GB2510401A - Monolithic biconvex fast axis collimator array - Google Patents

Abstract

A micro optical element 100 for use with an edge-emitting laser diode bar stack, has biconvex fast-axis collimators formed as a monolithic array of inlet lenses 222 on a first surface 118 and an array of outlet lenses 122 on a second opposite surface. The lenses form biconvex collimators acting along the fast axis. A method of manufacturing the micro-optical element 100 for use with a laser diode bar stack using a wavelength stabilized CO2 laser is also described.

Description

MONOLITHIC BICONVEX FAST-AXIS COLLIMATOR ARRAY

The present invention relates to micro-optical elements for high power diode lasers and in particular, though not exclusively, to a monolithic biconvex fast-axis collimator array.

High power diode lasers are used in applications such as pumping of solid state lasers and direcily in materials processing. In order to achieve the required power levels in a compact package, diode bars of emitters are arranged in stacks providing a two-dimensional array of emitters. The diode bars are typically a 10mm long semiconductor bar fabricated with 19 to 79 emitters having widths of 100 to 200pm. Fill factors typically range from 10% to 80% as a result of different combinations of emitter width and emitter pitch. For CW stacks the bar pitch is in the range 1 to 2mm, while for QCW stacks pitch can be smaller, in the range 300pm to 2mm. The semiconductor is solder-bonded to a heat sink which can include channels for water cooling. A common configuration of commercially produced units stacks ten bars, each bar emitting 50-100W, to build up a total laser power of 500-1000 W. Power levels up to 500W per bar are achievable on QCW systems.

Currently, research and development has concentrated on providing optical arrangements for improving the beam quality, as that produced directly from the emitters is unacceptable for many applications. For high-brightness applications and also for some medium-brightness applications, by which we mean those with divergence well below the ex-facet divergence, the beam must be, at least, collimated. Manufacturers typically attach an individual fast-axis collimator to each bar. The fast-axis refers to the vertical axis (perpendicular to the semiconductor wafer) where the beam diverges guickly (NA0.5) from an emitter region in the pm range. This is in contrast to the slow-axis, parallel to the face of the bars, where the emitter region is more typically 100 p.m (NA 0.05-0.1). The slow axis (x-axis) and the fast axis (y-axis) are perpendicular to each other and orthogonal to the propagation direction of the beam (z-axis) The most common fast-axis collimators are planc-acylindrical lenses, whioh are used to provide low aberration collimation for the high numerical aperture fast-axis beam. Biconvex acylinders are also used. Such collimators are manufactured in high-index glass to provide the maximum possible NA and thus maximum power delivery through the collimator. The high-index material minimises the maximum lens sag (surface profile) and the degree of the surface slope reguired to achieve the reqrired NA. Homogeneous cylindrical rod lenses i.e. fibre can be used when performance reguirements are non-critical as they provide poorer guality collimation but at a lower cost.

Due to the difficulty in positioning discrete collimators at each emitter and the inability of a planc-acylindrical lens positioned along each bar to correct for smile and facet bending, Us 2012/0140334 to the present Applicant's discloses a micro-optical element for use with an edge-emitting laser diode bar stack which is of single piece construction. The element comprises a plurality of spaced apart fast-axis collimators formed as a monolithic array, wherein the spacing between the collimators in the fast-axis varies across the micro-optic element. Such a monolithic array provides a surface of lenses with properties tailored to the geometry of the laser diode stack. A method of manufacturing the micro-optical element is also described wherein a laser is translated over a fused silica substrate to ablate portions of the substrate in a shot-by-shot raster regime. This laser micro-machining avoids the tooling and mask writing steps of alternative techniques providing faster fabrication and avoiding the costs of hard tooling and photomask generation.

By melting the surface of the silica in zones greater than the raster pitch, the residual pattern of the raster is removed and a smooth, polished surface is achieved. This step can also be achieved rapidly and prcvides a higher standard of smoothing than the mechanical polishing techniques used in conventional optical fabrication and on high-index glasses in particular.

However, a major disadvantage of this micro-optic element is that it is produced in fused silica which can be ablated easily. Conseguently, as this is a low-index material the NA achievable and therefore the efficiency of the element is limited and not as good as that from a high index material such as glass. The actual NA achievable in fused silica for this plano-convex arrangement of fast axis collimators at variable pitch is also low due to the practical limitation in the maximum slope achievable with good accuracy; the increased sensitivity of wavefront error to surface form error at high slope; and the performance limitations achievable with antireflection coatings which operate over a wide angular range.

