GB2139443A - Optical packet switching system - Google Patents

Abstract

In a packet switching system, the data bits are modulated on to a light beam, using one of several possible known techniques. Switching is accomplished in the optical switch using a SAW transducer working in the Bragg regime to direct the incoming signals to a desired trunk, or via local links. The switch may comprise a film of lithium niobate which may be piezo electrically excited to produce acoustic signals having a wavelength of the same order as the light. The light is thereby deflected, the angle of deflection depending upon the relationship between the acoustic and optical wavelengths. <IMAGE>

Description

SPECIFICATION Optical Packet Switching System The present invention relates to a packet switching centre in which optical techniques are used.

According to the invention there is provided a telecommunications switching centre, such as a packet switch, in which intelligence to be conveyed is modulated on to an optical beam using a suitable electro-optic, acousto-optic or magneto-optic device, and in which intelligence is switched between the trunks served by the switch by a multi-position switching device formed by an acousto-optic switch working in the Bragg regime.

An embodiment of the invention will now be described with reference to the accompanying drawings, in which Fig. 1 is a schematic representation of an acousto-optic switch working in the Bragg regime, while Fig. 2 is a schematic representation of an optical packet switch using switches such as that of Fig. 1.

In the optical communication system to be described, the optical switch element used confines the light to an optical waveguide in a material such as lithium niobate, since confining the light beam to a solid medium has the advantages that a number of devices can be integrated within that medium and the device is compact and rigid. With solid media there is a choice between bulk devices in which-the light is unconstrained and thin film devices in which the light is constrained to the film and sometimes to a channel within that film when it propagates as in a waveguide. In this case it is the thin film devices which are preferred.

Light sources used are usually semiconductor laser, which may be intensity modulated directly by varying the drive current. This is simple but introduces some problems as the wavelength of the laser's output depends on drive current level and temperature. The maximum achievable modulation frequency at present is in the 1-2 GHz region, and bandwidths of the 1 GHz order have been achieved.

Alternatively the laser output may be held constant and intensity modulated by an external device. Such devices are generally operated as on/off intensity modulators (typically with a 10 dB extinction ratio) and have much in common with, though simpler than, switching elements. It is possible to fabricate a modulator on the same chip as switch elements. Bandwidths approaching 10 GHz can be achieved with this type of modulator and frequency pulling is less likely to be a problem.

Incoherent receivers convert optical power directly into electrical current using a detecting device (e.g. an avalance photo diode (APD), or PIN FET. Coherent receivers use an optical frequency local oscillator and have a non-linear photo sensitive device as a mixer to produce an IF output, of in the case of a homodyne arrangement, a base band output. With detectors currently available for use in incoherent receiver applications, response times of the 0.1 ns order can be attained.

Coherent receivers are in a relatively early state of development and give improved performance in terms of signal to noise ratio over incoherent receivers. Their bandwidth is unlikely to be significantly different from that of an incoherent receiver as long as the same devices are used for mixers as are used as detectors for the incoherent receivers.

Switching of unregenerated signals at the end of a long haul transmission system is not needed in the present system, so incoherent receivers are used.

The system is based on waveguide devices, since they demand lower drive powers than the main alternatives and lend themselves to the sort of fabrication techniques associated with planar semiconductor devices.

In the present system, the light is propagated in the z-direction, through a waveguide in the form of a thin film. The waveguide is bounded in the x-direction by air on one-side and the substrate to which the thin film is attached on the other side. In the lateral y-direction, the wave may be effectively unbounded or may be confined to a channel. Such channels are achieved by locally increasing the refractive index of the thin film by a diffusion process.

Many current devices use lithium niobate which has the property that its refractive index can be changed by electrical or mechanical stress.

As it is a piezo-electric material these effects are not independent, mechanical stress causing electrical fields and vice versa. Stresses are set up as high frequency acoustic energy using electrical signal applied to electrode on the surface of the thin film.

The acoustic frequencies involved have a wavelength comparable with that of the light being switched. Thus sound with a wavelength of 1 micron has a frequency of the 3 GHz order in the sort of materials used for switches (velocity=3 x 1 03m/s).

For switches the main interest lies in devices operating in the Bragg regime, see Fig. 1. The guided light departs from orthogonality with the acoustic wave by an angle H called the Bragg angle. When the requisite amount of acoustic energy is applied at the "centre" frequency the beam is switched, in theory perfectly, through an angle 20. By varying the acoustic frequency about the centre frequency the angle through which the beam is switched varies proportionately but switching is less then perfect.

The Bragg angle is given by Sin B= 2A where A is the optical wavelength of the light being switched and A is the wavelength of the acoustic energy doing the switching. 0B is usually a small angle implying that A > R and acoustic frequencies typically in the 100--300 MHz region are used.

For a single inter digital transducer (IDT) there is a so called matching condition A/ < =A/nb where < is the length (in the Z direction) of the IDT, n is the refractive index of the film, and b is the width of the light beam in the unbounded, y, direction.

When this condition is fulfilled the bandwidth of the switch is limited equally by changes in Bragg angle and transit time and is given by Va Af=- b where Af is the bandwitch and Va is the velocity of sound of a surface acoustic wave in the waveguide medium. Thus switching cannot take place in a time less than it takes sound to travel across the light beam. This might be termed the "intrinsic" limit on speed. There may be an extrinsic limit set by the need to drive the IDT from an electrical source which is set by the characteristics of the drive circuitry and in particular the capacitance of the IDT.

