GB2304917A - Integrated optical devices - Google Patents
Integrated optical devices Download PDFInfo
- Publication number
- GB2304917A GB2304917A GB9518264A GB9518264A GB2304917A GB 2304917 A GB2304917 A GB 2304917A GB 9518264 A GB9518264 A GB 9518264A GB 9518264 A GB9518264 A GB 9518264A GB 2304917 A GB2304917 A GB 2304917A
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- United Kingdom
- Prior art keywords
- waveguide
- acoustic
- optical
- wave
- along
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- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
- G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour
- G02F1/11—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour based on acousto-optical elements, e.g. using variable diffraction by sound or like mechanical waves
- G02F1/125—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour based on acousto-optical elements, e.g. using variable diffraction by sound or like mechanical waves in an optical waveguide structure
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- Physics & Mathematics (AREA)
- Nonlinear Science (AREA)
- General Physics & Mathematics (AREA)
- Optics & Photonics (AREA)
- Optical Modulation, Optical Deflection, Nonlinear Optics, Optical Demodulation, Optical Logic Elements (AREA)
- Optical Integrated Circuits (AREA)
Abstract
An integrated optical device in the form of a tunable filter has an optical waveguide 15 extending therethrough and along which an optical wave may be propagated. An acoustic waveguide 16 is also formed in the device to extend generally parallel to the optical waveguide 15 so that an acoustic wave (typically having a frequency of the order of a few hundred MHz) will interact with an optical wave being propagated along the optical waveguide 15. The acoustic waveguide has a varying configuration or absorbtion means so that the power density of an advancing acoustic wave varies along the length of the waveguide (Figure 3B), so that the power transfer to the optical wave is controlled. In this way the transfer function of the filter is also controlled.
Description
INTEGRATED OPTICAL DEVICES
This invention relates to integrated optical devices. In particular, the invention relates to a method of controlling the transfer function of an integrated optical device, and also to an integrated optical device in the form of a tunable filter, wherein the transfer function thereof may be controlled.
It is known to provide an integrated optical filter having an waveguide for an optical signal, wherein the transfer function of the filter is defined by injecting into the filter a so-called acoustic wave (i.e. electromagnetic energy at a much lower frequency - typically in the range of a few hundreds of MHz than the frequency of the optical wave, and which is usually of sinusoidal form). The acousto-optic interaction between the colinear optical and acoustic signals results in a polarisation conversion of the signal. As a result if polarisation selective elements are added before and/or after the active section the passband on the output ports will be governed by whether this conversion has occurred. In this way it is possible to provide for the selective addition or subtraction of a wavelength division multiplexed channel from an optical communication network.Such an acousto-optical tunable filter is of considerable importance since it can provide the function of addition or subtraction simultaneously on multiple channels.
Essential elements in the specification of an acousto-optical tunable filter are the cross-talk level between the selected and unselected channels, and the flatness of the pass-band. The cross-talk and pass-band performances of an acousto-optical tunable filter can both be improved by a technique known as apodisiation, whereby the acousto-optic interaction is weighted as a function of the length of the waveguide over which that interaction takes place - the active region of the filter. In known acousto-optical tunable filters, this has been achieved by using an acoustic directional coupler in the active region. This has the result of providing a basic sinusoidal function or weighting into the acoustic power in the active arm of the directional coupler.
It is an aim of the present invention to provide both a method of controlling the transfer function of an acousto-optical tunable filter and also such an integrated optical device wherein the cross-talk and pass-band performances can be optimised.
According to a first aspect of the present invention, there is provided a method of controlling the optical transfer function of an integrated optical device in the form of a tunable filter having a optical waveguide, in which method an optical wave is propagated along said waveguide, an acoustic wave is propagated within an acoustic waveguide formed in the device essentially parallel to the length of the optical waveguide so as to transfer energy to the optical wave being propagated along the optical waveguide, the power density of the acoustic wave interacting with the optical wave being controlled to vary along the length of the acoustic waveguide so as thereby to control the energy transfer to the optical wave and so also the optical transfer function of the filter.
In the method of the present invention, a novel form of apodisation is employed, wherein the energy of the acoustic wave interacting with the optical wave is varied, along the active region. This may be achieved in a number of different ways: for example, the power density of the acoustic wave may be controlled by configuring the cross-sectional area (or mode size) of the acoustic waveguide, or the power density of the acoustic wave may be controlled by absorbing the energy of the acoustic wave, along the length of the acoustic waveguide.It is also possible to use both techniques simultaneously, but in the latter case, the power density of the acoustic wave may be controlled either by gradually absorbing at least some of the energy of the acoustic wave along the active region, or absorbing at least some of the energy at least at one pre-defined location along the length of the acoustic waveguide.
