GB2376141A

GB2376141A - Generating a compressed optical data pulse

Info

Publication number: GB2376141A
Application number: GB0130435A
Authority: GB
Inventors: John Mansbridge
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 2001-03-16
Filing date: 2001-12-20
Publication date: 2002-12-04
Also published as: GB0106475D0; GB0130435D0

Abstract

A data compression device comprises a pulse generator (1) for generating a chirped laser pulse (300), a spatially dispersive element (306) for generating a plurality of spatially distributed outputs from the input chimed laser pulse, a modulator (400) to modulate digital data onto each output and an inverted spatially dispersive element (307) to recombine the data modulated outputs. A method of data compression comprises generating a chirped laser pulse (300), inputting the pulse to a spatially dispersive element (306), generating a plurality of spatially distributed outputs from the input pulse; modulating digital data onto the distributed outputs; and recombining the data modulated outputs in an inverted spatially dispersive element (307).

Description

DATA COMPRESSION DEVICE AND METHOD This invention relates to a data compression appropriate device and method, in particular for use in optical systems.

Features of an optical TDM switch core are described in WOO 1/10165 and WO01/86768. These applications relate to a system whereby a chirped pulse is modulated with data, compressed into a short pulse and then time multiplexed onto a single optical fibre. Individual compressed pulses are then selected and decompressed at each of the exit ports of the system. Typically, the chirped pulse is derived from a central source and is distributed to the various data modulators via an optical fibre.

A problem with the TDM system of the prior art is that it tends to have a high power consumption, takes up a lot of space and is expensive to manufacture because of the number of optical fibre connections between components.

In accordance with a first aspect of the present invention, a data compression device comprises pulse generating means for generating a chirped laser pulse, a spatially dispersive element for generating a plurality of spatially distributed outputs from the input chirped laser pulse; modulating means to modulate digital data onto each output and an inverted spatially dispersive element to recombine the data modulated outputs.

In accordance with a second aspect of the present invention, a method of data compression comprises generating a chirped laser pulse, inputting the pulse to a spatially dispersive element, generating a plurality of spatially distributed outputs from the input pulse; modulating digital data onto the distributed outputs; and recombining the data modulated outputs in an inverted spatially dispersive element.

The device and method of the present invention allows for integration of components and hence reductions in manufacturing cost and size. The use of spatial dispersion removes the need to use components which are designed for high serial data rates, so reducing costs and overall power consumption.

Preferably, the spatially dispersive element comprises dispersion means, a plurality of first optical waveguides, output means for outputting a dispersed signal and a plurality of waveguides, each waveguide conveying a predetermined wavelength range of the dispersed signal to a respective modulator in the modulating means.

The spatially dispersive elements reduce costs by avoiding the need to align fibres during manufacture and the absence of fibres makes the device more robust.

Preferably, the modulating means comprises one of electro-optic Mach-Zehnder modulators, electro-absorption modulators and modulated semiconductor optical amplifiers.

Preferably, the device is integrated on a single substrate.

Alternatively, the spatially dispersive elements are integrated on a single substrate and the modulating means are provided on a substrate recessed on the spatially dispersive element substrate. This enables different technologies to be used for the modulators, which may be incompatible with the spatially dispersive elements, but still having the benefits of ease of manufacture, due to the limited number of components which need to be aligned.

Preferably, the spatially dispersive element comprises an arrayed waveguide grating.

Alternatively, for non-integrated forms of the device, the dispersion means comprises one of prisms, diffraction gratings, interference filters and Fabry-Perot filters.

The spatially dispersive element and inverted spatially dispersive element may be embodied separately, but preferably the spatially dispersive element and inverted spatially dispersive element comprise a common element.

Preferably, the device further comprises one or more mirrors to reflect the data modulated outputs back through the common element.

Preferably, in the method of data compression the chirped laser pulse is input to dispersion means; wherein outputs from the dispersion means are input to a plurality of first optical waveguides of differing length to generate a spread spectrum; wherein a plurality of second optical waveguides convey different wavelength ranges of the spread spectrum to modulating means; wherein data is modulated onto a signal in each wavelength range at the modulating means; and wherein the data modulated signals are recombined in an inverted spatially dispersive element.

An example of a data compression device and method in accordance with the present invention will now be described with reference to the accompanying drawings in which :- Figure 1 shows an example of a prior art data compression apparatus;

Figure 2 illustrates a data compression device according to the present invention; Figure 3 shows the first spatial dispersive element of Fig. 2 in more detail; Figure 4 illustrates the effect of combining two of the spatial dispersive elements of Fig. 3; Figure 5 shows the device of Fig. 2 in more detail; and, Figure 6 is an example of the device of Fig. 2 with elements implemented on different substrates.

