GB2442754A - Optical transmission apparatus and laser pulse duration expander - Google Patents

Abstract

An optical transmission apparatus comprising: a beam divider e.g. microlens array 203; fibre-optic bundle having a plurality of optical fibres 209a-d for receiving divided portions of the beam; and a beam combiner e.g. microlens array 211 for combining divided portions of the beam received from the bundle wherein at least two of the optical fibres have different optical path lengths e.g. due to different physical lengths or different refractive indices. The beam 201 may be from a pulsed laser. Also provided is a laser pulse duration expander having such a fibre-optic bundle so that input of a laser pulse with a first pulse duration results in the output of a combined pulse with a second pulse duration greater than the first. The laser may be a Q-switched laser.

Description

I

AN OPTICAL TRANSMISSION APPARATUS AND METHOD

This invention relates to an optical transmission apparatus, a method of optical transmission, a laser pulse duration expander and a Q-switched laser.

Thin-film-transistors (TFTs) are important building blocks for active matrix flat panel displays (FPDs) such as liquid crystal displays (LCDs). For such FPDs. TFTs are manutctured using thin film silicon as the active and/or doped layer. This silicon film is deposited onto commercial grade display glass as amorphous silicon in a layer between 50 to 500 nm thick but applications for this form of silicon are limited due to the low carrier mobility.

Improved FPD performance can be obtained by using polycrystalline silicon as the active and/or doped layer compared to amorphous silicon. This is because of the significantly improved earner mobility which for instance allows pixel-driving circuits to be monolithically integrated onto the display substrate (see, for example, "A. T. Voutsas, A new era of crystallization, advances in polysilicon crystallization and crystal engineering, Applied Surface Science 208-209 (2003) pp 250 -262").

Thin film amorphous silicon can be recrystallised to produce polycrystalline silicon by using a thermal annealing process. However when this type of thin film is applied onto a glass substrate there is an upper temperature limit due to the heat-sensitive nature of the glass which is in the region of 600 to 650 C, which gives rise to the name of this type of silicon film -low temperature polycrystalline silicon (LTPS).

Traditional methods of recrystallisation such as zone-melting-recrystallization (ZMR) cannot be used due to the high thermal loads that this method places on the substrate (see, for example, "J. S. im et a!, Controlled Super-Lateral Growth of Si Films for Microstructural Manipulation and Optimization, Phys. Stat. So!. (a) 166 (1998) pp. 603 -617"). Therefore new methods of recrystallisation have been developed many of which are laser based (see Voutsas).

One such recrystallisation method, known as sequential lateral solidification (SLS), is described by Im el a!. One of the factors that determines the success of this method is the pulse width of the incident laser beam onto the target amorphous silicon thin film.

Therefore, being able to control and in particular stretch the pulse width may give process benefits.

According to one known approach, US5960016 (University of Cahfornia) describes a pulse duration expander for ultra-short pulses (femtoseconds) as found in chirped pulse amplification systems The technique takes advantage of the large spectral bandwidth of ultra-short pulses in a high dispersion system to lengthen the duration of the pulses. However, this system is not suitable for nanosecond pulses as they have sub-nanometre bandwidths and so would not be appreciably stretched in a system of this type.

According to another known approach, US2005/0105579A1 (Sanyo), US6389045 (Lambda) and US7016388 (Klene ci a!) describe systems wherein a portion of an input beam is sent down a delay line and then recombined with the original pulse to produce a longer output pulse. Multiples of these stretchers may be used in series.

These documents describe different geometries for achieving this effect but aH are essentially the same. These systems use discrete optics (mirrors) to define a long cavity such that the light undergoes multiple reflections before emerging as a lengthened pulse. These systems suffer from losses at each reflective surface, which after many passes becomes significant. These systems have the disadvantage of being complex and difficult to setup.

The invention is set out in the claims. The claimed invention provides an approach for increasing the duration of a laser pulse using an array of optical fibres with different optical path lengths. This reduces the reliance on discrete optics for providing multiple beam paths and thereby reduces the energy lost due to optical interfaces. The claimed invention provides a means for controlling the laser pulse duration and thereby increasing the number of possible applications for Q-switched lasers.

