WO2017187151A1

WO2017187151A1 - Optical device

Info

Publication number: WO2017187151A1
Application number: PCT/GB2017/051143
Authority: WO
Inventors: Harish BHASKARAM; Wolfram Pernice; Chen ZENGGUANG; Carlos Andres RIOS-CAMPO
Original assignee: Oxford University Innovation Ltd
Current assignee: Oxford University Innovation Ltd
Priority date: 2016-04-27
Filing date: 2017-04-25
Publication date: 2017-11-02
Anticipated expiration: 2018-10-27

Abstract

An optical device (100) is disclosed, comprising: a first waveguide (4), a second waveguide (2), and a coupling waveguide (3). The first and second waveguide (4, 2) are substantially parallel. The coupling waveguide (3) comprises a modulating element (6) evanescently coupled thereto, and is disposed between the first and second waveguide (4, 2). The degree of optical coupling between the first and second waveguide (4, 2) via the coupling waveguide (3) varies depending on the state of the modulating element (6). The state of the modulating element (6) is switchable by an optical switching signal (305) carried by the coupling waveguide (3).

Description

OPTICAL DEVICE

The present invention relates to an optical device which may be used as a switch for controlling optical communication.

A high performance optical switch is a key component for future optical communication networks, and for enabling optical computing. It is necessary for optical routings with high data traffic. Several types of optical switches have been proposed for this purpose theoretically or in practice.

Mechanical optical switches are one type of optical switch that function to change the optical paths by mechanically moving optical components such as the optical waveguides or micro-mirrors. However, the switching speed is low, on the order of milliseconds (ms), and mechanical optical switches are typically very large and expensive.

Another type of optical switch are waveguide optical switches based on electro-optic materials (such as lithium niobate) . These can have very fast switching speeds, in the range of nanoseconds (ns) to picoseconds (ps) . A drawback of such devices are large device size (typically several millimeters (mm)) since a long coupling length waveguide is required to obtain a π (λ/2) phase shift of the signal because of the small electro-optic coefficients of the materials.

It has previously been proposed to address these problems by replacing the typical electro-optic materials (e.g. lithium niobate) with phase change materials (PCMs). PCMs have previously been used in optical storage products: compact disks (CD), digital versatile disks (DVD) and Blu-ray disks (BD). The optical properties of PCMs, such as refractive index, change drastically between their amorphous and crystalline states. Small size optical switches that use Ge₂Sb₂Te₅ (GST) as a PCM on top of Mach Zehnder Interferometer waveguides has been achieved with high switching speed ( 100 ns) and low power consumption.

Liang, Haibo, et al. "Electro-optical phase-change x 2 switching using three-and four-waveguide directional couplers. " Applied optics 54.19 (2015): 5897-5902 discloses an optical switch based on parallel waveguides and phase change materials. This document discloses that electrical actuation is preferred, but does not clearly describe the mechanism of switching and provides no experimental results.

An optical switch that overcomes or ameliorates at least some of the above mentioned problems would be useful.

According to a first aspect of the invention, there is provided an optical device, comprising: a first waveguide, a second waveguide, and a coupling waveguide, wherein:

the first and second waveguide are substantially parallel; and

the coupling waveguide comprises a modulating element coupled thereto, and is disposed between the first and second waveguide such that the degree of optical coupling between the first and second waveguide via the coupling waveguide varies depending on the state of the modulating element;

wherein the state of the modulating element is switchable by an optical switching signal carried by the coupling waveguide.

The modulating element may comprise a phase change material. The modulating element may comprise a material with a refractive index that is switchable between at least two stable values. Stable may mean substantially stable at NIST standard temperature and pressure conditions.

The modulating element may comprise a plurality of stable solid states, each corresponding with a different transmission, reflection or absorption characteristic of the waveguide.

The modulating element may comprise a material comprising a compound or alloy of a combination of elements selected from the following list of combinations: GeSbTe, VOx, NbOx, GeTe, GeSb, GaSb, AglnSbTe, InSb, InSbTe, InSe, SbTe, TeGeSbS, AgSbSe, SbSe, GeSbMnSn, AgSbTe, AuSbTe, and AlSb. The material may be doped with any element (e .g. C, Ni, Ce, Si etc).