Conventionally, a chirped pulsed laser 1 is used to generate a pulse of light 2 long enough to carry many bits on a single pulse. The pulse is then applied to a modulator 3 that modulates the pulse with data. The pulse is then compressed 4 into a short period of time to enable it to be time multiplexed onto an optical backplane. This is illustrated in Fig. 1. Using this approach it is possible to multiplex very large amounts of data (typically terabits per second) onto a single fibre optic cable. This approach enables a system to be implemented with the current generation of discrete electro-optic components. Thus the laser is connected to a first dispersive element using a fibre optical cable. The dispersive element is typically implemented as Bragg fibre grating, which consists of a length of fibre (typically of the order of half a metre in length). This is then connected to the modulator by another length of fibre and this in turn is connected to a second dispersive element by a further length of fibre. The advantages of this approach are that the laser, dispersive elements and the modulator can be manufactured by different manufacturers and each component can be selected from the most appropriate manufacturer to give the required performance.

However, there are strong requirements to minimise the power consumed by each of the elements shown in Fig. 1. The electrical drive power required to operate the modulator at data rates in the region of 10 Gb/s to 40 Gb/s is relatively high. In addition, there is a need to reduce the number of components that must be interconnected by optical fibres. Doing so will reduce the manufacturing costs of the system and reduce the space required for it. One approach to reducing the number of fibres is to use optical integration techniques. These involve the use of techniques similar to integrated circuits where a device is made from a number of elements fabricated on a single substrate and linked by tracks that carry the electrical signal

between the elements. The optical equivalent involves optical elements fabricated on a single substrate and linked by optical waveguides fabricated onto the substrate.

Because there are no fibre interconnections, the need to align the fibres with the discrete devices is eliminated and the manufacturing costs are substantially reduced. A device implemented in this way is more robust since there are no fibres that must be protected.

An implementation of a data compression device according to the present invention requires a new optical architecture. This is illustrated in Fig. 2. A chirped laser pulse 1 is applied to an element 306 called a spatially dispersive element shown in Fig. 2. This is an element that splits a single pulse of light into a number of spatially distributed beams according to the wavelengths present in the pulse. This is typically implemented as an arrayed waveguide grating (AWG) as illustrated in Fig. 3.

A short optical pulse 300 is arranged to have a wide optical bandwidth. That is, it comprises a spectrum of wavelengths that is relatively wide. This is an inherent property of a short pulse, although the optical bandwidth can be arranged to be wider than the inherent bandwidth. The pulse enters a first section 301 of the AWG and the light spreads out over the input to a second section containing a number of optical waveguides 302. The waveguides are each connected to a third section 303 of the AWG and are arranged to be of differing lengths so that the phase of the light as it enters the third section causes the interference between the pulses that have travelled through the different length paths in the waveguides 302. At the output of the third section 303 the interference effects result in the spectrum of the short pulse being spread across the output face of the section. Waveguides 304 are arranged to collect separate parts of the spectrum. As a result, the waveguides 304 each contain a pulse 305 that is a part of the spectrum of the original short pulse. Because the optical bandwidth of each of the output pulses 305 is a fraction of the bandwidth of the original pulse, each pulse is spread out in time.

Applying these pulses to another AWG that is the identical to the first, but reversed in direction will result in the inverse process and the pulse will be substantially returned to its original state. This is illustrated in Fig. 4. The output of the first AWG 306 is connected to the input of a second inverted AWG 307. The output 308 of the pair of AWGs is a short pulse 309 that is similar to the original short pulse, but with some degree of modification due to the limitations and imperfections of the

AWGs. Thus using this approach it is possible to split the short pulse into a number of pulses at different wavelengths and then to recombine the pulses.

In the present invention a modulator 400 is inserted in each of the waveguides between the AWGs 306,307. This is illustrated in Fig. 5. The modulators are used to impose separate digital data onto each of the separate pulses. If the modulators are used to turn the data on and off, the resulting set of pulses 401 is illustrated. Other modulation formats could be used such as frequency, phase or combinations of any of these modulations. When the pulses are recombined on the output 308 of the second AWG the original pulse is reformed with modulation on it that contains the data modulated by the modulators 400. The data modulated onto the pulse will cause the pulse to be somewhat spread out in time, but it will still be short compared with the time domain multiplex time slots used to multiplex the data together.