It is an object of the present invention to provide a monolithic fast-axis collimator array for use with a laser diode bar stack which alleviates or mitigates at least some of

the disadvantages of the prior art.

It is a further object of the present invention to provide a monolithic fast-axis collimator array for use with a laser diode bar stack which provides an increased NA and hence efficiency.

It is a yet further object of at least one embodiment of the present invention to provide an improved method of manufacturing a monolithic array of fast-axis collimators for use with a laser diode bar stack.

According to a first aspect of the present invention there is provided a micro-optical element for use with an edge-emitting laser diode bar stack, the element comprising a plurality of biconvex fast-axis collimators formed as a monolithic array, providing an array of inlet lens forms on a first surface of the element and an array of cutlet lens forms on a second surface, opposite the first surface, of the element.

In this way, the array dimensions are matched to the number of emitters and the benefits of mounting a single optical element at the end of the laser diode bars is achieved with the additional advantage that the NA can be optimised by the array of inlet lens forms. The first surface can be considered as an entrance surface and the second surface can be considered as an exit surface.

Preferably, the monolithic array is two-dimensional.

Alternatively, the monolithic array is one-dimensional. Thus elements can be prepared for any number of diode bars in a stack.

Preferably, each lens form describes a lens shape, with each inlet lens form having a matching outlet lens form, such that an array of biconvex lenses is formed in the monolithic structure. Preferably, each inlet lens form matches an emitter in the laser diode bar stack and thus a biconvex lens is effectively mounted at each emitter.

Preferably, the lens shape of eaoh inlet lens form is acylindric. Preferably also, the lens shape of each outlet lens form is acylindric. By adding acylindric terms, the output oollimation oan be optimised and/or the output beam profile can be controlled.

Preferably, the spacing between the lens forms in the fast-axis varies across the micro-optic element. In this way, each biconvex lens is tailored to an emitter. Preferably, the spacing on the y-axis provides a pitch in the range 100 microns to 2mm.

Preferably, each lens form has an equivalent focal length between 200 microns and 1000 microns. Using the technique of moving lithography, the focal length of each fast-axis collimator lens may be chosen to optimise the overall collimation for the bar that it collimates.

More preferably, each inlet lens form has a conic constant selected to optimise NA. In this way, the maximum angle of incidence can be limited to control the wavefront error and the antireflection coating performance.

Preferably each outlet lens form has a conic constant and a radius of curvature selected to provide a desired divergence angle and a desired divergence profile. The inlet lens may also have a conic constant selected to provide the desired divergence angle and the desired divergence profile. By selection of the conic constant, the compression factor' of the beam can be controlled i.e. the extent to which the beam is more peaky or more flat-top than a Gaussian beam, which can be quantified by the fourth standardised moment of the intensity distribution.

In an embodiment the desired divergence angle is near diffraction limit. In an alternative embodiment the desired divergence angle is defocussed. Ihe defocussed divergence angle may be selected from a group comprising 3 degrees, 6 degrees and 10 degrees FWHN.

In an embodiment the desired divergence profile is near Gaussian. In an alternative embodiment the desired divergence profile is squeezed such that the profile is more flat-top than Gaussian. In a further embodiment the desired divergence profile is stretched so as to be more peaky than Gaussian. The stretched profile may be as a consequence of a design that optimises NA for a given maximum surface slope.

The plurality of fast-axis collimators may have substantially the same characteristics and be located in a substantially regrlar array. This provides an array of identical fast-axis collimators on a fixed pitch between each array on the x-axis.

In this way, elements can be manufactured on a specific fixed pitch that is based on the actual mean pitch of a specific diode bar stack for each array on the x-axis.

Alternatively, fixed-pitch elements can be manufactured in a range of different pitches covering the typical process tolerance of bar pitch, and may be used on a production line as select-on-test components.

Preferably, a depth of the lens shape is in the range 5Om to 500pm. Preferably also, the first and second surfaces of the element is smoothed and polished. In this way, a precision micro-optical element is formed.

The above approaches do not deal with the problems of variable pitch between bars, variable bar-bar pitch along each bar, bar twist about the y-axis (fast-axis), bar smile, and other departures from the ideal geometry. Preferably, each fast-axis collimator has characteristics determined by a corresponding individual emitter location in a measured laser dicde bar stack. In this way, the position and focal length of the fast-axis collimator array is designed to match the precise locations, and in particular the locations in the x, y and even z axes, of the emitters in the laser diode bar stack, giving improved beam guality as the fast-axis collimatcrs are ideally matched to each emitter.