The first waveguide and second waveguide may be spaced apart from the coupling waveguide. Each of the first, second and coupling waveguides may be substantially parallel.

The first, second and/or coupling waveguide may comprise a material selected from: silicon, silicon nitride, gallium nitride, gallium arsenide, magnesium oxide, and diamond.

The modulating element may comprise a phase change material layer with a thickness of less than 40nm or 20nm.

The modulating element may be evanescently coupled to the coupling waveguide .

The modulating element may modify a transmission, reflection or absorption characteristic of the coupling waveguide dependant on its state .

The coupling waveguide may comprise an electrical conductor configured to switch and/or detect the state of the modulating element using an electrical signal that heats the modulating element. This may be an alternative arrangement to optically switching the modulating element using an optical signal carried by the coupling waveguide.

The electrical conductor may comprise a first electrode in contact with the modulating element, and a second electrode in contact with the modulating element, so that a conducting path is defined through the modulating element between the first and second electrode, so that the state of the modulating element is switchable by passing the electrical signal through the conducting path.

The modulating element may be sandwiched between the first and second electrode, so that the conducting path is substantially normal to the plane of the modulating element.

The first and second electrode may be arranged to define a lateral conducting path substantially parallel to a plane of the modulating element.

At least one of the first and second electrode may comprise indium tin-oxide, ITO. The coupling waveguide may comprise a resistor in thermal contact with the modulating element, so that the modulating element is switchable by passing the electrical signal through the resistor (with the electrical signal not passing through the modulating element).

The first, second and coupling waveguides may each be planar waveguides on a common plane.

According to a second aspect, there is provided a device according to the first aspect, configured as a switch, wherein when the device is configured such that, when the modulating element is in a first state, at least some light that enters the first waveguide will be coupled into the second waveguide, and when the modulating element is in a second state, light entering the first waveguide will substantially not be coupled into the second waveguide .

The device may be alternatively be configured as a modulator, such that modulation of the state of the modulating element results in modulation of the degree of optical coupling between the first waveguide and the second waveguide. According to a third aspect, there is provided an optical router, comprising: a plurality of input ports, a plurality of output ports, and at least one device according to the first aspect arranged to control the routing of optical signals between the input ports and output ports. According to a fourth aspect, there is provided a computer comprising the device of the first, second or third aspect.

A plurality of devices according to the first or second aspect may be used to (or configured to) control addressing of an optical storage medium.

The optical storage medium may be a non-volatile storage class memory.

According to a fifth aspect, there is provided a method of switching a device in accordance with any other aspect, comprising using an optical signal in the coupling waveguide to change the state of the modulating element. According to another aspect, there is provided a method of using a phase change material to make a device according to the first, second or third aspect. In some embodiments, an optical switch is provided, based on a three-waveguide directional coupler structure including a first and second parallel waveguide for signal transmission and one coupling waveguide between the first and second waveguide. The optical switch may comprises a substrate carrying the first, second and coupling waveguide. The modulating element of the coupling waveguide may comprise a thin layer of phase change material.

The dimensional parameters (e .g. width, length, height and gap) of the optical device may be designed for near-infrared (IR-A) signals. Near infra-red signals may be defined as having a wavelength ranging from 1500 nm to 1630 nm. In other embodiments, the dimensional parameters may be selected to provide a different range of wavelengths of operation, from ultraviolet to infrared.

The thickness of the modulating element thin layer phase change material may be from 5 nm to 50 nm, preferably 10 nm. The state of the modulating element electrically, optically or thermally switched since the refractive index of the phase change material can be electrically (e.g. by a current through the modulating element), optically (e.g. by an optical signal through the coupling waveguide), and thermally (e.g. by a resistive heater in the thermal contact with the modulating element) changed between amorphous and crystalline states.

Embodiments of the invention may provide a compact, ultra-fast, low-loss optical switch with low-energy consumption and wide operational range of wavelengths. It is to be understood that both the forgoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.

