GB2260666A

GB2260666A - Time division multiplexed diode lasers

Info

Publication number: GB2260666A
Application number: GB9120119A
Authority: GB
Inventors: Paul May
Original assignee: Sharp Corp
Current assignee: Sharp Corp
Priority date: 1991-09-20
Filing date: 1991-09-20
Publication date: 1993-04-21
Anticipated expiration: 2011-09-20
Also published as: JP2918010B2; JPH05206584A; GB9120119D0; GB2260666B

Abstract

An array of diode lasers generates a pulsed output at substantially the same frequency in response to an input signal having substantially that same frequency. A signal generator is connected to each of the lasers such that the distance travelled by input signals to each laser is different in order to allocate respective time slots to the output pulses. Applications to optical communication, optical parallel processing and optical sampling by the control of adjacent areas of a photocathode in an electron microscope or electron beam lithography in different time slots are disclosed. <IMAGE>

Description

TIME DIVISION #WtTIPLEXED DIODE LASERS This invention relates to time division multiplexed diode lasers and to their various applications in, amongst others, the fields of optical communications, optical parallel processing, optical sampling, and electron beam emission from a cathode source.

In time division multiplexing, a plurality of pulsed output signals of the same frequency are provided each having a different respective time slot. In conventional electronic signal time division multiplexing electronic means are used to introduce the different time slots to a plurality of pulsed output signals. However, to date, in optical transmission systems time division multiplexing has been accomplished by splitting a single source output pulse and introducing a time delay to respective branches of the split sources through the optical waveguides used to transmit the split signal. This technique is somewhat cumbersome.

Diode lasers provide a compact laser source suitable for a wide range of applications. It is known that, as well as the small signal frequency response of diode lasers, there is a resonance frequency that arises as a result of the dimensions of the diode cavity. This occurs at much higher frequencies. A diode laser operated so as to provide such a pulsed beam is known as a mode-locked diode laser. Figure 1 provides a schematic representation of the signal response of a diode laser of cavity length 4mm. As can be seen the small signal response falls off at a few GHz. There is, however, a resonance peak at around 10GHz. The frequency of the resonance peak is related to the round-trip distance of the laser cavity and is, accordingly dependent on the length of that cavity.

Monolithic mode-locked diode lasers have been developed recently through the use of split-contact Fabry-Perot lasers allowing modulation at this optical resonance frequency. Figure 5 illustrates in section and side perspective a typical construction of one material system for a double heterostructure diode laser 70. An n±GaAs substrate 72 is formed on one side thereof with an n-ohmic contact 74. On the other side of the substrate is a laminated structure comprising, in order, a n-AlGaAs first cladding layer 76, a GaAs active layer 78 and a p-AlGaAs second cladding layer 80. Over the top of the second cladding layer is formed a p-ohmic contact 82.As can be seen, the second cladding layer is formed with a longitudinal central ridge 84 which serves to localise the light generation to a central zone 86 in the active layer, and to encourage the light to propagate in the longitudinal direction X-X rather than to spread out in the whole of the plane of the active layer.

Other structures such as a separate confinement heterostructure are known. In separate confinement heterostructures there are two different cladding layers, one for optical and one for electrical confinement. In such structures the active region can be a single or multiple quantum well structure.

The p-ohmic contact of the illustrated laser diode 82 is split into a first, central portion 82a which defines a central gain region, a second portion 82b at one end which defines a RF modulation region, and third portion 82c at the other end which defines a saturable absorber region. In normal use of such a laser diode, a suitable dc bias is applied to the central contact portion 82a to cause laser light generation in the zone 86 of the central region, an RF pulsed signal is applied to the second contact portion 82b to pulse the laser at the desired sub-carrier frequency, and the signal to be modulated onto the sub-carrier is applied in the form of negative bias inhibiting pulses to the third contact portion 82c.

Typically the second contract will also be dc biased to lower the threshold of the laser cavity, and to ensure that the series resistance of the section of the diode is low (ie. that the electrical diode is switched on).

In some cases the laser contacts can be split in different ways. For example, for passive mode locking or active mode locking without modulation, two split contacts may be sufficient.

To actively mode-lock a diode laser such as that described above, a radio frequency signal is applied at the optical resonance frequency of the diode laser cavity to the split contact portion 82c of the diode laser.

Mode-locked diode lasers encouraged to pulse in this manner using an rf drive signal are known as actively mode-locked diode lasers.

Passive mode-locking on the other hand is a technique which provides a pulsed output from the laser without application of such radio frequency input signal.

To produce an output pulse in passive mode-locking the energy band gap at the pn junction across the active layer is reduced in the saturable absorber region by the application of a reverse bias. By reducing the energy band gap, more energy levels are available to absorb photons generated in the gain region of the laser. At low light intensities upper state (conduction band) energy levels are not fully populated by excited electrons, and all the light is absorbed. At high intensities these upper state levels become fully populated and the absorption is saturated - i.e. further photons pass through the material without absorption until these upper states become depopulated. The mechanism for depopulation in these structures is dominated by drift in the reverse bias field to the contacts, and not by electron-hole recombination.The former process is characterised by the drift velocity Vd=lLE where E is the field strength (Vcm-1), and p the mobility in cm2/V.s. For reasonable bias voltages (2V, say), and material thicknesses (across 2 pm) and using IL = 8500 cm2/V.s. for electrons in GaAs we get a value for Vd of 8.5 107 cm/s. This is greater than the saturation velocity which for this field strength is about 107 cm/s. The corners are swept out of the 2 Am region within 10 ps. Therefore, as long as the repetition rate of the mode locking pulses is less than 100 GHz the saturable absorber will recover. For optical guiding regions of 0.5 Am thickness (about the minimum to contain the mode), higher repetition rates are possible.

Because of absorbtion of low intensity pulses, a more stable output is achieved if a large reverse bias is provided at the saturable absorber. The greater the reverse bias, however, the more laser power is absorbed and the weaker the intensity of the pulse.

There is accordingly a trade-off between intensity and stability in the prior art passively mode-locked diode lasers.

It is only recently that a stable output has been achieved in passively mode-locked monolithic diode lasers. A technique known as Colliding Pulse Modelocking (CPM) has been used. (see M.C. Wu et al, IEDM 1990 pp 137-139). In this technique the saturable absorber of a monolithic diode laser is placed in the middle of the diode cavity to improve pulse shaping when two counter propagating pulses are coincident in this region.

Passively and actively mode-locked diode lasers generate an output pulse at the resonance frequency of the laser diode cavity. An external cavity arrangement can be used to provide a pulsed output at lower frequencies i.e. around 1 GHz. The pulse is typically of around lOps duration. One facet of the laser is antireflection coated, and an external mirror is used to define the cavity. A lens in a focal arrangement can be used to ensure that no mixing of light occurs from one laser stripe to the next of an array. Repetition rates of order 10 GHz and less are now possible.

Another technique used to obtain a pulsed output from a diode laser is known as gain-switching in which a diode laser is stimulated to emit by a very high powered, high frequency pulse. A short electrical pulse excitation introduced to the diode laser creates carrier inversion and causes stimulated emission to occur. Such a pulse can be generated by the application of a large amplitude sinusoidal voltage to a step recovery diode. The output pulses from the step recovery diode are generated at the frequency of the original sinusoidal signal. The output pulses typically have a duration of the order 50 - lOOps.