By applying the acoustic signal to a multiplicity of interdigital structures set at different angles relative to the optical beam the effects due to the change of Bragg angle with frequency may be reduced. However, transit time sets a limit on bandwidth of 2Va b A multiplicity of interdigital structures will however, increase the capacitance of the IDT and may reflect the problem back into the electrical drive circuits.

Bandwidth of the order of 100 MHz to 500 MHz are reported in the literature.

To date the main application of such Bragg cell devices has been in spectral analysis. An unmodulated light beam is deflected by an acoustic signal which is the spectrum to be analysed. The frequency spectrum is thus transformed into a spatial distribution of light. The light emitted from the Bragg cell is passed through a transform lens onto a photodiode array.

The output of this array is then a measure of the incident spectrum.

If the light is modulated with the intelligence to be switched and an acoustic wave applied of a frequency representing the destination of that intelligence an N-pole switch can be fabricated.

Clearly the value of N is of interest.

According to "Thin Film acoustic-optic devices", by E. G. H. Lean, J. M. White and C. D.

W. Wilkinsom, Proc. IEE, Vol. 64, No. 5, May 1967, pp 779-785, the number of resolvable spots, N is: N=tAf where t is the transit time of the acoustic wave and Af is the bandwidth due to the change in Bragg angle with frequency.

From this it appears that by making the beam wider in the y-direction one can increase resolution. However, the use of a wide beam increases transit time and reduced switching speed. There is a "trade off" between N and switching speed. N=450 has been achieved, which implies switching times of 2-3 microsecond order. Cross-talk may occur between poles of a multipole switch, but if digital information is conveyed by the optical beam relatively high cross talk levels (e.g. dB) are tolerable.

An N-pofe switch can be used as a demultiplexer or a multiplexer (i.e. a common channel can be split into N separate channels or N separate channels can be combined into one common channel). As will be seen it is of greater interest as a demultiplexer.

Fig. 2 represents a packet switch including an optical space switch based on the Bragg cell.

Incoming packets are buffered and stored in the Incoming Packet Store and are selected by the Control for modulating the light on the optical highway. The buffer stores are gallium arsenide shift registers, capable of working at 4.48 gb/s.

Packets are selected by the Bragg cell acting as an N-pole switch to be assigned to one out of N outgoing trunks or local links. The outgoing trunks carry the packets to distant packet exchanges and the local links are demultiplexed into a local outgoing packet store for transmission to the packet user.

By way of an example the following quantitative parameters will be assumed Link/trunk data rate =560 Mb/s Number of links/trunks =8 Data rate on optical highway =4.48 gb/s Full length user packet =128 octets (1024 bits) Packet envelope =8 octets Time to transmit packet =0.243 us Number of full length packets per sec =4x 1 06 Considerably more shorter length packets than the above could be handled but a limiting factor is the ability of the control to operate the switch fast enough. Packetised voice has been considered and if a packet length of 64 octets is assumed, this increases the number of packets handled per second to 7.5x106.

Video signals lend themselves to packetisation much more readily than speech because it is quite normal to store a frame of video information before transmission. Full length packets of 1 28 octets (or even larger) should be quite acceptable and it is envisaged that the transmission of video packets would be combined with some form of bandwidth compression. For example, if the video information were represented by a 10 mb/s digital signal, of the order of 400 video channels could be handled.

Claims (4)

1. A telecommunication switching centre, such as a packet switch, in which intelligence to be conveyed is modulated on to an optical beam using a suitable electro-optic, acoustic-optic or magneto optic device, and in which intelligence is switched between the trunks served by the switch by a multi-position switching device formed by an acousto-optical switch working in the Bragg regime.

2. A switching centre as claimed in claim 1, and in which each said modulator and each said acousto-optic switch is an integrated optic device, wherein an optical beam conveyed in an optical waveguide is modulated and then switched by an acoustic beam applied to the light beam substantially orthogonally from a surface acoustic wave transducer.

3. A packet switching centre for use in a telecommunications system, which includes an incoming packet store with a number of compartments one or more of which are allocated to one of a number of incoming trunks, an optical modulator to which the packets may be applied to be modulated on to a light beam, selection means for selecting under control of a control circuit which packets are to be modulated on to the beam, an optical switch for switching between the trunks served by the unit and arranged to route each packet modulated on to the light beam either to an outgoing optical trunk or to an outgoing packet store, a packet being routed to a compartment of an outgoing packet store if it is intended for a local trunk, and means to demodulate packets from the optical beam prior to their insertion in the respective compartments of the outgoing packet store, in which a light beam conveyed in an optical waveguide is modulated by an electric signal applied to its terminals, the modulation being effected by a suitable electro-electric, acousto-optic or magneto-optic device, in which the optical switch which effects the switching between the trunks is an integrated optic device working in the Bragg regime, wherein a light beam conveyed in an optical waveguide is switched between a number of outputs or inputs by an acoustic beam applied to that light beam substantially orthogonally from a surface acoustic wave transducer.

4. A packet switching centre substantially as described with reference to the accompanying drawings.