Embodiments of the invention will now be described, purely by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a schematic top and side view of an optical switch according to an embodiment; Figures 2 and 3 are simulated electrical field distributions in the optical switch of Figure 1 with the modulating element in an amorphous state (Figure 2) and a crystalline state (Figure 3);

Figure 4 is an optical microscope image of an optical system according to an embodiment;

Figures 5 and 6 are graphs of optical transmission characteristics measured from a system corresponding to Figure 4;

Figure 7 is an optical microscope image of an optical system according to an embodiment with a different optical switch length and modulating element position than Figure 4;

Figure 8 and 9 are a set of optical transmission characteristics measured from an optical system corresponding to Figure 7;

Figure 10 is a schematic illustrating optical switching of the modulating element;

Figure 1 1 is a schematic top view and sectional view of an arrangement for electrical switching of the modulating element;

Figure 12 is a schematic top view and sectional view of an alternative arrangement for electrical switching of the modulating element;

Figure 13 is a schematic of an nxn optical router implemented using devices according to an embodiment; and Figure 14 is a schematic of an alternative nxn optical routes implemented using devices according to an embodiment.

Figure 1 shows an optical device 100 according to an embodiment, comprising: substrate 1 , first waveguide 4, coupling waveguide 3, second waveguide 2. A cladding layer 5 is provided over the waveguides 2, 3, 4. The substrate 1 which can be any suitable substrate used for optical waveguide fabrication, such as silicon, silicon oxide, silicon nitride, quartz, etc. The substrate 1 in this example is silicon oxide on top of a silicon wafer.

The waveguides 2, 3, 4 are parallel, and may be fabricated with any appropriate process, which may comprise optical lithography, electron beam lithography, reactive ion etching, evaporation, etc. The waveguides 2, 3 and 4 in the following example embodiments were fabricated by electron beam lithography and reactive ion etching. The materials of the waveguides 2, 3, 4 could be any material that is appropriate for such a waveguide, for example: silicon, silicon nitride, polymers, etc. In the example embodiment, the waveguides 2, 3, 4, are made from silicon nitride.

The first waveguide 4 and second waveguide 2 define paths for optical signal transmission with the same length (L I), width (Wl) and height (HI) . The coupling waveguide 3 serves as an active switching waveguide . Ideally there will be no signal transmission inside the coupling waveguide. The coupling waveguide 3 has length (L2), width (W2) and height (H2). Preferably, W2 is smaller than Wl while L2 and H2 are equal to L I and HI respectively. The gap between the first waveguide 4 and the coupling waveguide 3 is G2 and the gap between the coupling waveguide 3 and the second waveguide 2 is Gl . Preferably, Gl is substantially equal to G2. The coupling waveguide 3 comprises a modulating element 6. The modulating element 6 comprises a thin layer of phase change material which is deposited on the top of the core of the coupling waveguide. A capping layer 7 is included, for example to protect the modulating element 6 (e.g. from the atmosphere). The capping layer 7 may be any materials that is chemically stable in the atmosphere, such as silicon oxide, silicon nitride, indium tin oxide (ITO), polymers, etc.

In the following example embodiments, the phase-change material of the modulating element 6 is Ge₂Sb₂Te₅ (GST) and the capping layer 7 is made from ITO. The modulating element 6 and/or capping layer could be deposited with any appropriate technique, for instance using physical or chemical methods, such as thermal evaporation, electron beam evaporation, sputtering, chemical vapour deposition, atomic layer deposition, etc., depending on the materials required. In the following example embodiments, RF sputtering was used for deposition of phase change materials and the capping layer 7.

The thickness, tl , of the phase change material may be in the range of 5nm to l OOnm. The thickness, t2, of the capping layer 7 may also be in the range of 5nm to l OOnm. More preferably, the thickness, tl and/or t2 are in the range of 5nm to 20nm, or around l Onm.

Figures 2 and 3 show theoretical simulations of the electrical field distribution in the optical switch of Figure 1. Figure 2 shows the case where the phase change material of the modulating element 6 is in an amorphous state, and Figure 3 shows the case where the phase change material of the modulating element 6 is in a crystalline state.

The parameters of the simulation model are shown in the Figures 2 and 4: L 1 = 1200 um, Wl = 1.0 um, W2=0.85 um, Hl=300 nm, G1=G2=500 nm, tl=t2= 10 nm.