In one embodiment, the acoustic wave is propagated substantially co-linearly with the optical wave.
Alternatively, the acoustic wave is propagated along an acoustic waveguide extending adjacent and substantially parallel to the optical waveguide, and induces acoustic coupling to a second acoustic waveguide extending colinearly with the optical waveguide. In the latter case, the power density of the coupled acoustic wave may be controlled by configuring the cross-sectional area of the second acoustic waveguide.
According to a second aspect of the present invention, there is provided a integrated optical device in the form of a tunable filter, comprising an optical waveguide extending through the device and along which an optical wave may be propagated, an acoustic waveguide formed in the device essentially parallel to the length of the optical waveguide, means for propagating an acoustic wave along the acoustic waveguide for the transfer of energy to an optical wave being propagated along the optical waveguide, the acoustic waveguide being configured to cause the power density of the acoustic wave interacting with the optical wave to vary along the length of the acoustic waveguide.
The cross-sectional area (mode size) of the acoustic waveguide may vary along the length thereof, so as thereby to vary the power density of an acoustic wave propagated therealong. Alternatively, or possibly in addition to constructing the acoustic waveguide with a varying mode size, means may be provided to absorb the energy of an acoustic wave being propagated along the length of the acoustic waveguide. Such means may be provided at least at one pre-defined location along the length of the acoustic waveguide to absorb at least some of the energy of the acoustic wave, at that location. In this way, the acoustic energy may have a step function, or may be abruptly stopped. Either effect can be achieved by laying or depositing at least one discrete piece of acoustic energy-absorbing material over the acoustic waveguide at the location where energy is to be absorbed.
The acoustic waveguide may be formed co-linearly with the optical waveguide. In an alternative embodiment, the acoustic waveguide is formed adjacent and extending substantially parallel to the optical waveguide, and a second acoustic waveguide is formed co-linearly with the optical waveguide, whereby an acoustic wave propagated along the first-mentioned acoustic waveguide couples to the second acoustic waveguide and transfers energy to an optical wave propagated along the optical waveguide. In such an arrangement, the cross-sectional area (mode size) of the second acoustic waveguide advantageously is configured to cause the power density of the induced acoustic wave interacting with the optical wave to vary along the length of the second acoustic waveguide, though measures similar to those described above could be used in association with the first-mentioned acoustic waveguide.
By way of example only, certain specific examples of the present invention will now be described in detail, reference being made to the accompanying drawings, in which:
Figure 1A is a diagrammatic representation of a known integrated optical component in the form of an acousto-optical tunable filter;
Figure 1B graphically represents the acoustic power density of the acoustic wave in the filter of
Figure lA;
Figure 2A also is a diagrammatic representation of a second known form of acousto-optical tunable filter but including an acoustic waveguide;
Figure 2B graphically represents the acoustic power density in the known component of Figure 2A;
Figure 3A is a diagrammatic representation of a first embodiment of acousto-optical tunable filter of this invention; ;
Figure 3B is a graphical representation of the acoustic power density within the acoustic waveguide in the embodiment of Figure 3A;
Figures 4A and 4B show two different ways of forming an acoustic waveguide in a substrate, in a device of this invention;
Figures 5A and 5B, 6A and 6B, 7A and 7B, and 8A and 8B are similar to Figures 3A and 3B, but of second, third, fourth and fifth embodiments of this invention, respectively;
Figure 9A shows the transfer function through an acousto-optical tunable filter of the prior art; and
Figure 9B shows the transfer function through an acousto-optical filter of the present invention, such as that shown in Figure 3A.
Figure 1 diagrammatically shows a known form of acousto-optical filter using no apodisation technique.
The filter is in the form of an integrated optical component having an optical waveguide 10 formed in a substrate (not shown) for example of lithium niobate, typically by a doping technique using titanium. An acoustic wave generator 11 is also formed on the substrate, that generator able to produce a wave 12 typically of a few hundred MHz (usually referred to in the art as an acoustic wave) whereby there will be interaction between the acoustic wave and light propagated along the optical waveguide. By appropriate selection of the frequency of the drive current to the acoustic wave generator, it is possible to select one or more frequencies input to the optical waveguide so as to pass through the filter and to de-select one or more other frequencies.
Figure 1B shows the acoustic power density of the acoustic wave which interacts with the optical signal passing along the optical waveguide. The acoustic power density falls rapidly initially and then tails away asymptotically, along the length of the optical waveguide, on account of the divergent acoustic wave.