These pulses are then applied directly to a diode laser optimally biased to just below the lasing threshold. The diode laser will typically emit pulses of duration of the order of 20ps at the repetition rate of the input signal. There is no dependence on cavity dimension as there is with mode-locked diode lasers which allows greater flexibility in the frequency at which a particular diode laser can pulse. However, the upper limit for diode lasers pulsed in this manner are frequencies of the order of 1 GHz. When high frequency or narrow pulse width are not essential, pulsed lasers formed in this manner can provide adequate optical signals.

One particular field in which laser diodes can be employed is optical communications. Optical communication systems are extensively used because of their comparatively wide bandwidth, small size and insensitivity to electrical interference. Figure 6 illustrates in schematic form the basic components necessary for an optical communication system. The arrangement shown provides one-way data communication from a transmitter A to a receiver B. The transmitter A receives on input line 50 an electrical input signal carrying the information to be transmitted, and includes a suitable electrical driver circuit 52 for deriving from the input electrical signal, driver signals for driving a diode laser 54.

The diode laser produces light which is modulated in accordance with the information, and which is coupled into one end of an optical fibre transmission line 56 through a first optical connector 58. The light travels along the transmission line 56 to the receiver B where it is applied, through a second optical connector 60, to a suitable photodetector 62 and thereby converted to an electrical signal. A receiver circuit 64 processes this signal and performs the required steps of demodulation etc to produce an output signal on output line 66 carrying the transmitted information in the required form.

Although the bandwidths currently achievable in such systems are sufficient for voice transmission in telephone networks, and for data transmission, for example machine-to-machine communication in Local Area Networks (LANs), there is increasing interest in the potential of optical communication systems to integrate voice, data and video transmissions in local loop subscriber networks such as Broadband Integrated Services Digital Networks (B-ISDNs). To enable such systems to compete with conventional cable television transmission systems that have bandwidths of the order of GHz, it is desirable to provide bandwidths in excess of 10 GHz and preferably as high as 100 GHz.

Data bandwidth requirements for local area networks are also likely to increase, especially for applications that require image transfers. Bandwidths in excess of 10 GHz may thus be necessary for this application as well.

Some suggested prior art optical transmission systems have utilised laser diodes operating in the small signal regime. In such systems the bandwidth is limited by the fall-off in the sub-carrier frequency response of the diode laser used to provide the optical signal. Time division multiplexing in the small signal regime has been used to provide bandwidths of the order of gigahertz. (See US patent 4,953,156).

Higher bandwidth systems have been proposed that provide an array of lasers of different wavelengths.

These are then multiplexed through use of a grating or holographic element. To produce stable wavelengths, however, special grating/laser structures have to be used, for example, Distributed, Feedback (DFB) and Distributed Bragg Reflector (DBR) which are expensive to fabricate. It is desirable, therefore, to provide a higher bandwidth optical communications system that is simple to fabricate.

If time division multiplexing is required at low repetition rates, but with similar time slot durations, an external cavity arrangement can be used.

One facet of the laser array is antireflection coated, and an external mirror is used to define the cavity.

A lens in a focal arrangement ensures that no mixing of light from one laser stripe to the next occurs.

Repetition rates of order 1 GHz and less are now possible. To fully utilise the bandwidth with, say, 10 ps time slots 100 independent lasers would be required.

A second field in which lasers are used is optical sampling. Generally in optical sampling, changes in an incident beam are monitored to provide information on a phenomenon under scrutiny.

Optical sampling provides a non-perturbative means by which electric fields and other phenomena can be probed. It is particularly desirable industrially to be able to probe integrated circuits with high temporal and spatial resolution. External electrooptical probing provides such a non-perturbative means.

Figure 9 illustrates one type of optical sampler that uses electro-optic interactions to build up a picture of waveforms on a circuit wafer. The electro-optical sampler 90 probes a circuit wafer 92. The illustrated probe 94 includes an extremely small electro-optic crystal 96 as a proximity electric field sensor. The technique used to determine the waveform of the field under consideration exploits the fact that a fringing field exists above the surface of a two-dimensional circuit between metalization lines at different potentials.

The electro-optic crystal 96 is birefringent, and 'dipping' the tip of the crystal 96 into an electric field causes the birefringence to change. This change is measured by directing an optical beam 98 through the tip so that it is reflected at its surface 100 and detecting the polarization change in the reflected beam 102 using a polarizer/detector arrangement. The field probed will vary in time and space. By using a laser beam as the optical beam reflected at the probe surface, spatial resolution of about 5 micrometers can be achieved.

Probes such as that described above, are used to build a picture of an entire waveform having a repetitive cycle, by serially sampling the field at a single point in space at different temporal positions of the waveform. To capture an entire waveform it must be sampled at a number of different temporal positions.

The resolution of the picture formed of a captured waveform will depend upon the accuracy of a signal determined for individual points of the waveform, and the number of points sampled. The accuracy of individual points, will be influenced by the duration of the pulse used for sampling, as well as by any noise present in the signal. The length of the pulse is a matter for design of the probe. However, the signal to noise ratio can be improved by taking a number of samples of the same point of a waveform.

The number of sample points required to accurately capture the waveform, will therefore be the number of times each point is sampled (N) multiplied by the number of different points (P) of the waveform sampled. As the optical sampling is serial, the time taken to capture the waveform with the desired resolution will be the repetition time of the pulse cycle multiplied by the number of samples taken (NxP).

As the waveform may fluctuate, excessive sample times may not substantially improve the resolution achieved.

If a number of samples of a given point on the waveform are required to improve the signal to noise ratio, the pulsing frequency of the beam must be synchronised with the frequency of the waveform to ensure that exactly the same point of the waveform is sampled each time. To subsequently sample different points of the waveform, a phase delay is introduced to the pulsed beam by, for example, physically altering the mirror directing the beam to the probe surface or, by introducing a delay by a motor driven optical delay line, or by purely electronic means.

With serial sampling, in order to build up a complete picture of a captured waveform, it is necessary for the waveform to repeat. Otherwise, only a single point on the waveform could be determined.

If the waveform being sampled is repetitive and there is no restriction on the sample time to produce a full picture of the waveform, ie fluctuations in the waveform can be ignored, or only low resolution is required, serial optical sampling techniques may be perfectly adequate. However, when there is not unlimited time in which to collect data and high resolution is required, or when the signal to noise ratio is poor, a faster mode of optical sampling may be desirable. It is, however, generally desirable to provide a faster means of sampling. In addition, a meaningful picture of a non-repetitive waveform cannot be captured using the serial sampling techniques at pulsing frequencies currently attainable. It is desirable, therefore to provide an optical sampling technique that allows a waveform to be sampled at a faster rate.

Other devices using optical sampling techniques such as electron microscopes suffer similarly from the slow acquisition of data that results from the serial sampling.

A third field of interest is optical lithography.

Optical lithography is the lithographic technique of first choice in the generation of patterned layers of semiconductor chips and the like as it readily lends itself to automation and repetition on an industrial scale. Optical lithography techniques generally require the generation of a mask for positioning between a light source and an optically sensitive material so that the optically sensitive layer becomes opaque and transparent in accordance with the pattern on the mask. The silicon layer can then be etched to provide a replica of the mask. As semiconductor chips become smaller and smaller, however, diffraction of optical beams passing through the mask causes them to diverge and reduces the accuracy of this method of reproduction. Under these circumstances either electron beam or X-ray lithographic techniques must be used to produce patterned silicon wafers.

