WO2005101115A1

WO2005101115A1 - Optical switch

Info

Publication number: WO2005101115A1
Application number: PCT/SE2005/000539
Authority: WO
Inventors: Marcin Swillo; Lars Thylén
Original assignee: Phoxtal Communications Ab
Priority date: 2004-04-19
Filing date: 2005-04-14
Publication date: 2005-10-27
Also published as: SE0401011L; SE0401011D0

Abstract

An optical switch for coupling light between two adjacent and parallel waveguides (2,3), which waveguides are formed in a dielectric material, where a membrane (4;7;25) is provided adjacent a first (3) of said waveguides, which membrane (4;7;25) is movable between a position away from the said first waveguide (3) and a position close to or abutting the said waveguide, thereby changing the refractive index of the said first waveguide (3), c h a r a c t e r i s e d i n, that the membrane (4;7;25) is made of a dielectric material.

Description

Optical switch

The present invention refers to an optical swit.-h. An optical switch is a basic element in recon igurable optical network systems. Typical solutions to realize light switching in present systems are based on: - specular reflection by mirror adjusting, where light can be reflected at variable angle or variabl-- posi- tion, - Mach-Zehnder interferometer (light can be switched between two output ports by applying phase shift of light propagated in one arm of^" an interferometer) - directional coupler, where coupling strength between waveguides is defined by distance between waveguides ⁽light switching between two output ports is realized by moving one- of coupled waveguides) .

The present invention of optical switch is based on a directional coupler. However, the distance between the two coupled waveguides is fixed. The light switching between output ports is obtained by a movable membrane suspended just above one of waveguides (fig.2) . If the distance between membrane and waveguide is large enouch the propagation constant of guided light is almost the same as without the membrane . In this position of the membrane, the waveguides are creating a symmetrical coupler ⁽fig.l). if light is launched only to the first waveguide (fig.2), the splitting ratio between the out_¬ put ports depends on length of the coupler. The length of the coupler corresponds to the maximum transmission of light from an input waveguide (IA) to a second waveguide (I₂). In this way a cross-state is realized.

If the membrane is pushed down and it touches or nearly touches the waveguide (fig.3), the coupler becomes asymmetric. When the membrane is closer to the waveguide, the propagation constant is increasing because the refractive index of the membrane is higher than air. The maximum increase of the propagation con- stant is obtained when the membrane is touching the waveguide core. The difference m refractive index between the waveguides results m a phase mismatch between guided modes and the light will not be coupled to the second waveguide. This means that the light launched to one of the input waveguides remains in the same waveguide at the output . The coupler is then m a bar state.

In both states, the bar state and the cross state, the light can be launched to any of the input ports. In the bar state the light will remain in the same waveguides in the output ports. In the cross state the light will be coupled to the opposite waveguides in the output ports .

If the switch utilizes a metal membrane, and the switch is in the bar state of the coupler, the light propagated m the waveguide touched by membrane is strongly attenuated. For this reason an optical switch with a metal membrane can not use both input ports when the coupler s in the bar state.

The present invention solves this problem. The present invention thus refers to an optical switch for coupling light between two adjacent and parallel waveguides, which waveguides are formed in a dielectric material, where a membrane is provided adjacent a first of said waveguides, which membrane is movable between a position away from the said first waveguide and a position close to or abutting the said waveguide, thereby changing the refractive index of the said first waveguide, and is characterised in, that the merα- brane is made of a dielectric material.