Embodiments of the invention will now be described by way of example with reference to the drawings of which: Fig. I is a schematic of a known high-power fibre-based delivery system; Fig. 2 shows a first embodiment of the present invention; Fig. 3 shows the input beam, microlens array and fibre bundle according to an embodiment of the present invention; Fig. 4 shows an example stretched pulse achieved from a range of fibre lengths; Fig 5 shows a skewed pulse resulting from an embodiment of the present invention; Fig. 6a and 6b show other example pulses resulting from further embodiments of the present invention; Fig. 7 shows an embodiment wherein the duration of the stretched pulse can be controlled.

In overview, there are provided input coupling optics for coupling an input beam into a plurality of optical fibres. Since the input beam is split into multiple paths of lower energy, each fibre carries less than the laser-induced damage threshold. The outputs of the fibres are recombined by output coupling optics to produce a recombined output beam. By varying the length of the fibres in the bundle, and hence the path length of the transmitted light, the duration of the output pulse is stretched.

A high-power fibre delivery system is described in EP0923749 (Lissotschenko & Hentze) and shown in Fig. 1. Referring to Fig. I, a microlens array 103 comprising a plurality of microlenses 105 is used to focus an input beam 101 from a laser (not shown) into a fibre bundle 107 comprising a plurality of optical fibres 109. Each microlens 105 is arranged to focus a portion of beam 101 into a respective fibre 109 A similar microlens array Ill is used at the exit to produce a collimated, recombined output beam 113. In a perfect system, the output beam 113 will have the same temporal profile as the input beam 101.

Referring to Fig. 2, there is shown a first embodiment of the present invention. The system comprises an input beam 201 from a laser (not shown) and an output beam 213. Input beam 201 is incident upon a microlens array 203 comprising a plurality of microlenses 205. The input end of a fibre bundle comprising a plurality of optical fibres 209a-d is positioned substantially at the focal plane 213 of array 203 Fibres 209 are of different length. The output plane of fibres 209 is positioned substantially at the focal plane 217 of a second microlens array 211 comprising a second plurality of microlenses 215. Second array 211 produces output beam 213.

As shown in Fig. 3, the microlens array 303 has the same pattern and pitch as the fibre bundle 305. The Input beam 301 covers a plurality of the microlenses comprising array 303. In the example shown in Fig. 3, light is coupled into each fibre of the bundle 305 by a corresponding microlens of the array 303.

In operation, input beam 201 from a laser (not shown) is incident upon microlens array 203. Each microlens 205 of the array 203 is arranged to focus a respective portion of input beam 201 into a respective optical fibre 209. A similar microlens array 211 is arranged to collect and recombine the beam portions emerging from fibres 209 into a single collimated beam 213.

Input beam 201 is split by microlens array 203 into many respective lower power beams. Each lower power beam is below the damage threshold for the fibre 209. The microlens array 203 may employ aspherics in order to reduce the effects of aberrations (such as spherical aberration) to improve the coupling of light into the fibres 209.

Since each fibre 209 is a different length, the light emerges from each fibre 209 at different times. Thus, the temporal profile of output beam 213 is made up of a series of pulses. The temporal profile of output beam 213 is, in part, determined by the time between pulses as described in greater detail below with reference to Figs 4, 5 and 6.

Referring to Fig. 4, which shows an assembly generally of the type described above with respect to Fig. 2, there is shown an input pulse 401, a fibre bundle 407 comprising a plurality of optical fibres 409a-e (in this case, five) and a combined output pulse 413 formed by the plurality of pulses 411 emerging from the fibres 409.

The microlens arrays, coupling light at the input and output of the fibres respectively, are not shown Tn order to stretch the duration of the input pulse 401, each fibre 409a-e in the bundle 407 has a different length This has the effect of producing an array of time-delayed pulses 411 at the exit of the bundle. When the beams are recombined by the output microlens array (not shown), the pulses add to give a stretched pulse 413.