X-rays have a shorter wavelength than visible light and are stopped by a mask formed of suitable material for example, a metal sheet. They can, therefore be used in a manner similar to that in which optical beams have been used in the past. However, the wavelength of such rays is still limited and diffraction will inevitably become a problem as chips having even smaller dimensions will no doubt be required in the future. X-ray lithography is extremely expensive and as it can only be a temporary solution to the problem of the limit of the applicability of optical lithography having been reached, it is important to find an alternative.

One alternative to X-ray lithography is electron beam lithography. Electrons have extremely short wavelengths and can be generated relatively cheaply.

However, electron beams can not be selectively inhibited by conventional mask materials as such materials tend to be opaque to electrons and suitable materials transparent to electrons for forming a mask are not available. Accordingly electron beams must write a pattern directly onto a wafer coated with electron resist such as PMMA. This creates problems when reproduction of a wafer pattern on a commercial scale is required. To date electron beams have been used industrially only in the one-off production of patterned wafers. This is because a single electron beam scanning the surface of a wafer is generally used to produce the pattern. Single electron beams, although slow are preferred as problems would be encountered if an array of electron beams were to be used to produce the pattern.The negative charge of a beam of electrons leads to divergence as a result of the Coulomb force when two such beams are in the close proximity required for utilisation in an array for the production of a patterned wafer.

When a single electron beam is used to write an entire pattern, a system such as that illustrated in Figure 10 is used. An electron source 110 provides a continuous beam of electrons which is directed into a vacuum chamber 112 and focussed and directed by an electron lens system 114. The beam direction is controlled by the lens system 114 and writes a pattern onto a wafer 116 coated with electron beam resist 118 located within the vacuum chamber 112. A scanning lens system 115 can be controlled by a computer to direct the beam. The computer can accordingly be programmed to produce a desired pattern. This system, although ideal for the generation of prototypes or individual customised chips, is very slow and would be prohibitively expensive for industrial application.

Electron beam lithography encounters a second problem that results in a reduced power output beam. As electrons are released continuously from the electron emitting surface there is inevitably a cloud of electrons at the surface of the electron emitter.

The Coulomb space charge effects are high and there is a barrier to further emission of electrons. The power of the emitted beam is therefore reduced.

There is, accordingly, a need for an improved lithographic technique suitable for producing small silicon chips and readily utilisable in large scale industrial applications.

The Coulomb interactions experienced between adjacent beams of electrons limits the use of electron beams in applications other than electron beam lithography.

For example, problems have to data been encountered in focussing a large number of electron beams to a single point to provide a high intensity electron beam that can be used for any number of applications including types of scanning and display apparatus. Problems are also encountered in electron microscopy where a single beam of electrons is used to scan because of the Coulomb interactions introduced if an array of beams is used to build up a picture of a surface more quickly.

There is accordingly a need for an improved electron beam emitter for use in a number of applications.

Optical sources also find utility in the field of optical processing. Typically in such systems emission from laser diodes is stimulated by electrical signals in accordance with stored data. With the desire for ever faster processors, parallel processing, that is, processing in which more than one event occurs during a clock cycle of the processor, is becoming increasingly desirable.

Optical processing enables vast increases in processing speeds to be contemplated and with parallel processing the speed at which functions can be performed on data can be further boosted.

A typical parallel processor will comprise an array of light sources coupled through a lens system to a modulator positioned in the image plane of the lens system. The modulator takes the form of a twodimensional array and performs a plurality of simultaneous operations on the plurality of outputs of the array of light sources. The light source outputs resulting from the operations performed on the laser beams input to the modulator, are coupled through a second lens system to a detector array. The bulkiness of the system can cause difficulties with design of apparatus and prevents optical parallel processors from being compact.

It is an aim of the present invention to alleviate at least some of the above-mentioned problems.

It is one aim of the present invention to alleviate at least some of the problems associated with prior art optical transmission systems.

It is another aim of the present invention to alleviate at least some of the problems associated with prior art optical sampling techniques.

It is another aim of the present invention to alleviate at least some of the problems associated with electron beam generation for application in, amongst others, the field of optical lithography.

It is another aim of the present invention to provide an improved optical parallel processor.

In accordance with a first aspect of the present invention there is provided an array of lasers comprising a plurality of diode lasers for generating a pulsed output at substantially the same frequency in response to an input signal having substantially that same frequency, means for providing an input signal to each of the plurality of diode lasers, and means for connecting the signal providing means to each of the plurality of diode lasers such that the distance travelled by input signals between the signal providing means and respective ones of the plurality of diode lasers is different for the allocation of a respective time slot to the output pulses from respective diode lasers within the pulsing frequency cycle of the pulsed outputs.

In one preferred implementation, the plurality of diode lasers comprise a plurality of mode-locked diode lasers. The input signal may then be a substantially sinusoidal radio frequency (rf) signal. The frequency of the pulsed outputs may be the resonating frequency of the diode cavity, or an external cavity may be provided in which case the frequency of the pulsed output will be the resonating frequency of the external cavity. The frequency of the rf input signal under these circumstances is substantially that of the resonating frequency of the external cavity.

In another preferred implementation the plurality of diode lasers comprise a plurality of gain-switched diode lasers. The input signal may then be a short electrical pulse excitation. The frequency of the pulsed output in this case is determined by the frequency of the input signal.

The pulse duration of the output of a pulsed output beam from a mode-locked diode laser is typically less than 1/10 of the cycle time. For typical photon lifetimes related to the number of round trips in a cavity before the particular photons in a pulse are emitted, the difference in frequency between the radio frequency input signal and the resonance of the diode laser can be of order 1%. Reference to 'substantially the same', 'substantially that of', and 'substantially' should be interpreted accordingly.

In a first preferred embodiment of the present invention the array of diode lasers is provided in an optical communications system, in which means are provided for modulating the output pulses of each of the plurality of diode lasers to carry information for transmission in an optical communications system.

In this embodiment, the array of lasers is preferably an array of actively mode-locked diode lasers and the means for supplying a signal is a rf source. Lines connecting the rf source to respective ones of the diode lasers are of different lengths to introduce a relative phase difference to the generated outputs from respective ones of the diode lasers.

In a second preferred embodiment of the present invention the array of diode lasers is arranged to provide a plurality of pulsed outputs for use with an optical sampler. Such a sampler can be used to probe, for example dynamic physical (e.g. carrier in semiconductors), electrical and optical phenomena.

The optical sampler preferably comprises means for directing each of the laser beams output from the array of diode lasers to probe a phenomenon under examination, and means for detecting changes in the characteristics of each of the laser beams in response to characteristics of the probed phenomenon, thereby obtaining information on the phenomenon at each of the time slots allocated to respective ones of the laser diodes.

The optical sampler may be an electro-optical sampler wherein the means for detecting changes comprises a birefringent crystal and means for detecting changes in polarization of laser beams reflected from the surface of the crystal in the presence of an electrical field.

The means for detecting is preferably a plurality of detectors each receiving the output pulse from a respective one of the plurality of diode lasers.