The lens shape may be interpolated along the slow-axis to give a continuous transition between lens forms. This is particularly suited for high slow-axis fill factors, where the beams from adjacent emitters may overlap before reaching the exit surface of the lens. This interpolation advantageously avoids sudden discontinuities in surface height or slope or curvature, so that the lens forms can be located closer together and the element positioned closer to the laser diode bar stack i.e. providing a higher fill factor and finer pitch.

Preferably, the element includes further optical characteristics such as bow-tie correction or slow-axis collimation. Additionally, each collimator on the fast-axis can be modified to one producing a fast-axis flat-top function in the far-field of each emitter. By making lens shapes upon the monolithic element, any lens-based optical characteristic can be provided in the element.

Preferably the monolithic element is made of a low-index material. More preferably, the monolithic element is made from fused silica. This advantageously allows the use of laser micro-machining to give rapid fabrication times.

Alternatively, other low-index materials known to those skilled in the art such as borofloat and BK7 may be used.

Optionally, the monolithic element may be formed from a high-index material which would advantageously give a lower sag.

According to a second aspect of the present invention there is provided a method of manufacturing a micro-optical element for use with a laser diode bar stack, comprising the steps: (a) providing a wavelength stabilized 002 laser with stable laser power operating on a laser line selected from the 00), spectrum; (b) providing a computer controlled acousto-optic modulator to give temporal control on laser pulses from the 002 laser; (c) providing a computer controlled X-Y translation stage; (d) locating a monolithic substrate on the translation stage; (e) operating the laser, acousto-optic modulator and the translation stage to ablate portions of the substrate in a shot-by-shot raster scan regime and form an array of predetermined lens forms on a first surface of the substrate; (f) turning the substrate over; and (g) operating the laser, acousto-optic modulator and the translation stage to ablate portions of the substrate in a shot-by-shot raster scan regime and form an array of predetermined lens forms on a second surface of the substrate, opposite the first surface.

In this way, a micro-optical element is produced which avoids the tooling or mask writing steps of alternative technigues and provides for faster fabrication.

Preferably, the method includes the step of aligning the first and second surfaces when the substrate is turned over. In this way, an array of bioonvex fast axis collimators can be formed.

Preferably, the substrate is fused silica. It has been realised that such a material is advantageous for laser micro-machining.

Preferably, the method includes the additional step of operating a laser, acousto-optic modulator and the translation stage to melt the silica in a raster scan regime to laser micro-polish the surface of the micro-optical element.

By melting the surface of the silica in zones greater than the raster pitch, the residual pattern of the raster is removed and a smooth, polished surface is achieved. This step can also be achieved rapidly and provides a higher standard of smoothing than the mechanical polishing technigues used in conventional optical fabrication and on high-index glasses in particular.

Preferably, the method includes the step of analysing the emitter geometries of the laser diode bar stack to determine shapes for the lens forms reguired. In this way, a tailored monolithic biconvex fast-axis collimator array is realised.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings of which: Figure 1 is a schematic illustration of a micro-optical element for a high power diode laser according to the prior art; Figure 2 is a schematic illustration of a high power laser diode bar stack; Figure 3 is a micro-optical element, viewed from the exit surface, for a high power diode laser according to an embodiment of the present invention; Figure 4 is the micro-optical element of Figure 3, viewed from the entrance surface, according to an embodiment of the present invention.

Figures 5(a) and (b) illustrate lens forms (a) of and the resulting beam distortion (b) through a micro-optical element

according to the prior art;

Figures 6(a) and (b) illustrate lens forms (a) of and the resulting beam distortion (b) through a micro-optical element optimised for numerical aperture and surface slope, according to an embodiment of the present invention; Figures 7 (a) and (b) illustrate lens forms (a) of and the resulting beam distortion (b) through a micro-optical element optimised for stretch, according to a further embodiment of the present invention; Figure 8 is a cross-sectional view through a portion of a micro-optical element for a high power diode laser according to a still further embodiment of the present invention; and Figure 9 is a schematic illustration of components and steps in a method of manufacturing a micro-opticat element for use with high power diode laser according to an embodiment of the present invention.

Referring initially to Figure 2 of the drawings there is illustrated an end of a high power diode laser, generally indicated by reference numeral 10. The end shows a laser diode bar stack 12 comprising four bars 14a-d of emitters 16a-e.