The modulators 400 can be of a number of technologies including electro-optic Mach-Zehnder, electro-absorption or modulated Semiconductor Optical Amplifiers (SOA). An advantage of the present invention is that for a system with N modulators 400 the data rate of each modulator is reduced by a factor N over a simple serial system. This reduction in the data rate for the individual modulators has a number of benefits such as the ability to use a wider range of technologies (i. e. ones that cannot cope with the serial data rate). The lower data rate means that more efficient modulators and electronic drive circuits can be used, reducing the overall power consumption for the system, and a much greater degree of integration can be achievedthe two AWGs and modulators can be implemented on a single substrate.

Although a major advantage of this approach is that the entire device can be integrated on a single substrate, the system could be implement as separate devices connected together. Thus, a technology could be used for the modulators that may be incompatible with the AWGs. This is illustrated in Fig. 6. This shows the two AWGs on separate substrates 500,502 and the modulators on a further substrate 501. Although this looses some of the integration advantages of the fully integrated approach, it is still relatively simple to manufacture since it only involves the alignment of three parts. The alignment and registration of the individual waveguides is defined by the manufacturing processes, which can be reproduced to very fine tolerances. Clearly, there are other possibilities such as the two AWGs being on the same substrate and the

modulators being on a substrate that is dropped into a recess or notch in the AWG substrate.

Another implementation of the invention replaces the second AWG 307 with a single mirror or a set of mirrors 601 that reflect the light back through the modulator and back through the first AWG 306. The depth of modulation can remain unchanged, as compared with the other implementations described above, by applying half the modulation before the light is reflected and half as it passes back through the modulator. This approach works provided that the mirror is close to the modulator and the modulator has not changed state by the time the light passes through on its return.

Use of mirrors allows the AWG 306 to be used as both the first and second spatially dispersive device. The modulated pulse 309 now exits the AWG 306 in the opposite direction to the applied pulse 300. The input and output pulses can be separated using an optical circulator (not shown). The advantage of this approach is that the first and second spatial dispersion means are guaranteed to match and there is a significant saving in space and the requirement to provide two separate AWGs.

Technologies other than AWGs may be used to implement the spatial dispersion in non-integrated forms. These include prisms, diffraction gratings, interference filters and Fabry-Perot filters.

Claims

CLAIMS 1. Data compression device, the device comprising pulse generating means for generating a chirped laser pulse, a spatially dispersive element for generating a plurality of spatially distributed outputs from the input chirped laser pulse; modulating means to modulate digital data onto each output and an inverted spatially dispersive element to recombine the data modulated outputs.
2. A device according to claim 1, wherein the spatially dispersive element comprises dispersion means, a plurality of first optical waveguides, output means for outputting a dispersed signal and a plurality of waveguides, each waveguide conveying a predetermined wavelength range of the dispersed signal to a respective modulator in the modulating means.
3. A device according to claim 1 or claim 2, wherein the modulating means comprises one of electro-optic Mach-Zehnder modulators, electro-absorption modulators and modulated semiconductor optical amplifiers.
4. A device according to any preceding claim integrated on a single substrate.
5. A device according to any of claims 1 to 3, wherein the spatially dispersive elements are integrated on a single substrate.
6. A device according to claim 5, wherein the modulating means are provided on a substrate recessed on the spatially dispersive element substrate.
7. A device according to any preceding claim, wherein the spatially dispersive element comprises an arrayed waveguide grating.
8. A device according to at least claim 2, wherein the dispersion means comprises one of prisms, diffraction gratings, interference filters and Fabry-Perot filters.
9. A device according to any preceding claim, wherein the spatially dispersive element and the inverted spatially dispersive element comprise a common element.

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10. A device according to claim 9, further comprising one or more mirrors to reflect the data modulated outputs back through the common element.
11. A method of data compression, the method comprising generating a chirped laser pulse, inputting the pulse to a spatially dispersive element, generating a plurality of spatially distributed outputs from the input pulse; modulating digital data onto the distributed outputs; and recombining the data modulated outputs in an inverted spatially dispersive element.
12. A method according to claim 11, wherein the chirped laser pulse is input to dispersion means; wherein outputs from the dispersion means are input to a plurality of first optical waveguides of differing length to generate a spread spectrum; who : in a plurality of second optical waveguides convey different wavelength ranges of the spread spectrum to modulating means; wherein data is modulated onto a signal in each wavelength range at the modulating means; and wherein the data modulated signals are recombined in an inverted spatially dispersive element.
13. A data compression device as hereinbefore described with reference to Figs. 2 to 7.
14. A method of data compression as hereinbefore described with reference to Figs. 2 to 7.