In a third preferred embodiment of the present invention the array is arranged to provide means for stimulating electron emission from a photocathode.

The allocation of different time slots to laser beams output from respective ones of the diode lasers means that adjacent beams of electrons emitted from the photcathode are displaced from one another in time.

There is accordingly no divergence of electron beams emitted from the photocathode as a result of coulomb interaction between electron beams. The pulsed outputs also allow higher power emission from photocathode to be possible.

One suitable material preferred for formation of the photocathode is a commercially available material S-l which is a complex alloy of caesium, oxygen, palladium and silver.

The apparatus for stimulating electron emission from a photocathode can be used in electron beam lithography.

The apparatus for electron beam lithography preferably comprises a mask for selectively preventing passage of light, positioned between the array of lasers and the photocathode so that electron emission is encouraged from the photocathode in accordance with the pattern on the mask.

The means for stimulating electron emission may be used in an electron microscope. The apparatus for an electron microscope preferably comprises means for detecting electrons scattered from a surface under investigation.

In a fourth preferred embodiment the array is arranged to provide signals for modulation in an electrooptical or optical processor.

In accordance with a second aspect of the present invention there is provided an optical transmission system comprising, a plurality of actively mode-locked diode lasers each adapted for generating a pulsed output at substantially the same resonance frequency determined by the dimensions of the cavities of respective diode lasers, and means for allocating a respective time slot within a resonance frequency cycle to pulses from respective diode lasers for transmission.

By utilising the high frequency pulses generated by the diode lasers to carry information, a large bandwidth signal for optical transmission is provided.

By time division multiplexing several such signals, the bandwidth is correspondingly increased. This aspect of the present invention resides at least partly in the recognition of this utility of diode lasers.

Preferably the means for allocating respective time slots comprises means for providing a signal with a frequency substantially equal to the resonance frequency of the diode lasers to one of the contacts of each of the plurality of mode-locked diode lasers, the phase of the input signal being different at each respective diode laser.

The phase of the input signal at each of the plurality of actively mode-locked diode lasers is varied by varying the length of the rf feed line from a rf frequency source to respective ones of the diode lasers of the array.

Preferably the mode-locked diode lasers are actively mode-locked monolithic diode lasers having a split cavity contact. The metal contact of the monolithic laser diode is split into two or three contacts.

Preferably a dc bias is applied to the longest contact for application to the gain region of the diode laser and a rf clock pulse signal with frequency substantially equal to the resonance frequency of the diode laser is applied to a second contact.

The quality of the mode-locking can be improved by application of a reverse bias to a third of the split contacts for application to the saturable absorber region. Information is preferably imposed on the output of each laser by selectively placing a suppressing voltage on the third contact, to remove the output pulse and deliver a "O". The suppressing voltage is removed to deliver a "1".

The rate at which a signal can be applied determines the bandwidth of the system. Because of the inertia in the system, at 10GHz it is unlikely that pulse suppression will be able to occur at the pulse frequency. The system is likely to require ten or so cycles before a pulse is suppressed. With the high frequencies possible using mode-locked diode lasers, the bandwidth reduction caused by the inertia in the system is compensated for, to at least some extent, by the increase in channel frequency. By increasing the number of time multiplexed channels the band width can be increased still further.If threshold detection is used (ie. in a digital system in which the decision as to whether to characterise a bit as 1 or O is based on whether it is below or above a particular threshold), then higher bandwidths will be possible as complete suppression of the pulses is not necessary.

Other means of applying a signal to each of the time division multiplexed carriers can be utilised instead of signal suppression.

In accordance with a third aspect of the present invention, there is provided an optical sampling apparatus comprising a plurality of diode lasers for generating a pulsed output at substantially the same frequency in response to an input signal having substantially that same frequency, each of the laser beams output from the plurality of diode lasers being directed to probe a phenomenon under examination, means for providing an input signal to each of the plurality of diode lasers, different time slots being allocated to the output pulses from respective diode lasers within the pulsing frequency cycle of the pulsed outputs, and means for detecting changes in the characteristics of each of the laser beams in response to characteristics of the probed phenomenon, thereby obtaining information on the phenomenon at each of the time slots allocated to respective ones of the laser diodes.

Preferably the means for providing an input signal to each of the diode lasers comprises means for connecting a signal source to each of the plurality of diode lasers such that the distance travelled by the input signals between the source and respective ones of the diode lasers is different.

In accordance with a fourth aspect of the invention there is provided, an electron beam emission apparatus comprising a cathode, and means for stimulating pulsed emission from a plurality of areas of the cathode such that the pulsed emission of electrons from respective ones of the plurality of areas of the cathode close enough to experience Coulomb interactions occupy different respective time slots.

The cathode is preferably a photocathode and the means for stimulating pulsed emission comprises at least one pulsed laser source.

In a first preferred embodiment of this fourth aspect, the at least one pulsed laser source comprises a plurality of diode lasers for generating a pulsed output at substantially the same frequency in response to an input signal having substantially that same frequency, and means for providing an input signal to each of the plurality of diode lasers, and means for connecting the signal providing means to each of the plurality of diode lasers such that the distance travelled by input signals between the signal providing means and respective ones of the plurality of diode lasers is different for the allocation of a respective time slot to the output pulses from respective diode lasers within the pulsing frequency cycle of the pulsed outputs.

In one preferred implementation, the plurality of diode lasers comprise a plurality of mode-locked diode lasers. The input signal may then be a substantially sinusoidal radio frequency (rf) signal. The frequency of the pulsed outputs may be the resonating frequency of the diode cavity, or an external cavity may be provided in which case the frequency of the pulsed output will be the resonating frequency of the external cavity. The frequency of the rf input signal under these circumstances is substantially that of the resonating frequency of the diode laser.

In another preferred implementation the at least one pulsed laser source comprises a plurality of gainswitched diode lasers. The input signal may then be a short electrical pulse excitation. The frequency of the pulsed output in this case is determined by the frequency of the input signal.

In a second preferred embodiment of the fourth aspect the at least one pulsed laser source plurality of comprises a plurality of independent passively modelocked diode lasers. As passively mode-locked monolithic diode lasers typically have a pulsed frequency of around 10 to 300 GHz (depending on cavity length) and a pulse duration of typically less than 1/10 of the round trip time), provided there is no linkage between the time slots allocated to adjacent passively mode-locked diode lasers, the random nature of the time slot distribution across the array will result in emission of electrons from areas of the photocathode liable to experience coulomb interactions occurring at different times.

In a third preferred embodiment, the at least one pulsed laser source is scattered by being passed through a diffuse screen. The screen splits the beam into a plurality of pulsed output beams spaced from one another both spatially and temporally. The pulse delays introduced by the diffuse screen introduce a random element to the plurality of pulsed outputs generated and emission from the photocathode will be random. Such a system will operate in a manner similar to that desired above in relation to the plurality of passively mode-locked diode lasers.

An electron emission apparatus of the type described above can further comprise means for selectively inhibiting emission of electrons from the photocathode in accordance with a described pattern for inclusion in an electron beam lithography apparatus.

The means for selectively inhibiting emission of electrons may then be an optical mask positioned between the array of lasers and the photocathode.

The means for selectively inhibiting emission may alternatively be means for selectively inhibiting light emission from lasers of the array in accordance with a desired pattern.

