GB2278926A - Optical devices - Google Patents

Abstract

An integrated optical device 10 has a pair of electrodes 15 and 16 across which a voltage is impressed to cause operation of the device. In order to achieve very high operational rates for example in modulators and switches, such a device 10 is provided with two drive circuits 21, one for each electrode 15 and 16 respectively. One drive circuit is arranged to provide a positive-going voltage, and the other to provide a negative-going voltage. The positive-going and negative-going voltages track one another, and each drive circuit need provide only about one half of the total drive voltage which otherwise would be required. This allows sub-unity nano-second switching times to be achieved. An integrated optical component may include a plurality of such devices 10, in which case the two electrodes 15 and 16 of each device are electrically-isolated from the electrodes of the other devices, and are electrically separately accessible from the exterior of the component package. <IMAGE>

Description

OPTICAL DEVICES This invention relates to a method of operating an integrated optical component comprising a plurality of optical devices, and to such optical components themselves.

Integrated optical components, comprising a plurality of optical devices integrated within a single package, are increasingly being used within fibre optic systems for controlling the propagation of light along a fibre optic. For example, such devices may be used for modulating, switching or sensing light being propagated within a fibre optic. The usual technology for the production of an integrated optical component including a number (and typically several tens) of devices within a single package is by etching the devices into a substrate, using a technology similar to that widely employed in the semi-conductor industry.

As such, there is formed a single, common base electrode which, during operation of the component, is held at circuit ground. Each device has an operating electrode which is led to the exterior of the package, for the application of a suitable driving voltage thereto.

Developments in integrated optical components have allowed high modulation bandwidths and fast switching times, but the industry is calling for even greater bandwidths and yet faster switching times. Currently, a bandwidth of 1 or 2 GHz of 1 or 2 GB/s can be achieved in modulators, and switching speeds of a few ns can be achieved. The limiting factor is the slew rate of the drive electronics, which provide the required drive voltage to the device electrodes, rather than the devices themselves.

Typical optical devices integrated into a single package require drive voltages within the range of 3 to 5V, though some devices may require voltages as high as 150V - for example, a polarisation-independent optical switch. Currently, the dominant electronics technology employed to drive high-speed optical devices is emitter-coupled logic (ECL) which is able directly to provide a 0 7V voltage swing. Whilst this voltage swing may be increased to about 2V with only a small loss in the rise-time, if higher drive voltages are required (as is the case with most integrated optical devices), it becomes increasingly difficult to achieve the required voltage swing with a very fast rise-time.

As such, the limit on the operating speed is normally imposed by limitations in the drive electronics technology.

Though it is known that it is possible to reduce the required drive voltage for an optical device, physical limitations make it impractical to reduce the drive voltage down to that which is obtainable directly from ECL technology. For example, a typical device which requires a drive voltage of 5V may have an electrode length of perhaps 7 to 8mm; if the device is to operate at 3.5V, then the electrode length may have to be increased by about 50%. If operation at a voltage as low as 2V is required, then the electrode length will have to be increased by about 150% to approximately 20mm. Such electrode lengths are quite impracticable and in any event would also greatly limit the number of separate devices which may be included within a single integrated package.

The present invention stems from research into the operation of integrated optical devices at higher speeds than those currently achievable with the known technology, described above.

According to one aspect of the present invention, there is provided a method of driving an integrated optical device having a pair of electrodes across which a voltage is impressed to cause operation of the device, in which method operation of the device is caused by applying a positive-going voltage to one electrode and simultaneously applying a negative-going voltage to the other electrode.

It will be appreciated that an integrated optical device used in the method of the present invention must permit separate access to each of the two electrodes of the device. It will further be appreciated that two separate drive circuits are required, one for each electrode. Nevertheless, the voltage which has to be delivered to each electrode within the available time for causing device operation is only one half of that voltage which otherwise would have the single electrode of a device, using the conventional technology. This half voltage may be achieved much more quickly - and perhaps ten times as quickly - as the full required voltage swing. Thus, at the expense of a more complex integrated component and twice as many drive circuits, it is possible to obtain bandwidths of 1OGHz, or subunity nano-second switching times.

Most preferably, the applied positive-going and negative-going voltages track one another - that is to say, the magnitude of the two voltages at any instant should be substantially the same, and the rate of change of those voltages also should be substantially the same. In this way, the apparent operation of the device will be essentially the same as the operation of a similar device when driven in the conventional manner, by pulling the single electrode of such a conventional device up to the full operating voltage.

The drive circuits for the two electrodes of the device should be arranged to hold those electrodes at the same voltage prior to the application of the positive-going and negative-going voltages, to operate the device. Conveniently, that same voltage comprises circuit ground, though for some operations or drive circuit configurations, it may be convenient to clamp said same voltage to some other non-zero voltage.

In order to allow direct operation of the device using ECL drive circuit techniques, it is preferred for each device to be configured to permit its operation with a total drive voltage within the range of 4 to 5V, achieved by applying positive-going and negative-going voltages in the range of +2 0 to +2 5V.

According to a second aspect of this invention, there is provided an integrated optical component for use in a method of this invention as described above, in which component there is provided a substrate on which is formed a plurality of individual optical devices, each device being electrically isolated from the others and having a pair of electrodes across which a voltage must be impressed to cause operation of that device, both electrodes of each device being separately accessible externally of the integrated optical component.

