of the overcladding do not match the mode index of the waveguide throughout the average index range from 1.470 to 1.50 as the ESBG is switched on. This will ensure low insertion losses within the wavelength span of interest throughout operation.

When the ESBG is switched, the index modulation of the Bragg grating increases thus increasing the out-coupling of a particular wavelength of light out of the forward propagating waveguide mode. This Bragg grating coupling creates the narrow notch shown in FIG. 4. This figure shows the attenuation within the wavelength span of interest - the attenuation is high at the design wavelength of 1592 nm. and low everywhere else. The ESBG must have a state of very low index modulation so that the attenuation peak can be switched off when desired. This low index modulation state can be either the completely unswitched state or a partially switched state.

The design wavelength (Bragg wavelength) is determined by the Bragg grating pitch, the mode index of the waveguide core mode, and the mode index that the light is coupled to. In this particular embodiment, the light is coupled into one of the modes of the overcladding. Note that coupling only occurs in the presence of a Bragg grating since the mode indices of the overcladding and the waveguide core are sufficiently different.

The Bragg grating will couple light from the forward propagating waveguide core mode into one or more of the reverse propagating overcladding or waveguide core modes. If light is coupled into more than.one mode, then more than one attenuation peak will appear in the spectrum. This can be seen in FIG. 5 where the simulated wavelength response of a device is plotted as a function of the tilt angle of the Bragg grating. Note that an untilted grating has Bragg planes normal to the propagation axis of the waveguide. Multiple peaks appear for devices with tilt angles of 0, 2.8 and 8.5 degrees. To obtain only a single peak, the Bragg grating must be tilted about 5.7 degrees. This is because the tilt causes the Bragg condition to be satisfied for coupling to only one mode. The properties of tilted Bragg gratings are described in more detail in a paper by T. Erdogan and J. E. Sipe entitled "Tilted fiber phase gratings," J. Opt. Soc. Am. A13, pages 296-313 (1996). An additional benefit of using a higher index overcladding layer is the ability to couple into a mode other than the reverse propagating waveguide core mode as is typical of most Bragg grating-based devices. By coupling into an overcladding mode instead of a waveguide core mode, light is prevented from coupling back into the input of the device and hence return losses are minimized.

A method of fabricating a waveguide filter device in accordance with the invention will be described with reference to FIG.6. At step 300, a substrate is provided. At step 310, a waveguide is formed near to the surface of said substrate, said waveguide having a mode index given by N₀. At step 320, an overcladding layer is applied to said substrate surface, said overcladding having an average index Noc where N_σc is greater than N₀. At step 330 a cover plate is provided, said cover plate having a refractive index given by NO_P, where N_Cp is lower than N₀. At step 340, switching electrodes are deposited on said cover plate. At step 350, said cover plate is applied to said overcladding layer.

While the invention has been shown and described above with respect to selected structures, processes and applications, it should be understood that these structures, processes and applications are by way of example only and that one skilled in the art could construct other structures and applications utilizing techniques other than those specifically disclosed and still remain within the scope of the invention.

Claims

CLAIMSWhat is claimed is:

1. A component for filtering a guided light beam comprising: a substrate containing at least one waveguide core disposed near to the surface of said substrate, said waveguide core having an input port for receiving an input light signal and an output port for outputting a filtered light signal; an overcladding layer overlaid on said substrate; and a cover plate overlaid on said overcladding layer; wherein said overcladding layer and said cover lie within the evanescent coupling field of said waveguide core; wherein the average refractive index of said overcladding layer is greater than the waveguide core mode index; wherein the average refractive index of said cover plate is lower than the waveguide core mode index.

2. A component as claimed in Claim 1 wherein said overcladding layer is a variable refractive index material.

3. A component as claimed in Claim 1 wherein said overcladding layer is a Polymer Dispersed Liquid Crystal.

4. A component as claimed in Claim 1 wherein said overcladding layer is an Electrically Switchable Bragg Grating.

5. A component as claimed in Claim 1 wherein said Electrically Switchable Bragg Grating has tilted Bragg planes.

6. A component as claimed in Claim 1 wherein said cover plate is fabricated from fused silica.

7. A component as claimed in Claim 2 wherein said waveguide is a buried waveguide.

8. A component as claimed in Claim 2 wherein said waveguide core is in contact with said overcladding layer.

9. A component as claimed in Claim 2 wherein said waveguide is a coupler half.

10. A component as claimed in Claim 4; wherein said Electrically Switchable Bragg Grating has an average unswitched refractive index of approximately 1.470 and an average switched refractive index of approximately 1.500; wherein said core has a mode index of approximately 1.447; wherein said cover glass has an average refractive index of approximately

1.444.

11. A component as claimed in Claim 4; wherein said Electrically Switchable Bragg Grating has an average unswitched refractive index of approximately 1.470 and an average switched refractive index of approximately 1.500; wherein said Electrically Switchable Bragg Grating layer is approximately 4 microns in thickness; wherein said Electrically Switchable Bragg Grating layer has Bragg planes tilted at angles of approximately 5.7 degrees; wherein said core has a mode index of approximately 1.447; wherein said cover glass has an average refractive index of approximately 1.444.

12. A method of fabricating a component for filtering a guided light beam comprising the steps of: providing a substrate; forming an optical waveguide comprised of at least one core disposed near to the surface of said substrate; forming an overcladding, wherein the average refractive index of said overcladding layer is greater than the waveguide core mode index; providing a cover plate, wherein the refractive index of said cover plate is less than said waveguide core mode index; forming electrodes on said cover plate; and applying said cover plate to said overcladding layer; wherein said overcladding layer and said cover plate lie within the evanescent coupling region of said waveguide.

13. A method as claimed in Claim 12, wherein said overcladding layer is an Electrically Switchable Bragg Grating.

14. A method as claimed Claim 13, wherein said Electrically Switchable Bragg Grating has tilted Bragg planes.

15. A method as claimed Claim 12, wherein said cover plate is fabricated from fused silica.

16. A method as claimed in Claim 12, wherein said waveguide is a buried waveguide.

17. A method as claimed in Claim 12, wherein said waveguide core is in contact with said overcladding layer.

18. A method as claimed in Claim 12, wherein said waveguide is a coupler half.

19. A method as claimed in Claim 13; wherein said Electrically Switchable Bragg Grating has an average unswitched refractive index of approximately 1.470 and an average switched refractive index of approximately 1.500; wherein said core has a mode index of approximately 1.447; wherein said cover glass has an average refractive index of approximately

1.444.

20. A method as claimed in Claim 13; wherein said Electrically Switchable Bragg Grating has an average unswitched refractive index of approximately 1.470 and an average switched refractive index of approximately 1.500; wherein said Electrically Switchable Bragg Grating layer is approximately 4 microns in thickness; wherein said Electrically Switchable Bragg Grating layer has Bragg planes tilted at angles of approximately 5.7 degrees; wherein said core has a mode index of approximately 1.447; wherein said cover glass has an average refractive index of approximately

1.444.