This is a simplified stack as the range of values is indicated hereinbefore. The pitch between emitters 16 and the bars 14 is deliberately small so that a high fill-factor exists to provide the reguired high power density.

As discussed previously, the bar stacks 12 produce beams of high divergence in the fast axis (y-z plane) . Collimation of each beam on the fast axis is reguired to provide a useful output from the stack 12. A prior art micro-optical element used to achieve this collimation is illustrated in Figure 1.

This is as described in the Applicant's patent application fJS2O12/0140334 which is incorporated herein by reference.

Referring now to Figure 1 of the drawings there is illustrated a micro-optical element, generally indicated by reference numeral 11, to be positioned at an end of a laser diode bar stack 12 of a high power diode laser 10, according to the

prior art.

Element 11 is a monolithic structure of fused silica with dimensions of approximately 1cm1. One face 18 is machined flat, while machined on the opposing front-face 20 is a two dimensional array of lens forms 22. The number and location of each lens form 22 matches the emitters 16 of the bar stack 12.

Each lens form 22 defines a lens shape 24, being primarily convex with a depth (in the propagation axis z) of between 5Opm and 500pm. Each lens shape 24 defines a fast-axis collimator as is known in the art. A continuous surface 26 is provided between the lenses on the fast-axis. Surface 26 is formed as a trough of sufficient depth to define the separate lens shapes 24. Though not shown as well, a oontinuous surfaoe 28 is provided between the lenses on the slow-axis also.

Surface 28 is formed as a trough of suffioient depth to define the separate lens shapes 24.

This micro-optical element U, having a two dimensional array of fast-axis collimators is positioned in front of the emitters 16, with the plane surface 18 acting as an entrance surface for the beams from each emitter 16 and the surface of lens forms 22 acting as the exit surface 20, from which the collimated beams exit.

The lens forms 22 do not form a regular array upon the surface 20, as each fast axis collimator is positioned to match the exit trajectory of the beam from an individual emitter 16.

Thus the irregularity refleots the misaligned emitters 16 in each bar 14 of the laser stack 12. The indivIdual calculations from each emitter 16 are used to provide the positions of and lens shapes 22 over the surface 20 to provide a highly oollimated output beam to the laser diode stack 12. The troughs in spaces 26,28 are all smoothed allowing both definition to the lenses 22 while removing sudden discontinuities.

It is known that the laser 10 provides an array of high numerical aperture fast-axis beams from the emitters 16. As discussed hereinbefore fused silica is used to as it suits the micro-machining process to create the element 11. However, the low-index of silica is a constraint. Accordingly, the array structure limits the physical aperture and hence limits the numerical aperture for a specific effective focal length. This means that the element is inefficient at transferring power.

In effect the resulting divergence properties of the beams have been taken as more important than the efficiency of the element 11.

While a maximum theoretical numerical aperture can be calculated for the element 11, of an array of plano-convex lenses, the actual achievable in the design is much lower, due to the practical limitation in the maximum slope achievable with good accuracy; the increased sensitivity of wavefront error to surface form error at high slope; and the performance limitations achievable with antireflection coatings which operate over a wide angular range.

Reference is now made to Figure 3 of the drawings which illrstrates a micro-optical element, generally indicated by reference numeral 100, which is a monolithic biconvex fast-axis collimator array according to an embodiment of the present invention. Like parts to the element 11, shown in Figrre 1, have been given the same reference numeral with the addition of 100, to aid clarity.

As can be seen in Figure 3, the exit surface 120 appears identical to the exit surface 20 of the prior art. Here there is a two-dimensional array of lens forms 122 providing a fast axis collimator at each of the emitter 16 positions. However, on the entrance surface 118, the plane profile is now replaced with a two-dimensional array of lens forms 222.

The arrangement of lens forms 222 on the entrance surface 118 are best seen in Figure 4 which shows the element 100 viewed from this entrance surface 118. On the surface 118 there is a two-dimensional array of lens forms 222 providing a convex lens at each of the emitter 16 positions. The lens forms 122,222 are paired to align a biconvex lens on the optical axis of each beam from each emitter 16 in the stack 12. The effect of these lens forms 222 is shown with the aid of Figures 5 to 7.