Light is introduced in the left port of the first waveguide 4, and the electrical field distributions in Figures 2 and 3 demonstrate how the light is transported in the optical switch. In Figure 2 (with the phase change material in a first, amorphous state), the light is completely localized in the first waveguide 4 at the input port and the light intensity in the first waveguide 4 decreases and is coupled to the second waveguide 2 with increasing transmission length. When the transmission length reaches the coupling length (CPL, in Figure 2), the light is completely exchanged to the second waveguide 2.

With the phase change material in the second, crystalline state, as shown in Figure 3, the light in the first waveguide 4 has little exchange with the second waveguide 2, and is substantially only transported in the first waveguide 4.

The simulation therefore shows that the modulating element 6 can actively switch the light between two channels. When the modulating element 6 is in the first state, light is switched from the first waveguide 4 to the second waveguide 2 after the coupling length. With the modulating element 6 in the second state, light is not switched from the first waveguide 4, but continues along the first waveguide 4 without being coupled to the second waveguide 2.

The electrical field in the coupling waveguide 3 in both Figure 2 and 3 is extremely small which means that the device will have very low losses. With careful selection of the parameters, such as L I , Wl , W2, HI , Gl , G2, tl and t2, the performance of a device according to an embodiment can be further improved with high signal-to-noise ratio, small coupling length, little energy loss, high switching speed and low energy consumption.

Figure 4 shows an example of an optical system 1 1 1 comprising an optical device 100 according to an embodiment. The optical system 1 1 1 comprises apodized grating couplers 8, 9, 10. The grating couplers comprise an input coupler 8, first output coupler 9 and second output coupler 10. The input grating coupler 8 is connected (via a waveguide) with the input of the first waveguide 4, and the first output coupler 9 is connected to the output of the first waveguide 4 (via a waveguide) . The second output coupler 10 is connected (via a waveguide) to the output of the second waveguide 2. The optical device 100 is here configured to switch light entering the optical system 1 1 1 between the first and second output couplers 9, 10. The dimensional parameters of the optical device in this example are : L l=60 um, Wl = 1.0 um, W2=0.85 um, Hl=300 nm, G1=G2=450 nm, tl =t2= 10 nm.

The transmission power from the first and second output couplers 9, 10 were measured as Pou_ti and P_out2 - Pin is the initial power measured from the output of the laser (which is coupled into the device 1 1 1 at the input coupler 8) . The transmission ratios Tl and T2 are calculated as:

Figure 5 shows a graph 120 of measurement data for T l and T2 with respect to wavelength for modulating element 6 in a first (amorphous) and second (crystalline) state. Curves 121 , 122, 123, and 124 respectively show: T l first state; T2 first state; T l second state; T2 second state . After fabrication of the system 1 1 1 , the phase change material (GST) was in an amorphous state. The transmission ratios T l and T2 were measured (i.e . the data of 121 and 122). After that, the sample was placed on a hot plate at 250 °C for 5 mins to switch the GST material of the modulating element 6 to the crystalline state. The transmission Tl and T2 was measured again to obtain the remaining data 123, 124

On switching from the first (amorphous) state to the second (crystalline) state, Tl increased at all wavelengths tested, and T2 decreased at all wavelengths tested ( 1500- 1630nm) . Notably, T2 decreased by around 20db over a broad wavelength range, with only weak wavelength dependence (at the wavelengths tested).

Figure 6 shows a graph 130 of the fractional transmission ratio (T 1/T2) and the total transmission (T 1+T2) as a function of wavelength for the embodiment of Figure 4. Curves 13 1 , 132, 133, and 134 respectively show: (T 1/T2) first (amorphous) state; (T1/T2) first state; (T 1+T2) second (crystalline) state; (T1+T2) second state .

The fractional transmission ratio T1/T2 increased -20 dB with the transition of the modulating element from the first state to the second state (from curve 13 1 to 132). Moreover, no significant change was found of the total transmission (T1+T2) (curves 133, 134), which means the energy loss in the optical device 100 is negligible. Furthermore, the working wavelength region is very large (- 100 nm) . Embodiments of the invention can be designed for any optical wavelength with the appropriate dimensional parameters. Altogether, the example embodiment shown has achieved a substantially lossless optical switch 100. The design parameters could be further optimized for an increased on-off ratio of the optical switch.