In the following, the invention will be described in more detail, partly in connection with exemplifying embodiments of the invention, where - Figure 1 shows a cross-section of the optical switch in the center of the coupler for cross-state - Figure 2 shows a top view of the optical switch - Figure 3 shows a cross-section in the center of the coupler of the optical switch, for bar state. - Figure 4 an optical switch for a pre-defined wavelength with movable membrane is shown - Figure 5 shows the crosssection of an optical switch for pre-defined wavelength movable membrane in the center of the M-Z interferometer, with the membrane state far from waveguides - Figure 6 The cross-section of the optical switch for^' pre-defined wavelength with movable membrane in the center of the M-Z interferometer (membrane state: close to waveguides) - Figure 7 shows a top view of the optical switch for pre-defined wavelength with movable membrane - Figure 8 illustrates a simulation of the Bragg grating transmission for: statel- the membrane touching the waveguide, state2- the membrane far from the waveguide - Figure 9 shows a switchable add-drop filter with 2x2 switch - Figure 10 shows a switchable add-drop filter from fig.7 with optical switch with movable membrane. -

In figure 1 and 2 an optical switch 1 for coupling light between two adjacent and parallel waveguides 2, 3 is shown. The waveguides 2, 3 are formed in a dielectric material. The switch comprises a membrane 4, that is provided adjacent a first 3 of said waveguides. The membrane 4 is movable between a position away from the said first waveguide 3, as shown in Fig 1, and a position close to or abutting the said waveguide 3, as shown in figure 3, thereby changing the refractive index of the said first waveguide 3.

According to the invention the membrane 4 is made of a di- electric material.

According to a preferred embodiment of the invention, the membrane 4 is made of the same or similar material as the core of the waveguides or cladding 5 in which the waveguides 2, 3 are formed.

According to another preferred embodiment of the invention there is provided a support 6 on which the membrane rests between the side of the membrane 4 facing the substrate 5 and the substrate 5, giving a distance between the membrane and the substrate -when the membrane 4 is in a stressless state. The thickness and refractive index of the dielectric membrane has to be defined in a state when the membrane is touching the waveguide, i.e. the bar state of the coupler .

The light guided in a waveguide which is touched by the dielectric membrane can be easy coupled to the membrane. This will results in significant loss of transmitted light to the output port. In order to avoid this loss the propagation constants of guided modes in waveguide and membrane, when the membrane is touching the waveguide, should be separated as much as possible. The stronger the phase mismatch between modes in waveguide and membrane, the lower the loss of the light transmitted to output port will be. The phase mismatch between those modes depends on the thickness and refractive index of the membrane.

As an example the coupler switch presented in fig.l can have the following parameters. The thickness of the membrane is 1 micron, the refractive index is 1.46, the distance between the membrane and the waveguide is 1 micron for the cross state of the coupler, the refractive index of the cladding is 1.445 microns and the for the core is 1.45588 microns.

The waveguide dimension are aι width of 5 microns and a height of 6 microns. The distance between the waveguides is 6 microns.

The only moving element in tb-is optical switch is the membrane. When the membrane -Ls touching the core of a waveguide, it is altering the propagations constants of the guided modes. It is important that all the propagation constants of the modes of a waveguide with touched membrane are enough different from the propaga- tion constant of the mode of the waveguide without the membrane .

In the presented example, when the membrane is touching the waveguide, i.e. the bar state of the coupler the overlap between modes is negligible because of the phase mismatch between modes - The larger the difference between the propagation constants of those modes, the lower the coupling between the waveguides i.e. lower crosstalk between the output ports will occur.

The guided light in a waveguide touched by the membrane is not coupled to the modes of the membrane, but it is only partially guided in the membrane region. This will ensure a minimum propagation loss caused by light leal- ing to the membrane.

The membrane is moving up and down by any means of electrostatic force, or thermal induced stress in the membrane. The position of the membrane can be bistable, monostable or adjusted continuously with respect to the waveguide core.

According to another preferred embodiment of the invention the membrane 7 is located adjacent a wave guide 8, 9 provided with a Bragg grating 10, 11, where the Bragg grating is arranged to reflect incoming ligth when the membrane 7 is at a distance from the wave guide 8, 9 and arranged to pass the light when the membrane 7 is close to or abutting the wave guide at the Bragg grating, please see fig. 4.