An electron beam apparatus of the type described above may further comprise means for directing the electrons onto a surface and detecting electrons transmitted or reflected by that surface for inclusion in an electron microscope arrangement.

The electron beam emission apparatus of the types described above may also further comprise means for focussing the pulsed emissions from the plurality of areas of the photocathode to provide a single high intensity beam.

In accordance with the fifth aspect of the present invention there is provided an electron beam lithography apparatus comprising a photocathode, means for stimulating pulsed emission of electrons from the photocathode and means for controlling the emission of electrons from the photocathode in accordance with a desired pattern.

The invention will now be described in greater detail, by way of example only, with reference to Figures 1 13 of the accompanying drawings of which: Figure 1 is a schematic representation of the frequency response of a diode laser with a diode cavity length of around 4mm; Figure 2 is a schematic representation of an array of actively mode-locked monolithic laser diodes; Figure 3 is a section through a microstrip line suitable for providing a radio frequency signal to each of the plurality of diode lasers of the array of Figure 2; Figure 4 is a schematic plan view of a plurality of microstrip lines carrying a rf signal to a plurality of diode lasers; Figure 5 is a section and side perspective of one type of diode laser; Figure 6 is a schematic representation of the basic components of an optical communication system;; Figure 7 is a schematic representation of the optical communication system of a second aspect of the present invention; Figure 8 is a schematic representation of a digital signal that may be carried by the system of Figure 7; Figure 9 is a schematic representation of known optical sampler; Figure 10 is a schematic representation of a known apparatus for electron beam lithography; Figure 11 is a schematic representation of an optical sampler in accordance with a third aspect of the present invention; Figure 12 is a schematic representation of an apparatus for electron beam lithography in accordance with a fourth aspect of the invention; and Figure 13 is a schematic representation of an optical parallel processor according to a fifth aspect of the present invention.

Several embodiments of the present invention will be described below. Each operates through the utilisation of an array of diode lasers arranged to produce pulses at the same frequency but with different phases. Figures 2 to 4 illustrate an array of diode lasers and means for supplying a rf signal to each of the diode lasers of the array so that the phase of the signal differs when it reaches respective diode lasers of the array in accordance with the present invention. This embodiment of the invention illustrated in Figures 2 to 4 is suitable for use with any of the other embodiments of the invention described below although other arrangements may be used in any of the embodiments described.

Each of the lasers 10 of the array 12 depicted in Figure 2 is a monolithic mode-locked diode laser having a split contact divided into three parts 14, 16, 18. Each diode laser has a saturable absorber and a gain region. A rf signal is input to the laser 10 through the first contact 14. A positive bias is applied to the gain region through the second, longer, contact 16. Information can be superimposed on the output pulses from the diode lasers by altering the voltage supplied to the saturable absorber by way of the third contact 18.

It is important to the operation of the present embodiment of the invention that the feed lines from a rf source to respective ones of the plurality of diodes 10 of the array differ in length so that the output pulses excited from the laser differ in phase.

In Figure 2 this is illustrated as a time delay on the rf signal of lOps between adjacent diode lasers of the array. Figure 3 is an illustration of a microstrip line 20 suitable for providing a rf signal to the diode lasers 10 of the array. The microstrip line 20 comprises a metal back plane 22, a substrate 24, formed for example from GaAs, a laser region 26 and a dielectric 28. A metal line 30 is located on the surface of the dielectric 28. The metal line 30 sees the back metal contact as a conducting back plane and the substrate acts as a dielectric.Typically the effective dielectric constant will be about three and the speed of an electrical pulse along the metal line 30 will, accordingly be around lO10cm/s. In order to generate a time delay of the order of lOps between the outputs of adjacent lasers a difference in the line length from the rf source for adjacent lasers will be around lmm.

Figure 4 illustrates one feed line configuration that can be adopted to provide the requisite difference in line length from the rf source to four adjacent diode lasers that may form part of a larger array. A rf input line 32 is provided that branches into four subbranches 34(a)-(d). Each of the sub-branches 34(a) (d) differs in length from an adjacent branch by lmm.

This arrangement is particularly advantageous when insitu facet etching techniques are used to provide the contacts of the diode lasers. The microstrips 30 can then be etched to have a sinusoidal configuration, the amplitude of the sinusoidal path of each microstrip line 30 being determined by the length of the path required. The lengths of lines AE, BE, CE, and DE differ by lmm. AE, the longest contact differs in length from AD the shortest contact by 3mm. A quarter wavelength coupling section is provided at point E for impedance matching of the rf input and the four laser feeds to ensure no reflections at E.

If instead of in situ facet techniques, for example, cleaving is used to define the laser facets, a similar configuration of the feed lines to the respective diode lasers can be used and wire bonds can provide the connection from the end of the rf line to the shorter section of the respective diode lasers.

A second embodiment of the present invention will be described with reference to Figures 7 and 8 of the drawings. The array of diode lasers 12 depicted in Figure 7 is similar to that illustrated in Figure 2 and described in relation to that drawing. Like numerals designate like parts. The array 12 of Figure 7 is, however, arranged to provide a plurality of data carrying sub-carriers for use in an optical transmission system. Information is imposed on the output of each laser by application of a large negative voltage, typically between -2 and -5V on the third contact 18. The output signal is a 'zero' when a negative suppressing voltage is applied to the third contact and a 'one' if the suppressing voltage is removed. The rate at which this signal can be applied determines the overall data rate for the structure.

Figure 8 shows a digital signal that can be carried by the system of Figure 2.

A single rf source (not shown) is used to provide the rf signal to each of the diode lasers 10 of the array 12. The signal lines 20 from the rf clock source to the lasers 10 of the array are of different lengths such that the phase of the rf signal introduced to a diode laser differs from that of the other diode lasers of the array 12. A diode laser of cavity length of the order 4mm will mode-lock at about a 10GHz repetition rate. As the output pulse from an actively mode-locked diode laser is of the order of lOps, ten time slots can be used to carry data on an optical transmission line without there being any overlap between the channels. To provide a lOps time delay between the rf signal at adjacent diode lasers a difference in length of the feed lines of around lmm is required.The lead arrangement of the first embodiment illustrated in Figures 2 to 4 would provide the required time delay. Thus the total bandwidth of 100 GHz is possible if each laser can be modulated at 10 GHz. This may be possible if complete suppression of the pulse is not required through the use of thresholding detection techniques. Such bandwidths would enable thousands of television channels to be carried by a single optical fibre. These bandwidths could also be used for 3-D image transfer in local area network configurations.

Once generated, the output signals from the diode lasers are introduced through suitable collecting optics 22 to an optical transmission medium. For minimum dispersion this is preferably a single mode optical fibre. For relatively short distance transmissions (of the order of 1Km) multi-mode optical fibre, could be used instead. The signals are transmitted until they reach a detector designed to detect the signals carried to transmit them for demodulation.

With currently available techniques, laser diodes with lengths as small as 100 microns can be produced. A diode cavity of this dimension has a resonance frequency of about 350GHz.

The use of high frequency channels may in itself generate problems in the detection of transmitted signals. This is because currently available electronic detection equipment is only operable at frequencies of the order of 50GHz. The electronics that makes operation at these high frequencies possible are at the forefront of current technological achievement and are correspondingly expensive.