Figure 9A shows a typical optical signal after passing through such a tunable filter as is shown in
Figure 1, and after passing through six such tunable filters. It can be seen that there are significant side bands after passing through the first such filter, but after passing through six filters, the side bands have essentially disappeared but the channel of interest has become extremely narrow and this may lead to subsequent processing errors on account of the narrowed band width.
Figure 2A shows a modified form of the device shown in Figure 1A. Here, there is formed on the substrate an acoustic waveguide 13 for the acoustic wave produced by the generator 11, but in all other respects the device is essentially the same as that shown in Figure 1A. However, as the acoustic wave is constrained within the acoustic waveguide, the acoustic power density of that wave is substantially constant along the length of the component, as shown in Figure 2B. Though such a device may give an improved transfer function as compared to the device of Figure 1A, the device still suffers from cross-talk and pass-band degradation.
Figure 3A diagrammatically shows an integrated optical component in the form of a acousto-optical tunable filter of this invention, which differs from that of Figure 2A in that the acoustic waveguide is configured to control the power density of the acoustic wave propagated along that waveguide. In the embodiment of Figure 3A, the optical waveguide 15 and the acoustic waveguide 16 are formed coaxially with the optical waveguide extending centrally along the acoustic waveguide, the acoustic wave generator 17 being formed on the substrate at the entrance to the acoustic waveguide. The mode size of the acoustic waveguide (in effect, the cross-sectional area of the acoustic waveguide, assuming a substantially constant depth for that waveguide in the substrate) is initially restricted and then enlarged, in order that the acoustic power density may be as shown in Figure 3B.
In this way, there may be the greatest inter-action between the acoustic wave and the optical signal propagated along the optical waveguide, at approximately 50% of the propagation distance.
The acoustic waveguide may be formed in the substrate of the integrated component in one of two ways, as shown in Figures 4A and 4B. In the case of
Figure 4A, a dopant is used which lowers the refractive index of the substrate, as seen by the acoustic wave, two areas 18 of dopant being employed one to each side of the optical waveguide 15. The acoustic waveguide comprises the undoped region of higher refractive index between the two areas 18 of dopant. In Figure 4B, a dopant which raises the refractive index of the substrate, as seen by the acoustic wave, is employed to define the waveguide itself, with a different dopant being employed axially along that waveguide, to define the optical waveguide 15.
Figures 5A and 5B are similar to Figures 3A and 3B, but of a second embodiment of this invention.
Here, a significantly more complex acoustic waveguide 20 is formed in the substrate, in order that the acoustic power density may significantly vary along the length of the optical waveguide 15 within the component. By reducing the acoustic power density substantially to zero at least at two points along the length of the optical waveguide, the interaction with an optical signal may be controlled in order to optimise both pass-band performance and also to minimise cross-talk to other channels.
Figures 6A and 6B show a third embodiment of this invention, similar to that of Figures 5A and 5B, but employing acoustic wave absorbent means 21 at two points along the length of the acoustic waveguide.
Such means may comprise material deposited on the surface of the substrate over the top of the acoustic waveguide, which material absorbs acoustic energy thereby to attenuate the acoustic wave being propagated along the acoustic waveguide. In this way, the acoustic power density may be varied in order to optimise the filter performance.
Figure 7A shows an alternative form of tunable filter of this invention, wherein the acoustic power density of an acoustic wave inter-acting with an optical signal may be varied along the length of the optical waveguide. Here, an optical waveguide 22 is formed in a substrate in the usual way, and an acoustic waveguide 23 is formed coaxially with the optical waveguide 22, waveguide 23 being defined by two doped areas 24. A further acoustic waveguide 25 is defined parallel to acoustic waveguide 23, by one of the doped areas 24 and a further doped area 26. An acoustic wave generator 27 is formed on the substrate within the further acoustic waveguide 25, so as to launch an acoustic wave along that waveguide. The energy of that acoustic wave couples with waveguide 23, to give rise to an acoustic wave within that waveguide 23, as shown in Figure 7B. By varying the configurations of the waveguides 23 and 25, the acoustic power density which inter-acts with an optical signal propagated along optical waveguide 22 can be controlled.
Figure 8A shows a modified form of the tunable filter shown in Figure 7A. Here, the configuration of at least one of the waveguides (in this example, the waveguide 23 coaxial with the optical waveguide 22) is varied in order to change the optical coupling characteristics between the two acoustic waveguides, thereby controlling the inter-action between the acoustic wave and an optical signal being propagated along the optical waveguide. Clearly, other configurations of acoustic waveguide could be employed, using for example the measures described above with reference to Figures 3A, 5A and 6A.