Figure 7 shows a further example optical system 1 12, comprising an optical device 100 configured as a switch. The optical system 1 12 has a similar structure to that of Figure 4, with dimensional parameters of: L l=80 um, W l = 1.0 um, W2=0.85 um, Hl=300 nm, G1=G2=450 nm, 11 =t2= 10 nm. One significant difference with the system 1 12 of Figure 7 is that the phase change material of the modulating element 6 is misaligned, with some part of the modulating element 6 sitting in the gap between the first waveguide 4 and the coupling waveguide 3. Figure 8 shows a graph 140 of measurement data for T l and T2 with respect to wavelength for modulating element 6 in a first (amorphous) and second (crystalline) state. Curves 141 , 142, 143, and 144 respectively show: T l first state; T2 first state; T l second state; T2 second state .

Both Tl and T2 change significantly when the modulating element 6 changes state (from a first amorphous state to a second crystalline state). Tl increases significantly at all wavelengths tested as the modulating element 6 transitions from the first state to the second state. T2 decreases significantly at all wavelengths tested as the modulating element 6 transitions from the first state to the second state.

Figure 9 shows a graph 150 of the fractional transmission ratio (T 1/T2) and the total transmission (T 1+T2) as a function of wavelength for the embodiment of Figure 7. Curves 15 1 , 152, 153, and 154 respectively show: (T 1/T2) first (amorphous) state; (T1/T2) first state; (T 1+T2) second (crystalline) state; (T1+T2) second state .

Again, (T1/T2) increased -20 dB when the modulating element 6 was switched from the first (amorphous) state to the second (crystalline) state. However, in this example, the total transmission (T 1+T2) has changed (by as much as 20 %) dependent on the wavelength. This is a result of energy loss in the phase change material in the gap between the first waveguide 4 and coupling waveguide 3.

If low losses are an important factor, it may therefore be desirable to align the modulating element 6 accurately with the core of the coupling waveguide 3, or at least to avoid placing phase change material in the gap between the first waveguide 4 and coupling waveguide 3, and the gap between the second waveguide 2 and coupling waveguide 3. One way to ensure that the modulating element 6 is accurately aligned with the coupling waveguide 3 is to co-pattern the modulating element 6 with the core of the coupling waveguide (e .g. with a single lithographic mask). An alternative is to use an appropriate design rule to ensure that the modulating element 6 cannot extend beyond the edge of the coupling waveguide core .

The above examples have shown that embodiments are suitable for providing optical switching with low energy loss. In some embodiments, the modulating element 6 could be patterned as a discrete island (or more than one discrete island), rather than a film that covers most of the upper surface of the coupling waveguide 3 core . Optical switching of the modulating element 6 may be used, using an optical signal carried by the coupling waveguide 3. The modulating element 6 may be evanescently coupled to the coupling waveguide 3, so that an optical switching signal carried by the coupling waveguide 3 heats the modulating element. The coupling waveguide includes an input port 3 15 and an output port 3 16.

Figure 10 illustrates switching of the state of coupling waveguide 3 using an optical signal 305. The coupling waveguide 3 includes a modulating element 6 in the form of an island of material on the surface of the waveguide core 301. An encapsulation layer (e.g. ITO) may be present, but this is not shown for clarity.

An optical switching signal 305 within the coupling waveguide core 301 is evanescently coupled to the modulating element 6, thereby heating the modulating element 6. A nanosecond (or shorter) optical pulse may be used to switch the modulating element 6.

For amorphization, the PCM (e .g. GST) of the modulating element 6 may be melted and then cooled down rapidly to preserve this disordered state. On the other hand, heating the PCM above the crystallization temperature (but below the melting temperature) for a few nanoseconds may enable recovery of the atomic ordering and thus crystallization. The properties (e .g. transmission, absorption, reflection) of the coupling waveguide 3 may therefore be modulated by varying the state of the modulating element 6.