In some circumstances it may, therefore, be desirable to generate output pulses at frequencies of around 10GHz. A time multiplexed system operating at these frequencies will provide ample bandwidth for most applications and still provide a substantial improvement on conventional optical transmission systems.

If bandwidths as high as 100GHz are, however, desired, until electronic equipment has been developed that operates to the required frequency, 'all optical' deserialization techniques are required for implementation. This can be achieved in a linear fashion through the use of directional couplers. (See M. Shabeer et al., PROC ICOC, Toulon France, 1988 (SPIE vol 963)).

Although high bandwidth optical transmission systems according to the present invention could be utilised in a number of different ways, one use of particular interest is in video subscriber systems. With the bandwidths of the order of 100GHz possible with the present invention, thousands of video channels can be transmitted simultaneously to enable an end subscriber to detect any number of channels or perhaps to choose individual films out of thousands broadcast continuously.

A time slot could be allocated to a reference signal and the receiving terminals would be provided with means to demodulate the signals carried by one or more particular time slot. End users could then be charged for a decoder to read each of the signals carried by respective time slots.

The system of the second embodiment of the present invention could also be used to transmit information around local area networks. One or more particular time slot would then be allocated to each terminal making up the network. Communication between one or more terminals could then be carried simultaneously on the transmission lines.

In such a system one time slot would be allocated as a reference slot. A master clock generated by one of the nodes of the network would fill this slot and the timing of all the other nodes would be relative to this reference signal and they would position all the time slots relative to the received reference time slot.

A third embodiment of the present invention will be described with reference to Figure 11 of the drawings.

In this embodiment an array of mode-locked monolithic diodes such as that described in relation to Figures 2 to 5 is provided for use with an optical sampling probe. By using a plurality of outputs within one cycling period, the number of samples that can be taken in a give time period is increased over that possible with conventional serial samplers by a factor N, where N is the number of diodes in the array. The array of diode lasers of the present invention can be used to improve the sampling rate of any optical sampler. However, the principles will be described below in relation to the electro-optical sampling probe of Figure 11.

The probe of Figure 11 is similar to conventional probes in that it comprises an electro-optic crystal tip that incurs a change in birefringence when influenced by an electric field. The optical sampling probe in accordance with one example of the third embodiment of the present invention can be seen in Figure 9. An array of diode lasers 120 similar to that illustrated in Figure 2 of the drawings sends lasers pulses at predetermined intervals through a lens 122 for focussing. As described above, each laser diode is pulsed at the same frequency determined by the cavity dimensions of the laser diode. There is, however, no output from the laser diode for the majority of a cycle, and so by arranging the outputs from the various diode lasers of the array to occur in sequence within one cycle, a far greater number of samples can be taken in a given time period.

A series of pulses are delivered to the surface of the probe 124 for reflection. A detector lens 126 directs each reflected pulse to an array of detectors 128 to determine the waveform of the electric field. Each detector of the array of detectors is assigned to a particular diode laser and provides information on the waveform at a particular time slot of the waveform cycle. In this way each of the array of detectors 128 looks in parallel at a different point of the waveform. If the output pulses from adjacent diode lasers of the array are close enough together in time to provide the required resolution, and the waveform signal is large enough, a waveform can be captured in a single cycle.This provides a considerable advantage over conventional optical samplers which necessitate a waveform being captured over a number of sampling cycles, as with this embodiment of the present invention there is no need for the optical beams of the sampling probe to be synchronised to the frequency of the waveform to be captured. This can substantially reduce the electronics required in a sampling system.

However, even if the signal-to-noise ratio is not sufficiently high for the waveform to be accurately captured in a single cycle, or if greater resolution is required, the optical probe 124 of the third embodiment of the present invention still allows a waveform to be captured at a faster rate than conventional optical sampling systems. If the same number of samples are taken, the time for taking the samples is reduced by the number of time slots allocated within the repetition cycle. This is likely to improve resolution as fluctuations in the waveform over a smaller number of cycles is likely to be reduced. If instead, samples are taken for the same period as would a conventional system, the number of samples collected is much greater by the number of time slots allocated within the repetition cycle i.e.

the number of diode lasers in the array. This is likely to allow improvements in resolution and signalto-noise ratio to be achieved.

Similar improvements in signal-to-noise ratio and sample rate are achievable using pulsed diode arrays to allocate respective time slots within a cycle period to respective diode lasers of the array in other types of optical sampler. Such optical samplers may rely on changes in different characteristics of a laser beam to provide information on physical, electrical or optical phenomena.

A fourth embodiment of the present invention will now be described with reference to Figures 12 of the drawings. In this embodiment, a one dimensional array of diode lasers such as that described in relation to the first embodiment of the present invention is used to generate electron emission from a photocathode.

This electron emission apparatus can be used for a number of different applications. However the principles will be discussed below in relation to an electron beam lithography system.

In Figure 12 an array of diode lasers 130 is provided for generating emission from a photocathode. The pulsed output beams from the laser diode array are passed through a collimating lens 132 to provide parallel beams. The collimated beams then pass in turn through an optical mask 134 and a demagnifying lens 136. The beams are then incident on a photocathode 138 formed of S-1 so that electron emission occurs only at positions corresponding to the portions of the mask 134 through which the pulsed laser beams are able to pass. The photocathode 138 is located in a vacuum chamber 140 and electrons emitted by the photocathode are focussed onto a silicon wafer 142 coated with a layer of electron resist 144, such as PUMA.

The arrangement of this embodiment allows an optical mask to be used to apply a pattern to a silicon wafer using electron beams. As a plurality of electron beams are emitted from the photocathode 138, the throughput of such a device is far greater than the conventional single electron beam controlled by an electron lens system. However, if there is continuous emission from the photocathode, both of problems describe above in relation to prior art devices i.e. a reduction in output power due to Coulomb space charge effects and divergence of adjacent beams resulting in a lack of resolution due to Coulomb forces would be encountered.

Nevertheless, by using an array of the kind described in relation to Figure 2 of the drawings, these problems can be overcome. The adjacent diode lasers of the array of diode lasers in Figure 12 provide output pulses at intervals separated in time. Each pulse of laser light, therefore, causes emission from the photocathode at a time when adjacent areas of the photocathode are not emitting. This ensures that there are no Coulomb forces between electron beams patterning adjacent areas of the wafer so that the reproduction of the pattern defined by the optical mask is precise. Careful arrangement of the diode lasers is necessary to ensure that diode lasers causing electron emission from parts of the photocathode liable to interact through the Coulomb force, have different allocated pulsing time slots.

This is effected through a suitable arrangement of the rf feed lines to the laser diodes. Of course laser diodes having the same phase can stimulate emission from areas of the photocathode that will not interact through the Coulomb force. In this way an array of laser diodes large enough to span one dimension of an entire wafer pattern can easily be provided without problems of interference between the beams. The array is then scanned across the photocathode to deliver the entire pattern to the silicon wafer.

The pulsed output of the photocathode as a result of pulsing the diode lasers stimulating emission has the second advantage of reducing build up of electrons near the emitting surface of the photocathode. This reduces the Coulomb space charge effects and allows the pulsed beam of emitted electrons to be of greater intensity than a continuously emitting beam. There is an optimum duty cycle at which the average power of the electron beam is at a maximum.