Figure 9B shows the transfer function of a typical acousto-optical tunable filter using the apodisation technique of the present invention, as described above.
As will be appreciated, the pass-band performance is much enhanced after the optical signal has been passed through a single filter, and the cross-talk to adjacent wave-lengths is significantly reduced. Provided the apodisation is optimised, even after passing through six such filters, very little degradation of the passband performance will take place.
The precise apodisation to be employed (and so in effect the acoustic waveguide configuration) may be determined empirically. However, the drawings of embodiments of this invention may show typical acoustic waveguide configurations which may be found to give significant improvements in the transfer functions of acousto-optical tunable filters.
Claims (17)
1. A method of controlling the optical transfer function of an integrated optical device in the form of a tunable filter having a optical waveguide, in which method an optical wave is propagated along said waveguide, an acoustic wave is propagated within an acoustic waveguide formed in the device essentially parallel to the length of the optical waveguide so as to transfer energy to the optical wave being propagated along the optical waveguide, the power density of the acoustic wave interacting with the optical wave being controlled to vary along the length of the acoustic waveguide so as thereby to control the energy transfer to the optical wave and so also the optical transfer function of the filter.
2. A method as claimed in claim 1, in which the power density of the acoustic wave is controlled by configuring the cross-sectional area of the acoustic waveguide.
3. A method as claimed in claim 1 or claim 2, wherein the power density of the acoustic wave is controlled by absorbing the energy of the acoustic wave, along the length of the acoustic waveguide.
4. A method as claimed in any of the preceding claims, wherein the power density of the acoustic wave is controlled by absorbing at least some of the energy of the acoustic wave at least at one pre-defined location along the length of the acoustic waveguide.
5. A method as claimed in any of the preceding claims, wherein the acoustic wave is propagated substantially co-linearly with the optical wave.
6. A method as claimed in any of claims 1 to 4, wherein the acoustic wave is propagated along an acoustic waveguide extending adjacent and substantially parallel to the optical waveguide, and induces acoustic coupling to a second acoustic waveguide extending colinearly with the optical waveguide.
7. A method as claimed in claim 6, in which the power density of the coupled acoustic wave is controlled by configuring the cross-sectional area of the second acoustic waveguide.
8. A method of controlling the optical transfer function of an integrated optical device in the form of a tunable filter having a optical waveguide and substantially as hereinbefore described with reference to the accompanying drawings.
9. An integrated optical device in the form of a tunable filter, comprising an optical waveguide extending through the device and along which an optical wave may be propagated, an acoustic waveguide formed in the device essentially parallel to the length of the optical waveguide, means for propagating an acoustic wave along the acoustic waveguide for the transfer of energy to an optical wave being propagated along the optical waveguide, the acoustic waveguide being configured to cause the power density of the acoustic wave interacting with the optical wave to vary along the length of the acoustic waveguide.
10. An integrated optical device as claimed in claim 9, wherein the cross-sectional area of the acoustic waveguide varies along the length thereof, thereby to vary the power density of a wave propagated therealong.
11. An integrated optical device as claimed in claim 9 or claim 10, wherein means are provided to absorb the energy of an acoustic wave being propagated along the length of the acoustic waveguide.
12. An integrated optical device as claimed in claim 11, wherein means are provided at least at one predefined location along the length of the acoustic waveguide to absorb at least some of the energy of the acoustic wave.
13. An integrated optical device as claimed in claim 12, wherein at least one discrete piece of acoustic energy-absorbing material is laid over the acoustic waveguide at the location where energy is to be absorbed.
14. An integrated optical device as claimed in any of claims 9 to 14, wherein the acoustic waveguide is formed co-linearly with the optical waveguide.
15. An integrated optical device as claimed in any of claims 9 to 14, wherein the acoustic waveguide is formed adjacent and substantially parallel to the optical waveguide, and a second acoustic waveguide is formed co-linearly with the optical waveguide, whereby an acoustic wave propagated along the first-mentioned acoustic waveguide couples to the second acoustic waveguide and transfers energy to an optical wave propagated along the optical waveguide.
16. An integrated optical device as claimed in claim 15, wherein the cross-sectional area of the second acoustic waveguide is configured to cause the power density of the induced acoustic wave interacting with the optical wave to vary along the length of the second acoustic waveguide.