Crystallization of amorphous PCM may enhance optical absorption in the modulating element 6 at the design wavelength, for instance by one order of magnitude . This increase in optical absorption results in an increase in the proportion of the energy of a switching signal 305 in the coupling waveguide 3 that is absorbed by the modulating element 6. This renders both amorphization and crystallization transitions possible with optical switch signal pulses 305 of comparable length and power. Full recrystallization with a single light pulse 305 may require optimisation of the pulse (e.g. duration and power).

If the phase transition occurs before the end of the pulse, the continued optical energy supply may heat up the PCM further, to the melting temperature, and cause immediate reamorphization. Temperature variations across the device may mean that this cannot be prevented completely since all parts of the PCM material may not crystallize simultaneously. To address this issue, an erase transition (making the PCM material more crystalline) may be based on stepwise partial recrystallization using a train of consecutive pulses.

In order to prevent reamorphization of already crystallized regions, the individual pulse energies may be gradually decreased from pulse to pulse (e.g. by approximately 5 % of the initial pulse energy). The initial pulse energy may correspond with the pulse energy used for a transition from crystalline to amorphous. The energy of the final pulse determines which state is achieved, and therefore what transmission characteristic of the coupling waveguide 3 is achieved. The final state of the modulating element 6 can be fully crystalline or an intermediate state (i.e. partially amorphous and partially crystalline), for instance by stopping a transition before complete amorphization or crystallisation has occurred.

High reproducibility of the transition operations may be achieved by an initial conditioning step. A conditioning step may comprise performing a switching cycle a few times on the as-deposited and subsequently annealed GST. Within the first few cycles the read-out transmissions, which initially vary slightly from cycle to cycle, stabilize to a fixed value.

To achieve optical switching by an optical signal carried within the coupling waveguide 3 another input/output apodized grating coupler can be added to an optical system like that shown in Figures 4 and 7. This additional grating coupler may be used to introduce a switching signal to the coupling waveguide 3 to switch the state of the modulating element 6. In another embodiment, electrical switching of the state of the modulating element 6 may be employed. Electrically switchable embodiments may be advantageous in optical switching or routing in multiplex. Figure 1 1 illustrates a coupling waveguide 3 according to an embodiment in which the state of the modulating element 6 may be altered by an electrical signal. The modulating element 6 is evanescently coupled to the waveguide 101. A resistor 323 is provided in thermal contact with (e.g. on top of, or adj acent to) the modulating element 6. When an electrical current is passed through the resistor 323, it will heat up the modulating element 6, which results in the modulating element 6 changing state (as previously described).

The resistor 323 may comprise part of a conducting track, for example a metal or semiconductor track. In some embodiments the resistor 323 may comprise a material that is substantially transparent at the optical probe signal wavelength. The resistor 323 may, for example, comprise a resistor track patterned over the modulating element 6. A dielectric layer or insulating layer may be interposed between the resistor 323 and the modulating element 6. The state of the modulating element 6 may be determined optically, using a probe signal within the coupling waveguide 3.

Figure 12 illustrates an embodiment in which the state of the modulating element 6 can be read and/or written electrically (i.e. by an electrical signal) . The state of the modulating element 6 may further be read and/or written optically, by optical signals carried by the coupling waveguide 3 (as described above) . The coupling waveguide 3 in this embodiment comprises a waveguide core 301 , modulating element 6, first electrode 321 and second electrode 322.

The first electrode 321 and second electrode 322 are both in electrical contact with the modulating element 6, so that a voltage difference applied between the first and second electrodes 321 , 322 results in a current through the modulating element 6. Where the modulating element 6 comprises a layer of material, it may be convenient for the first electrode 321 to be disposed under the layer, and the second electrode 322 to be disposed on top of the layer. Alternatively, a lateral arrangement of electrodes may be used, in which the flow of current through the layer of the modulating element 6 is substantially in the plane of the layer. The resistance of the modulating element 6 may be inferred from its voltage -current characteristics via the first and second electrodes 321 , 322. The state of the modulating element 6 may thereby be inferred from an electrical probe signal applied to via the first and second electrodes 321 , 322. Furthermore, the state of the modulating element 6 can be varied by Joule heating the modulating element 6 by applying a voltage difference between the first and second electrodes 321 , 322.

At least one of the first and second electrodes 321 , 322 may comprise an optically transparent material, such as ITO. The optical reading and/or writing of the modulating element 6 may be substantially as described above, with reference to other embodiments.