It is envisaged that for one-off semiconductor wafer manufacture a single pulsed electron beam could be used in conjunction with a scanning lens arrangement to provide a useful electron beam lithography system.

A pulsed laser beam could be used to stimulate emission from a photocathode and generate an electron beam for writing a pattern on a wafer with electron resist. Because of the reduction in Coulomb space charge effects as a result of pulsing, the overall beam intensity could be higher than that possible with a continuously emitting beam.

As well as the 1-D array of diode lasers for stimulating emission as described above, the array of diode laser could be two dimensional with adjacent diode lasers emitting during different time slots of a cycle comprised of a stack of l-D arrays. With modelocked diode lasers pulsing at around 10 GHz, tens of diodes in an array can be provided each having a different allocated time slot. This will provide a sufficient number of time slots for arrays of any size to be generated without problems of beam interaction.

The 2-D edge could then be used so as to deliver the entire mask pattern onto the silicon wafer at one time.

Although the actively mode-locked diode lasers of the array described in relation to Figure 2 of the drawings, provide a fast and effective means for producing a patterned silicon wafer, pulsing a laser beam or other light source at any reasonable frequency would provide the same result. A pulsed array of light sources other than the monolithic diode lasers specifically described are also envisaged for use in this embodiment of the invention.

One such arrangement is an array of independent passively mode-locked diode lasers. By spacing the diode lasers sufficiently to ensure that there is no leakage between the diode lasers to encourage emission in-phase, and that linkage through back reflection is prevented, the phases of the output beams of a plurality of diode lasers having the same cavity dimensions will be random. As tens of different time slots within the frequency cycle of the pulsed outputs are available, there will be no interaction between electron beam emissions stimulated from the photocathode and other beams emitted from proximate areas of the photocathode.

Another approach to stimulating random emission of electrons to avoid Coulomb interaction is envisaged using a pulsed output from a single laser that is scattered as it is passes through a diffuse screen.

The diffuse screen splits the laser pulse into a plurality of pulsed outputs with random time slot allocations. These outputs can be used in the same way as the random outputs of the passively mode-locked array of diode lasers to stimulate emission from the photocathode.

The electron beam emission apparatus described in relation to the electron beam lithography apparatus of Figure 12 can be used instead to provide a high intensity electron beam that can be used, for example in scanning and photo emission applications. Any of the means for generating a plurality of pulsed outputs from a photocathode with respective different allocated time slots described above can be used in this application. To form a high intensity beam from the arrangements described, a lens system is provided to focus each of the plurality of electron beams to a single point. Because of the different time slot allocations, no Coulomb force interactions that tend to cause the beam to defocus are encountered.

The high intensity beam is substantially continuous and also benefits from the higher intensity of individual beams as a result of their being pulsed to avoid Coulomb space charge effects. A high intensity electron beam of this type can be used in any desired application.

The array of pulsed electron beams with different respectively allocated time slots can also be used to provide a plurality of electron beams for use in an electron microscope with the advantage of faster information acquisition by a factor equivalent to the number of diode lasers in the array or the number of different areas of emission from the photocathode.

A fifth embodiment of the present invention will now be described with reference to Figure 13 of the drawings. An array of laser diodes 150 provides a pulsed output at the clock cycle of an optical processor. The pulsed outputs of the array of diode lasers are time division multiplexed and digitally modulated in accordance with data to be processed. In one particular arrangement the data carried by the signals from the array of time division multiplexed diode lasers represent a vector Ii. Outputs from the lasers of the array 150 are focussed through a lens 152. A modulator 154, for example an acousto-optic modulator is provided in the focal plane of the lens 152. In this particular embodiment, the modulator is one dimensional and performs a single operation serially on data with repetition cycle far higher than the clock cycle of the processor.

The output beams from the respective diode lasers of the array pass through the modulator 154 and are operated on in accordance with a waveform applied to the modulator 154. The beams passing through the modulator 154 are focussed through a lens 156 onto a detector array 158, in which each detector detects the output beam of a respective one of the diode lasers of the array.

The repetition waveform in one embodiment represents a vector. This can be a binary vector in which case the detector array is arranged so that only 'l's', for example, are detected because of diffraction caused by the acousto-optic modulator. If the signal represented by the outputs of an array of diode lasers is a vector, and the repetition wave input to the modulator represents another vector, vector/vector multiplication can be achieved. Other functions can be performed on data representing different entities using the system of this aspect of the present invention.

Basically by using a modulator that operates at a faster rate than the clock cycle of the processor, operations on a number of data elements are performed within one clock cycle. The modulator is accordingly able to function essentially serially so that higher dimensional arrays that add to the complexity of the system are not required. This results in a speed up in processing by the number of diode lasers that are time division multiplexed. In addition, if still faster processing is required a one or even two dimensional array modulator can be provided and an additional speed-up factor of the number of elements in the array can be enjoyed. In this instance a number of time division multiplexed arrays would be necessary to provide data to the modulator.

In embodiments of this aspect of the present invention, the frequency of the pulsed outputs of the diode lasers is not critical; what is important is that the modulator has a higher modulation bandwidth than the clock cycle of the processor, ie the bandwidth of the lasers. This will generally be true when the modulator operates using non-carrier effects such as electro-optic modulation.

Diode lasers other than those described in relation to the embodiment illustrated in Figure 2 of the drawings are therefore envisaged.

Each of the embodiments of the invention described above provides a practical application for an array of diode lasers arranged to produce output pulses in sequence within a pulsing cycle common to each of the diode lasers. This type of arrangement provides a wide range of applications specific ones of which are described above.

Claims

CLAmS:

1. An array of lasers comprising a plurality of diode lasers for generating a pulsed output at substantially the same frequency in response to an input signal having substantially that same frequency, means for providing an input signal to each of the plurality of diode lasers, and means for connecting the signal providing means to each of the plurality of diode lasers such that the distance travelled by input signals between the signal providing means and respective ones of the plurality of diode lasers is different for the allocation of a respective time slot to the output pulses from respective diode lasers within the pulsing frequency cycle of the pulsed outputs.

2. An array of lasers according to claim 1 wherein the plurality of diode lasers comprise a plurality of mode-locked diode lasers.

3. An array of lasers according to claim 2 wherein the input signal is a substantially sinusoidal radio frequency signal.

4. An array of diode lasers according to claim 2 or claim 3 wherein the frequency of the pulsed outputs is determined by the resonating frequency of the diode cavity.

5. An array of diode lasers according to claim 2 or claim 3 wherein the frequency of the pulsed outputs is determined by the resonating frequency of an external cavity.

6. An array of lasers according to claim 1 wherein the plurality of diode lasers comprise a plurality of gain-switched diode lasers.

7. An array of lasers according to claim 6 wherein the input signal is a short electrical pulse excitation.

8. An optical communications system, comprising an array of diode lasers according to any one of claims 1 to 7 further comprising means for modulating the output pulses of each of the plurality of diode lasers to carry information for transmission in an optical communications system.

9. An optical sampling probe comprising an array of diode lasers according to any one claims 1 to 7, and means for detecting changes in the characteristics of each of the laser beams in response to characteristics of a probed phenomenon, thereby obtaining information on the probed phenomenon at each of the time slots allocated to respective ones of the laser diodes.