17. An integrated optical device as claimed in claim 9 and substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9518264A GB2304917A (en) | 1995-09-08 | 1995-09-08 | Integrated optical devices |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9518264A GB2304917A (en) | 1995-09-08 | 1995-09-08 | Integrated optical devices |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| GB9518264D0 GB9518264D0 (en) | 1995-11-08 |
| GB2304917A true GB2304917A (en) | 1997-03-26 |
Family
ID=10780338
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB9518264A Withdrawn GB2304917A (en) | 1995-09-08 | 1995-09-08 | Integrated optical devices |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2304917A (en) |
Cited By (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO1999006876A1 (en) * | 1997-07-31 | 1999-02-11 | Pirelli Cavi E Sistemi S.P.A. | Acousto-optic devices having an acoustic attenuator structure |
| EP0969310A1 (en) * | 1998-06-04 | 2000-01-05 | PIRELLI CAVI E SISTEMI S.p.A. | Waveguides for use in acousto-optical mode converters and method for making the same |
| US6195476B1 (en) | 1997-07-31 | 2001-02-27 | Pirelli Cavi E Sistemi S.P.A. | Acousto-optic devices having an acoustic attenuator structure |
| US6282332B1 (en) | 1998-06-04 | 2001-08-28 | Pirelli Cavi E Sistemi S.P.A. | Waveguide structures in particular for use in acousto-optical mode converters and method for making same |
| US6718110B2 (en) | 1998-06-04 | 2004-04-06 | Corning Incorporated | Indiffused optical waveguide structures in a substrate |
Citations (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5218653A (en) * | 1991-11-08 | 1993-06-08 | Bell Communications Research, Inc. | Polarization converter with apodized acoustic waveguide |
| US5400171A (en) * | 1993-10-01 | 1995-03-21 | Bell Communications Research, Inc. | Acousto-optic filter with near-ideal bandpass characteristics |
| US5446807A (en) * | 1994-06-23 | 1995-08-29 | Bell Communications Research, Inc. | Passband-flattened acousto-optic polarization converter |
| EP0718662A1 (en) * | 1994-12-20 | 1996-06-26 | Alcatel N.V. | Acousto-optic device for optical filtering |
-
1995
- 1995-09-08 GB GB9518264A patent/GB2304917A/en not_active Withdrawn
Patent Citations (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5218653A (en) * | 1991-11-08 | 1993-06-08 | Bell Communications Research, Inc. | Polarization converter with apodized acoustic waveguide |
| US5400171A (en) * | 1993-10-01 | 1995-03-21 | Bell Communications Research, Inc. | Acousto-optic filter with near-ideal bandpass characteristics |
| US5446807A (en) * | 1994-06-23 | 1995-08-29 | Bell Communications Research, Inc. | Passband-flattened acousto-optic polarization converter |
| EP0718662A1 (en) * | 1994-12-20 | 1996-06-26 | Alcatel N.V. | Acousto-optic device for optical filtering |
Non-Patent Citations (1)
| Title |
|---|
| INSPEC abstract number A9207-4280C,B9204-4170-004 &IEEE 1990Ultrasonics symposium vol. 2 p.605-8 * |
Cited By (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO1999006876A1 (en) * | 1997-07-31 | 1999-02-11 | Pirelli Cavi E Sistemi S.P.A. | Acousto-optic devices having an acoustic attenuator structure |
| US6195476B1 (en) | 1997-07-31 | 2001-02-27 | Pirelli Cavi E Sistemi S.P.A. | Acousto-optic devices having an acoustic attenuator structure |
| AU742321B2 (en) * | 1997-07-31 | 2001-12-20 | Corning O.T.I. S.P.A. | Acousto-optic devices having an acoustic attenuator structure |
| EP0969310A1 (en) * | 1998-06-04 | 2000-01-05 | PIRELLI CAVI E SISTEMI S.p.A. | Waveguides for use in acousto-optical mode converters and method for making the same |
| US6282332B1 (en) | 1998-06-04 | 2001-08-28 | Pirelli Cavi E Sistemi S.P.A. | Waveguide structures in particular for use in acousto-optical mode converters and method for making same |
| US6718110B2 (en) | 1998-06-04 | 2004-04-06 | Corning Incorporated | Indiffused optical waveguide structures in a substrate |
Also Published As
| Publication number | Publication date |
|---|---|
| GB9518264D0 (en) | 1995-11-08 |
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| Date | Code | Title | Description |
|---|---|---|---|
| WAP | Application withdrawn, taken to be withdrawn or refused ** after publication under section 16(1) |