The magnitude of the delay in any individual fibre can be calculated using the following simple formula: nLV delay = -

C

where n is the refractive index of the fibre, zM is the difference in length of the fibre compared to a reference length and c is the speed of light. The duration of the stretched pulse will approximately be given by the delay difference between the longest and shortest fibre in the bundle. This is only approximate because it does not take into account the duration of the input pulse and the precise input pulse profile.

The temporal profile of the output pulse can be controlled by varying the range of fibre lengths. In an embodiment, shown in Fig. 5, a skewed pulse with a peak skewed to the beginning of the stretched pulse 501 is achieved by having a large portion of the fibres with lengths close to the shortest length thereby producing output pulses 503.

In an alternative embodiment, a pulse with a peak skewed to the end of the stretched pulse can be achieved by having a large portion of the fibres with lengths close to the longest length. Thus, the shape of the stretched pulse can be controlled by modifying the distribution of fibre lengths within the bundle. The skilled person will be aware that the temporal profile of the pulse can be tailored to application requirements Referring to Figs 6a and 6b, the stretched pulse temporal profile will also be affected by the duration and shape of the input pulse. As shown in Fig 6a, if the input pulse duration is of the order of the delay difference between fibres or longer, as shown by pulses 601, the stretched pulse 603 will have a smooth temporal profile. As shown in Fig. 6b, if the input pulse duration is shorter than the delay difference between fibres, as shown by pulses 605, the stretched pulse 607 will have a "spiky" profile. The temporal separation of the spikes in the profile is determined by the difference in delay between fibres in the bundle (see equation above). The skilled person will be aware that either case may be preferable depending on the exact application requirements. For example, for some applications such as laser-roughening, an irregular profile may be preferable.

Fig. 7 shows another embodiment wherein the output pulse duration can be controlled by altering the position of the input beam relative to the fibre bundle. Fig 7 shows the input beam 701, microlenses 707 and fibres 703 in cross-section. Each fibre 703 has a respective microlens 707 and the lens-fibre pairs are arranged in a regular array 705. The embodiment of Fig. 7 shows a square array 705 and a circular beam 701.

The diameter of the beam 701 is less than the width of the array 705 so input beam 701 covers only a portion of the array 705.

The fibres 703 of the bundle are arranged by length in a predetermined way; the shorter lengths are arranged at the centre of the bundle and the length increases in a spiral pattern around the edge of the array 705 as indicated by arrow A. The duration of the stretched pulse is changed by moving the position of the input beam 701 on the fibre bundle 705. As the input beam 701 is moved around in the direction of arrow A, longer fibres of the array 705 are used. This increases the maximum time delay between pulses emerging from the fibres and therefore the overall output pulse duration.

The skilled person will realise that the distribution of fibre lengths in the bundle can be tailored to the pulse durations or range of pulse durations required. Furthermore, the skilled person will be aware that the geometry of the system can be modified to accommodate any size and shape of array and input beam and that the arrow of increasing fibre length (arrow A in Fig. 7) can follow any path. The skilled person will also be aware that the arrangement of fibres at the input does not need to be the same as the arrangement at the output.

The fibre bundles of Figs 2 or 4, comprising varying lengths of individual fibres, may be fabricated in the following way. A pre-form is made which contains the fibres in the required array. When this pre-form is drawn a fibre bundle with a regular pattern will be produced. After the fibre bundle has been cut to the correct length, the individual fibres at one end can be peeled back and cut to the appropriate range of lengths for a given stretch factor and pulse shape The peeled fibres can then be reformed to produce an array suitable for coupling by the microlens array.

Any pulsed laser is suitable, for example the Starlase AO2G from Powerlase which produces radiation at a wavelength of 532 nm. The output power is 90 W with a repetition rate of 10 kHz and a pulse duration of 50 ns. The pulse duration of this laser may not be long enough for the most demanding poly-silicon annealing applications; however, in embodiments of the present invention, the pulse duration is increased sufficiently to produce a laser pulse which is suitable for this application.