10. An optical sampling probe according to claim 9 wherein the means for detecting is a plurality of detectors each receiving the output pulse from a respective one of the plurality of diode lasers.

11. An apparatus for stimulating electron beam emission comprising a photocathode, an array of lasers according to any one of claims 1 to 7 for stimulating the emission of electrons from the photocathode.

12. An apparatus for stimulating electron beam emission according to claim 11 further comprising means for selectively inhibiting emission of electrons from the photocathode in accordance with a desired pattern for inclusion in an electron beam lithography apparatus.

13. An apparatus according to claim 12 wherein the means for selectively inhibiting emission of electrons comprises a mask for selectively preventing passage of light emitted from the lasers from stimulating emission from the photocathode.

14. An apparatus according to claim 12 wherein the means for selectively inhibiting emission comprises means for selectively inhibiting light emission from lasers of the array in accordance with a desired pattern.

15. An electro-optical or optical processor comprising an array of lasers according to any one of claims 1 to 7 wherein

16. An optical transmission system comprising, a plurality of actively mode-locked diode lasers each adapted for generating a pulsed output at substantially the same resonance frequency determined by the dimensions of the cavities of respective diode lasers, and means for allocating a respective time slot within a resonance frequency cycle to pulses from respective diode lasers for transmission.

17. An optical transmission system according to claim 16 further comprising means for modulating the output pulses of each of the plurality of diode lasers to carry information for transmission.

18. An optical transmission system according to claim 16 or 17 wherein the means for allocating respective time slots comprises means for providing a signal with a frequency substantially equal to the resonance frequency of the diode lasers to one of the contacts of each of the plurality of mode-locked diode lasers, the phase of the input signal being different at each respective diode laser.

19. An optical transmission system according to claim 18 wherein the length of rf feed lines from a rf frequency source to respective ones of the diode lasers of the array differs to provide the different phases of the input signal at each of the plurality of actively mode-locked diode lasers.

20. An optical transmission system according to any one of claims 16 to 19 wherein the actively mode-locked diode lasers are actively mode-locked monolithic diode lasers having a split cavity contact.

21. An optical transmission system according to claim 17 to 20 and any claim dependent therefrom wherein the means for modulating comprises means for selectively inhibiting output pulses in accordance with a data signal.

22. An optical sampling apparatus comprising a plurality of diode lasers for generating a pulsed output at substantially the same frequency in response to an input signal having substantially that same frequency, each of the laser beams output from the plurality of diode lasers being directed to probe a phenomenon under examination, means for providing an input signal to each of the plurality of diode lasers, different time slots being allocated to the output pulses from respective diode lasers within the pulsing frequency cycle of the pulsed outputs, and means for detecting changes in the characteristics of each of the laser beams in response to characteristics of the probed phenomenon, thereby obtaining information on the phenomenon at each of the time slots allocated to respective ones of the laser diodes.

23. An optical sampling apparatus according to claim 22 wherein the means for detecting comprises an electro-optical crystal for placing in an electric field to be sampled.

24. An optical sampling apparatus according to claim 22 or 23 wherein the means for detecting is a plurality of detectors each receiving the output pulse from a respective one of the plurality of diode lasers.

25. An optical sampler according to any one of claims 22 to 23 further comprising means for connecting a signal source to each of the plurality of diode lasers such that the distance travelled by the input signals between the source and respective ones of the diode lasers is different.

26. An electron beam emission apparatus comprising a cathode, and means for stimulating pulsed emission from a plurality of areas of the cathode such that the pulsed emission of electrons from respective ones of the plurality of areas of the cathode close enough to experience Coulomb interactions occupy different respective time slots.

27. An electron beam emission apparatus according to claim 26 wherein the cathode is a photocathode.

28. An electron beam emission apparatus according to claim 27 wherein the means for stimulating pulsed emission comprises at least one pulsed laser source.

29. An electron beam emission apparatus according to claim 28 wherein the at least one pulsed laser source comprises a plurality of diode lasers for generating a pulsed output at substantially the same frequency in response to an input signal having substantially that same frequency, and further comprising means for providing an input signal to each of the plurality of diode lasers, and means for connecting the signal providing means to each of the plurality of diode lasers such that the distance travelled by input signals between the signal providing means and respective ones of the plurality of diode lasers is different for the allocation of a respective time slot to the output pulses from respective diode lasers within the pulsing frequency cycle of the pulsed outputs.

30. An electron beam emission apparatus according to claim 29 wherein the plurality of diode lasers comprise a plurality of mode-locked diode lasers.

31. An electron beam emission apparatus according to claim 28 wherein the at least one pulsed laser source comprises a plurality of gain-switched diode lasers.

32. An electron beam emission apparatus according to claim 28 wherein the at least one pulsed laser source comprises a plurality of independent passively modelocked diode lasers.

33. An electron beam emission apparatus according to claim 28 wherein the at least one pulsed laser source passes through a diffuse screen scattering spatially and temporally.

34. An electron beam emission apparatus according to any one of claims 27 to 33 further comprising means for selectively inhibiting emission of electrons from the photocathode in accordance with a desired pattern for inclusion in an electron beam lithography apparatus.

35. An electron beam emission apparatus according to claim 34 when dependent upon any one of claims 29 to 33 wherein the means for selectively inhibiting emission of electrons comprises an optical mask positioned between the plurality of lasers and the photocathode.

36. An electron beam emission apparatus according to claim 34 when dependent upon any one of claims 29 to emission comprises means for selectively inhibiting light emission from respective ones of the plurality of lasers in accordance with a desired pattern.

37. An electron beam emission apparatus according to any one of claims 26 to 33 further comprising means for directing the electrons emitted onto a surface and detecting electrons transmitted or reflected by that surface for inclusion in an electron microscope arrangement.

38. An electron beam emission apparatus according to any one of claims 26 to 33 further comprising means for focussing the pulsed emissions from the plurality of areas of the photocathode to provide a single high intensity beam.

39. An electron beam lithography apparatus comprising a photocathode, means for stimulating pulsed emission of electrons from the photocathode and means for controlling the emission of electrons from the photocathode in accordance with a desired pattern.

40. An optical/electro-optical processor comprising a plurality of diode lasers for providing a pulsed output at a clock cycle of the processor, means for allocating a respective time slot to the output pulses from respective diode lasers within the clock cycle of the processor, means for modulating the outputs from respective ones of the diode lasers of the array in accordance with a first data signal, means for supplying the modulated outputs of the plurality of diode lasers in series to a modulator, and means for modulating the modulated outputs of the diode lasers in accordance with a second data signal to provide a modulated signal, wherein the modulate has a larger bandwidth than the bandwidth of the output of each of the diode lasers.

Amendments to the claims have been filed as follows comprises a mask for selectively preventing passage of light emitted from the lasers from stimulating emission from the photocathode.

15. An electro-optical or optical processor comprising an array of lasers according to any one of claims 1 to 7 wherein said array is arranged to provide signals for modulation in said electro-optical or optical processor.

23. An optical sampling apparatus according to claim 22 wherein the means for detecting comprises an electrooptical crystal for placing in an electric field to be sampled.

36. An electron beam emission apparatus according to claim 34 when dependent upon any one of claims 29 to 33 wherein the means for selectively inhibiting emission comprises means for selectively inhibiting light emission from respective ones of the plurality of lasers in accordance with a desired pattern.