According to yet another embodiment the membrane 7 is positioned adjacent the two wave guides 8, 9, each having a Bragg grating 10, 11, m an optical switch at the center of a mach- zender interferometer, please see fig.7. In fig. 7 referens numeral 12 indicates the aforementioned electrode to operaste the membrane and numeral 13 indicates a wire bonding pad.

10

In Fig. 4 an optical switch consisting of a M-Z interferometer (based on two 50/50 couplers) and two identical Bragg gratings (one m each arm of interferometer) i5 is shown.

The optical signal is coming to input port. The light, where the wavelength doesn't satisfy the Bragg condition is transmitted through the grating to the output Ό port. If the Bragg condition is satisfied the light is reflected from the grating and then transmitted to the drop port. In the same way, light coming to Add port and which is reflected by the Bragg grating is transmitted to the output port.

>5 The switching process is controlled by the position of a membrane placed above both waveguides, creating a M-Z interferometer.

10 In fig. 7 a top view of the optical switch for predefined wavelength with movable membrane is presented. As an example the electrode on the top layer is used in order to apply an electrostatic force between the top electrode and silicone substrate. The silicon substrate acts as a second electrode. The electrostatic force is

5 pushing the membrane down until it will touch the waveguide. When the electrostatic field is off, the strain generated in membrane is pulling the membrane back to the previous position, which is far from waveguide . o If the distance between the membrane and the waveguide is large enough, as shown in fig. 5, the propagation constant of guided light is the same as without the membrane. In this position of the membrane, the wave-

5 length of light reflected by the Bragg grating is defined by the propagation constant of guided mode in the waveguide and the grating period. If the membrane is moved closer to the waveguide core, as shown in fig.6, the propagation constant of guided mode is higher. Then o the wavelength which is satisfying the Bragg condition increases. The maximum change of wavelength satisfying the Bragg condition is obtained when the membrane is touching the waveguide core.

5 In this way, the two position of the membrane, namely far from waveguide and close to the waveguide, corresponds to two different wavelengths which are reflected by the Bragg grating.

Ό As mentioned above, the refractive index of the dielectric membrane is higher than air. This means that, the membrane can guide light as a slab waveguide. It s important that the propagation constants for all modes existing in the memorane are far enough from propagation constant of the guided mode m waveguide. In this way, tne light guided through the waveguide is not coupled to the membrane due to phase mismatch of modes. The membrane is only modifying the propagation constant of guided mode in waveguide.

If the membrane is far from both waveguides and the light is incoming to input and/or add port and its wavelength satisfy the Bragg condition of both gratings, then the light is reflecting by the gratings and is transmitted to the output port or the drop port respectively. If for the same wavelength the membrane is moved close to both waveguides, this will change the

Bragg condition of both gratings, and light w ll come to the input port and/or the add port, then the light will pass through the gratings and be transmitted to the drop port and the output port respectively.

If the light with pre-defined wavelength which satisfy the Bragg condition is reflected to drop port, then the strength of the gratings defines crosstalk to drop port .

In fig.8 there is presented a simulation of transmission through a 10mm long Bragg grating with sinus appo- d zation and maximum refractive index modulation 1.4-10^"3. The solid and dash line represents trans is- sion when the membrane is touching, please see fig.6 and far, fig. 9, from the waveguides respectively. The change of effective refractive index caused by the membrane is ^~n_efectιve-0 - 0007 , for a refractive index of the membrane of 1.46, a refractive index of the core of 1.45588, of the cladding of 1.445. The waveguide dimensions are : 5 microns wide and 6 microns high.

As one can see, the isolation between add and drop port for the grating presented in fig. 10 is ca . -60dB. The shift in operating wavelength of the Bragg grating caused by the membrane corresponds to approximately one ITU channel, in this example.

Due to the high isolation between add and drop port, the optical switch with movable membrane can be directly applied in switchable add-drop filter presented in fig . 9.

In fig.9 referens numerals 14 - 17 indicates wave guides, 18 a 2X2 switch, 19, 20 Bragg gratings and 21 - 24 reflectors.