Features of the example embodiments described with reference to Figures 1 1 and 12 may be combined. For example, an arrangement that includes both a resistor 323, enabling heating of the modulating element 6 by thermal conduction (without passing current through the modulating element 6) may be combined with first and second electrodes 321 , 322. Such a device may be electrically written via the resistor and/or first and second electrodes 321 , 322. Furthermore, the device may be read and/or written optically, by optical signals in the coupling waveguide 3.

Electrical reading and/or writing may be more straightforward to interface with an electrical controller. Optical reading and/or writing may offer faster speed. Depending on the application, different combinations of electrical and optical reading and writing may be appropriate .

In another embodiment, an n x n optical router 160 is developed with the schematic diagram shown in Figure 13. The optical router has 2 X (n— 2) + 1 (n > 2) optical switches 100. There are n different input ports (I I₂, I_n) of the optical switches with n output ports (0₁, 0₂, O_n) on the other side of the signal path. In use, optical signals may be fed to each of the input ports, for routing to the output ports. The optical router 160 is configured to connect each one of the input ports (ii, I₂, I_n) with one of the output ports (0₁, 0₂, O_n), in any permutation, so that Ij may be connected to O_s, for example, and I₈ connected with a different output port. Each optical switch 100 has two states: bar 161 and cross 162, corresponding to the state of the modulating element 6 on the coupling waveguide 3. The state of the modulating element 6 is controllable using electrical or optical control signals 305 (as previously described) . The bar state 161 means an optical signal entering the first waveguide 4 will substantially not be coupled into the second waveguide 2 and that an optical signal entering the second waveguide 2 will substantially not be coupled into the first waveguide 4. The bar state may correspond with the modulating element 6 being in the crystalline state . The cross state 162 means an optical signal entering the first waveguide 4 will be completely (or substantially) coupled into the second waveguide 2 on the output of the optical switch 100 and vice versa. The cross state 162 may correspond with the modulating element 6 being in an amorphous state .

The optical routes 160 comprises waveguides connecting input port /; to output port Oj and I₂ to 0₂ etc. The optical switches 100 are disposed between these waveguides, and are offset from each other along the length of the waveguides connecting the input ports(/i, I₂, I_n) and output ports (0₁, 0₂, O_n), so that a signal leaving a switch disposed between a first and second waveguide (e .g. between Ιι-Ο_γ and I₂-0₂) can enter an adjacent switch (e.g. between I₂-0₂ and I₃-0₃) . Depending on different combinations of controlled optical switches 100, a specific input signal can be routed at any output port. The switches 100 may be configured to be controlled optically, electrically, electrically or electro-thermally, as described above . The optical router 160 may provide for substantially lossless, high-speed switching and arbitrary routing, suitable for future on-chip routing. An optical router according to an embodiment may be used in the emerging field of cognitive computing and could be operated with feedback attributes to model weighting (e.g. synaptic weighting).

In another embodiment, a further x n optical router 170 is proposed with the schematic diagram shown in Figure 14. The optical router 170 has n— 1 (n≥ 2) optical switches 100. There are n different input ports (Ii, I₂, I_n) of the optical switches with n output ports (0₁, 0₂, O_n) on the other side of the signal path. The optical switch has two states: bar 161 and cross 162 corresponding to the state of the modulating element 6 of the coupling waveguide 3 which is controllable with electrical or optical control signals 305. In this embodiment, the optical switches 100 are positioned adjacent to each other, rather than offset.

Compared with the optical router 160 in Figure 13, the optical router 170 has less optical switches 100 which may result in smaller dimensions, faster routing speed and lower energy consumption. The optical coupling among different optical paths in optical router 170 may be stronger than that in optical router 160, the signal to noise ratio may be decreased while the energy loss in the coupling waveguide 3 could be increased.

Similar to the optical router 160, the optical router 170 may also be suitable for the applications of on-chip optical routing and cognitive computing.

Other arrangements of switches may be used to provide routing functionality in a router. For example, optical switches 100 may be arranged in a crossbar architecture, with an nxn matrix of switches provided between n input and n outputs.