Alternatively, the input pulse may have a duration of 30 to 100 ns; the output pulse may have a duration of 100 to 500 ns; and the pulse repetition rate may be in the range 4 to 20 kHz The laser light is coupled into the fibres using an array of microlcns such as those available from LIMO GmbH, Germany. The focal length of the microlens may be in the range of a few millimetres to a few centimetres. Short focal length microlenses are good for compact design, but the small spot size which results on the fibre may cause damage. Thus, longer focal length microlenses may be more suitable for high power beams, but these systems will be physically longer. The size of microlens array may be chosen to match the size of the input beam.

Any suitable optical fibres may comprise the fibre bundle, for example silica core fibres with a fluorine-doped cladding.

The parameters of the optical components should be chosen with consideration to the damage threshold of the optical fibres in the bundle. The well-known formula co = 2J7t,12 / nw may be used for this purpose. By way of example: a fibre illuminated with a 2.5 mm input beam diameter (2w); by a lens with input focal length of 50 mm (I); at an input wavelength of 1064 nm (A.o); with an input beam quality M2 of 20; and an input pulse energy of I mJ, experiences an average laser fluence of 44 J/cm2 at the focus. Since the typical damage threshold of silica is 10 J/cm2, the fluence calculated above is likely to damage the fibre According to embodiments of the present invention, if the same laser beam is divided into a 5x5 microlens array (each lens with a focal length of 20 mm), the fluence in each fibre would be only 1.7 J/cni2, which is well below the above stated damage threshold The length of the optical fibres in the bundle primarily determines the output pulse duration. For example, with an input pulse duration of 50 ns, using a fibres with a core refractive index of 1.5, a length difference (between the shortest and longest fibres in the bundle) of 90 metres would produce an output pulse of approximately 500 ns. To achieve a symmetric output pulse shape from a bundle of twenty one individual fibres (for example), the fibre lengths would be spread equally with a length difference between adjacent fibres of 4.5 metres.

The fibres could be mounted in any appropriate manner, for example using standard SMC connectors or an adapted bayonet fitting for high laser power applications, such as available from Optoskand AB, Sweden The input beam may be aligned to an input coupling housing that contained the microlens array and a suitable bayonet socket for the fibre bundle to locate into. The microlens array may be mounted on an X-Y translation mount so that the input beam can be aligned into the fibre bundle. The same arrangement may be required at the output.

The output of the pulse stretcher may be further sent to a beam shaping system that re-shapes the beam into a long thin shape with excellent homogeneity. This beam may be in the region of S to 50 microns by tens of millimetres to several centimetres. The beam shape may be designed so that the fluence is sufficient so that a single pulse (or a few pulses) cause crystallisation of polysilicon. This may be in the region of 0.1 to 2 i/cm2 but may need to be determined by experiment. The length of the homogenised beam may be determined by the fluence needed for crystallisation.

The parameters that may be suitable for polysilicon annealing: pulse duration 200 to 500 ns; pulse fluence 0.1 to 2 mi/cm2; and line focus dimensions 5 to 50 microns by tens of centimetres. The overall length of the fibre bundle may be chosen with respect to the logistics of the placement of the laser with respect to the beam shaping optics.

The number of fibres may be determined by the exact pulse profile required. The number of fibres may also be determined by the input power of the laser and the damage threshold of the fibre: the higher the power of the input laser, the more fibres required in the bundle to ensure they are not damaged.

The laser, pulse stretcher and beam shaper may be held stationary and the thin film coated glass substrate moved at a predetermined speed. This may be a granite system to avoid distortions caused by thermal expansion. A control system may be used to combine and control the various sub-systems An entire FPD may be crystalhsecl by moving the line across the screen until the whole screen has been covered. The line may be long enough to fill the screen in one dimension (although it may be also possible to stitch' lines together to cover larger areas). Consecutive lines may be overlapped. The final configuration may depend on the specific requirements, for example silicon crystal size and electron mobility.