An integrated optical component according to this aspect of the present invention may be operated in accordance with the method described above, since the two electrodes of each device of the component may be appropriately driven by a pair of drive circuits, one for each electrode wholly independently of all of the other devices integrated into the component. Thus, there is no electrode common to all of the devices within the component. Though this leads to an increased manufacturing complexity and a larger number of pins for a given number of devices within one package, nevertheless each device of the component may be operated at a significantly higher speed as compared to the conventional technology.

According to a further aspect of the present invention, there is provided an integrated optical component as described above, in combination with a respective pair of drive circuits for each individual device of the component, each pair of drive circuits being arranged simultaneously to supply positive-going and negative-going voltages to the two electrodes respectively of the associated device.

By way of example only, the invention will now be described in further detail and one specific embodiment thereof given, reference being made to the accompanying drawings, in which: Figure 1 is a schematic diagram of a conventional optical device together with its drive circuit; and Figure 2 shows schematically a device of this invention together with its drive circuit.

Illustrated in Figure 1 is an optical device 10 which comprises one of a number of similar devices all integrated together within a single package, to form an integrated optical component. The illustrated device 10 is in the form of a modulator for light propagated along a fibre optic, the device being provided with appropriate couplers 11 and 12, to couple the device to an input fibre optic 13 and an output fibre optic 14.

The device comprises a base electrode 15 and an operating electrode 16. The device is manufactured by a similar technology to that employed for the manufacture of electronic integrated circuits, and so by etching of a substrate to forming the base electrode 15, and the other parts of each device. By manufacturing a plurality of such devices on the substrate, all of these devices have their base electrodes linked together, and which are coupled to an external "ground" pin 17 on the package.

The drive electrode 16 is connected to an external pin 18 of the package, there being one such pin for each device integrated within the component. Each drive electrode 16 is connected to a driver circuit 19 individual to that device, which provides the full required voltage swing for operating the device.

Though the voltage at which the device will operate depends upon the physical geometry thereof, typically the devices within a component are configured to operate at 5V.

The driver circuit 19 is triggered by a control signal and when triggered, delivers the required voltage to the operating electrode 16 of the device.

This serves to place the device in the OFF state, where no light can be propagated through the device. Removal of the voltage, so that the two electrodes are at the same (ground) potential turns the device to the ON state, where light can be propagated therethrough.

Turning now to Figure 2, there is illustrated one device of an integrated optical component of this invention, together with the drive circuit therefor.

The physical configuration of the device is substantially the same as that of the device of Figure 1, and like items are given the same reference numbers.

However, in the case of the device of Figure 2, the base electrode 15 of each device is electrically isolated from the base electrodes of all of the other devices, and the base electrode for each device is connected to an individual pin 20 of the package. As such, the two electrodes of each device are connected to a respective pair of pins of the package so that each device is electrically wholly separate from all of the other devices within the package.

The driver 21 for the device includes a pair of drive circuits, one for each of the two electrodes of the device. Each of these drive circuits is required to deliver one half of the overall operating voltage, one circuit providing a negative-going voltage and the other a positive-going voltage. As such, when the two drive circuits are delivering voltage to the device, the total voltage across the electrodes is the required operating voltage.

The drive circuits within the driver 21 are arranged to track each other as closely as possible, both as regards instantaneous voltage and rise time.

As a consequence, the operation of the device of Figure 2 will be indistinguishable from that of Figure 1, but since each of the drive circuits of the driver 21 have to deliver only one half of the voltage of the circuit 19 of Figure 1, much faster slew rates can be obtained, leading to faster device operation.

A typical device constructed and arranged in accordance with Figure 1 (the prior art) may require an operating voltage of 5V. Using state-of-the-art ECL drive circuit techniques, a bandwidth of 1 or 2 GHz can be achieved. However, by employing the techniques of this invention as exemplified by Figure 2, each drive circuit has to deliver only 2 5V, and using the same ECL drive circuit techniques, a bandwidth of up to 1OGHz can be achieved.

Claims (1)

1. A method of driving an integrated optical device having a pair of electrodes across which a voltage is impressed to cause operation of the device, in which method operation of the device is caused by applying a positive-going voltage to one electrode and simultaneously applying a negative-going voltage to the other electrode.

2. A method as claimed in Claim 1, in which the magnitude of the applied positive-going voltage is substantially equal to the magnitude of the applied negative-going voltage.

3. A method as claimed in Claim 2, in which the rate of change of the applied positive-going voltage is substantially equal to the rate of change of the applied negative-going voltage.

4. A method as claimed in any of the preceding Claims, in which the first and second electrodes are held at the same voltage prior to applying the positive-going and negative-going voltages thereto.

5. A method as claimed in Claim 4, in which said same voltage comprises circuit ground.

6. A method as claimed in any of the preceding Claims, in which the maximum applied voltages to the electrodes are in the range of +2 0 to +3 0 volts.

7. An integrated optical component for use in a method as claimed in any of the preceding Claims, in which there is provided a substrate on which is formed a plurality of individual optical devices, each device being electrically isolated from the others and having a pair of electrodes across which a voltage must be impressed to cause operation of that device, both electrodes of each device being separately accessible externally of the integrated optical component.

8. An integrated optical component as claimed in Claim 7 in combination with a pair of drive circuits for each used device of the component, the drive circuits of a pair being associated one with each electrode respectively of the device, the two drive circuits of each pair being arranged simultaneously to supply positive and negative-going voltages to the two electrodes respectively of the associated device.

9. The combination as claimed in Claim 8, wherein each drive circuit is arranged to supply two equal and opposite voltages in the range of +2 0 to +3.0 volts.

11. The combination as claimed in Claim 8 or Claim 9, in which each drive circuit has a bandwidth in the range of up to 10 GHz.