Although specific embodiments have been described, these are not intended to limit the scope of the invention, which should be determined only with reference to the appended claims.

Claims

1. An optical device, comprising: a first waveguide, a second waveguide, and a coupling waveguide, wherein:

the first and second waveguide are substantially parallel; and

the coupling waveguide comprises a modulating element evanescently coupled thereto, and is disposed between the first and second waveguide such that the degree of optical coupling between the first and second waveguide via the coupling waveguide varies depending on the state of the modulating element;

2. The device of claim 1 , wherein the modulating element comprises a phase change material.

3. The device of claim 2, wherein the modulating element comprises a material with a refractive index that is switchable between at least two stable values.

4. The device of claim 3, wherein the modulating element comprises a plurality of stable solid states, each corresponding with a different transmission, reflection or absorption characteristic of the waveguide.

5. The device of any preceding claim, wherein the modulating element comprises a material comprising a compound or alloy of a combination of elements selected from the following list of combinations: GeSbTe, VOx, NbOx, GeTe, GeSb, GaSb, AglnSbTe, InSb, InSbTe, InSe, SbTe, TeGeSbS, AgSbSe, SbSe, GeSbMnSn, AgSbTe, AuSbTe, and AlSb.

6. The device of any preceding claim, wherein first waveguide and second waveguide are each spaced apart from the coupling waveguide.

7. The device of any preceding claim, wherein each of the first, second and coupling waveguides are substantially parallel.

8. The device of any preceding claim, wherein the first, second and/or coupling waveguide comprise a material selected from: silicon, silicon nitride, gallium nitride, gallium arsenide, magnesium oxide, and diamond.

9. The device of any preceding claim, wherein the modulating element comprises a phase change material layer with a thickness of less than 40nm or 20nm.

10. The device of any preceding claim, wherein the modulating element is evanescently coupled to the coupling waveguide; and the modulating element modifies a transmission, reflection or absorption characteristic of the coupling waveguide dependant on its state.

1 1. The device of any preceding claim, wherein the coupling waveguide comprises an electrical conductor configured to switch the state of the modulating element using an electrical signal that heats the modulating element.

12. The device of claim 1 1 , wherein the electrical conductor comprises a first electrode in contact with the modulating element, and a second electrode in contact with the modulating element, so that a conducting path is defined through the modulating element between the first and second electrode, so that the state of the modulating element is switchable by passing the electrical signal through the conducting path.

13. The device of claim 12, wherein the modulating element is sandwiched between the first and second electrode, so that the conducting path is substantially normal to the plane of the modulating element.

14. The device of claim 13, wherein the first and second electrode are arranged to define a lateral conducting path substantially parallel to a plane of the modulating element.

15. The device of any of claims 12 to 14, wherein at least one of the first and second electrode comprises indium tin-oxide, ITO.

16. The device of any of claims 10 to 15, wherein the coupling waveguide comprises a resistor in thermal contact with the modulating element, so that the modulating element is switchable by passing the electrical signal through the resistor (with the electrical signal not passing through the modulating element).

17. The device of any preceding claim, wherein the first, second and coupling waveguides are each planar waveguides on a common plane.

18. The device of any preceding claim, configured as a switch, wherein when the device is configured such that, when the modulating element is in a first state, at least some light that enters the first waveguide will be coupled into the second waveguide, and when the modulating element is in a second state, light entering the first waveguide will substantially not be coupled into the second waveguide . 19. The device of any of claims 1 to 17, configured as a modulator, wherein the device is configured such that modulation of the state of the modulating element results in modulation of the degree of optical coupling between the first waveguide and the second waveguide . 20. An optical router, comprising: a plurality of input ports, a plurality of output ports, and at least device according to any preceding claim arranged to control the routing of optical signals between the input ports and output ports.

21. A computer comprising the device of any of claims 1 to 20.

22. The computer of claim 21 , wherein a plurality of devices according to any of claims 1 to 20 are using to control addressing of an optical storage medium.

23. The computer of claim 21 or 22, wherein the optical storage medium is a non- volatile storage class memory.

24. A method of switching or modulating a device according to any of claims 1 to

19. comprising using an optical signal in the coupling waveguide to change the state of the modulating element.