A device as shown in Fig 9 is described in detail in applicants Swedish patent No 0301286-1.

The typical solution for a 2x2 switch used in a device presented in fig. 9 is a thermal switch based on a Mach-Zehnder interferometer. However, the isolation between the add and the drop port is limited by the quality of two couplers used in this interferometer.

A typical isolation is -20dB for a single thermal switch and about -40dB for two cascaded switches. The optical switch with a movable membrane can be applied in a switchable add-drop filter as 2x2 switch, please see fig. 10. In fig. 10 the referens numerals 14 - 24 indicates the same thing as in figure 11. However, the switch 18 according to the present invention includes a membrane 25.

The additional phase shift is added after one of the Bragg gratings, in order to exchange the output port for the add port. The isolation between the add and the drop port can be improved up to -60dB if using the grating presented above.

Many embodiments have been described above as well as different applications for the inventive coupler.

However it is apparent that the man skilled in the art can modify the coupler to fit with other implementations and applications .

Therefore the invention shall not be restricted to the above described embodiments, but can be varied within the scope of the attached claims.

Claims

Claims .

1. An optical switch for coupling light between two adjacent and parallel waveguides (2,3), which waveguides are formed in 5 a dielectric material, where a membrane (4; 7; 25) is provided adjacent a first (3) of said waveguides, which membrane (4;7;25) is movable between a position away from the said first waveguide (3) and a position close to or abutting the said waveguide, thereby changing the refractive index of the w said first waveguide (3) , c h a r a c t e r i s e d i n, that the membrane (4;7;25) is made of a dielectric material.

2. Optical switch according to claim 1, c h a r a c t e r - i s e d i n, that the membrane (4; 7; 25) is made of the same

15 or similar material as the core of the waveguides or cladding (5) in which the waveguides (2,3) are formed.

3. Optical switch according to claims 1 or 2, c h a r a c t e r i s e d i n, that the thickness of the 0 membrane (4;7;25) is in the order of 1 micron.

4. Optical switch according to claims 1 or 2, c h a r a c t e r i s e d i n, that the propagation constants, for different modes, of the membrane (4;7;25) differs

25 from the propagation constant of the said first waveguide (3) to such extent that substantially no light is coupled from said first waveguide (3) to the membrane (4;7;25).

5. Optical switch according to claims 1, 2 or 3,

30 c h a r a c t e r i s e d i n, that the refractive indexes of the membrane (4;7;25), the cladding (5) and the core of said waveguides have (2,3) the following ratio : 1.46, 1.445 and 1.45588 respectively.

6. Optical switch according to claims 1, 2, 3, 4 or 5, c h a r a c t e r i s e d i n, that the membrane (4; 7; 25) is moved between said two positions by an electrostatic force 5 created by an electric voltage applied between the membrane (4;7;25) and the substrate (5). ^■

7. Optical switch according to claims 1, 2, 3, 4, 5 or 6, c h a r a c t e r i s e d i n, that between the side of the w membrane (4;7;25) facing the substrate (5) and the substrate (5) there is provided a support (-6) on which the membrane (4;7;25) rests, giving a distance between the membrane (4; 7; 25) and the substrate (5) when the membrane is in a stressless state.

15 8. Optical switch according to any of the preceeding claims, c h a r a c t e r i s e d i n, that the membrane (7) is located adjacent a wave guide (8,9) provided with a Bragg grating (10,11), where the Bragg grating (10,11) is arranged 0 to reflect incoming ligth when the membrane (7) is at a distance from the wave guide (8,9) and arranged to pass the light when the membrane (7) is close to or abutting the wave guide at the Bragg grating (10,11) . 5 9. Optical switch according to claim 8, characterized in, that the membrane (25) is positioned adjacent two wave guides (16,17), each having a Bragg grating (26,27), in an optical switch (18) at the center of a mach-zender interferrometer .

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