The claimed invention combines the ease of use and flexibility of fibre delivery with the advantage of increasing the output pulse duration for those applications that are necessitate longer pulses, such as laser-annealing of silicon. The system allows pulses from standard Q-switched lasers to be used in applications otherwise prohibited by their short pulse duration. The system is monolithic and very easy to align with few discrete optics. This significantly reduces the number of optical interfaces thereby reducing energy losses from the system The claimed invention stretches the laser pulse in time as well as providing the flexibility and ease of use associated with fibre delivery. The system will stretch an input pulse with any duration and does not rely on spectral bandwidth. Furthermore, the system is monolithic and does not require any additional setup than the usual fibre input alignment. Sincc fibre-delivery is frequently preferred anyway, the claimed invention does not present significant design challenges.

Diode pumped solid-state Nd:YAG lasers are generally limited in the range of temporal pulse durations they can produce at a given repetition rate (for example ns at 10 kHz repetition rate) The present invention generally opens up the possibilities for applications for this type of laser.

In a modification, instead of using fibres of varying lengths, each fibre could have a slightly different refractive index by varying the concentration of an appropriate dopant. This would have the effect of slowing down the pulses in the more highly doped fibres (for example) and therefore producing the same effect as having varying lengths of fibre, namely varying the path length between fibres.

The skilled person will be aware that any method of temporally delaying a pulse in an individual fibre in the bundle will cause the output pulse to be stretched.

Claims (18)

1. An optical transmission apparatus comprising a beam divider, fibre-optic bundle having a plurality of optical fibres for receiving divided portions of the beam; and a beam combiner for combining divided portions of the beam received from the bundle wherein at least two of the optical fibres have different optical path lengths

2. The apparatus of claim I wherein the at least two different optical path lengths result from different physical lengths.

3. l'hc fibre bundle of claim I wherein the at least two different optical path lengths result from different refractive indices.

4. The apparatus as claimed in any preceding claim wherein the arrangement of fibres at an input end is different to the arrangement of fibres at an output end.

The apparatus as claimed in any preceding claim further compnsing a pulscd light source for generating the beam.

6. The apparatus as claimed in claim 5 wherein the pulsed light source is a laser.

7. The apparatus as claimed in claim 5 or 6 wherein the input light pulse has a duration in the order of 1 0 to I 0 seconds

8 The apparatus as claimed in any preceding claim wherein the output beam has a duration in the order of 10-8 to 106 seconds

9. The apparatus as claimed in any preceding claim wherein the beam divider comprises a plurality of lenses.

10. The apparatus of claim 9 wherein each lens is arranged to focus a portion of the light into a respective optical fibre.

11 The apparatus as claimed in any preceding claim wherein the beam combiner comprises a plurality of lenses.

12. The apparatus of claim 11 wherein each lens of the beam combiner is arranged to collect light from a respective optical fibre of the bundle.

1 0

13. The apparatus as claimed in any preceding claim wherein the pulsed light source illuminates a subset of the fibres of the fibre bundle array.

14. The apparatus of claim 13 comprising a selector for varying the illuminated subset.

15. A method of optical transmission comprising coupling at least a portion of a light pulse into at least two optical fibres wherein at least two of the optical fibres have different optical path lengths and recombining the outputs of the optical fibres to produce a combined output beam.

16. The method according to any of claim 15 wherein the input light pulse has a duration in the order of I 0 to I seconds.

17. The method according to any of claim 15 wherein the output beam has a duration in (he order of 10.8 to I 06 seconds.

18. A laser pulse duration expander comprising a fibre optic bundle arranged to transmit a laser pulse having a first pulse duration via at least two optical channels with different optical path lengths and combining the output from the optical channels to produce a combined output pulse with a second pulse duration wherein the second pulse duration is greater than the first.

19 A Q-switched laser comprising a fibre bundle delivery system having at least two optical fibres with different path lengths and arranged to increase the pulse length of the laser output.

20, A method or apparatus substantially as described herein with reference to the drawings