Referring to Figure 5(a) there is illustrated a plot of the lens shape 24 with distance 34 from the emitter 16 on the x-axis and distance 32 from the optical axis on the y-axis. A straight vertical line 18 represents the planar entrance surface 18 while the curve providing a slope at 65 degrees describes the convex lens form 22 of the fast axis collimator, for the plano-convex arrangement of the prior art element 11 shown in Figure 1. A ray trajectory 30 is illustrated to show the effective numerical aperture achievable on entry to the surface 18 and the resulting collimation on exit through the lens form 22. A plot of output ray height 36 over input ray height 38 for the arrangement of Figure 5(a) is shown in Figure 5(b) where the plot 40 shows the compression of the outer rays as che input ray height 38 increases. Thus illrstrating that the numerical aperture and hence the efficiency of this prior art element 11 is constrained by the required working distance, distance from emitter to entrance surface, and the critical thickness of uhe fused silica material used.

An increase in numerical aperture and efficiency can be seen for the element according to an embodiment of the present invention, in equivalent Figures 6(a) and 6(b) . In Figure 6(a) the entrance surface 118 now has a lens form 222 being convex to a depth of 43 microns. This provides a slope on entry giving an increased numerical aperture. The slope of 47 degrees on the exit surface 120 created by a convex lens shape 22 with a depth of 97 microns, is also optimised within the same working distance and critical thickness of the prior art element 11. Figure 6W) now illustrates the beam distortion as the stretch of the outer rays 42 from a straight line 40.

Advantageously, it has been found that the bioonvex arrangement of the lens forms 122,222 can be selected to oontrol the compression faotor' or stretch' of the beam.

This can be considered as the extent to which the beam is more peaky or more flat-top than a Gaussian beam. In mathematical terms it is guantified by the fourth standardised moment of the intensity distribution. In manufacturing terms, it can be varied by changing the conic constant of the surface profile of a lens form and also by changing the even terms in a polynomial that contributes to the description of acylinder shape. In particular we vary the conic constant of the lens form on the entrance and exit surfaces to modify the output profile at the exit and also it can be used to modify the efficiency. rae tailor the conic constant at the entrance surface in order to provide adequate performance against two criteria: limiting the maximum angle of incidence which affects both the wavefront error and the antireflective coating performance; and controlling the compression factor.

We know that a high index plano-convex fast axis collimator significantly compresses the tails of the beam, so by controlling the conic and higher order aspheric terms we can bring this closer to a Gaussian, if desired.

A biconvex fast-axis collimator cptimised for stretch, according to an embodiment of the present invention, is shown in Figures 7 (a) and 7 (b) . In Figure 7 (a) the entrance surface 118 has a lens form 222 being convex to a short depth of 16 microns. This provides an increased slope on entry giving an increased numerical aperture. The increased slope of 61 degrees on the exit surface 120 created by a convex lens shape 22 with a depth of 137 microns, is closer to the planc-ccnvex case, see Figures 5 (a) and (b) , but the combination of a biconvex lens arrangement gives an ideal beam distortion profile showing a linear relationship 40 between the output ray height 36 over input ray height 38 as shown in Figure 7 (b).

While Figures 6 and 7 show designs for individual bioonvex fast axis collimators, as would be found in an embodiment of an element of the present invention, Figure 8 shows a portion of a oross-section through an element to illustrate the array and the smoothing which is reguired to distinguish each lens shape 122,222 on the surfaoes 118, 120.

Referring to Figure 8, there is provided an element oreated from a substrate 52 having an initial thickness of 0.40 mm. A critical thickness 50 of 0.370 mm is selected together with a working distanoe 70 of 0.090 mm. Lens forms 122 are machined in an array on the exit surface 120 at a pitch 54 of 0.43mm to mimic the pitch of the laser bars 14. Troughs 126 to a depth of around 0.12 mm are machined between the lens forms 122.

This creates the array of fasu-axis collimators on the exit surface 118. On the entrance surface 118, lens forms 222 are machined opposite the lens forms 122, though these will be of a lesser depth to create the biconvex lens through the substrate 52. While the gap between the clear apertures of the lens forms 122 of the collimators was small, only 65 microns, the gap between the clear apertures of the lens forms 222 on entry is comparable to the diameters of the machined lens forms 222. Conseguently, troughs are not provided, but instead a dimple 60 is formed to maintain a minimum thickness of the substrate 52, this being around 0.245 mm. Of note is that both the entrance and exit surfaces are smooth continuous surfaces with no discontinuities present. The cross-section shown egually applies in the x and y axis of the element 100.

In the embodiment of the present invention, shown in Figure 8, the lens shapes are uniform across the array and assume a fixed pitch. In ccnstructing a high pcwer diode laser, a manufacturer could choose such a uniform array of biconvex fast-axis collimators frcm a set manufactured in a range of different pitches ccvering the typical prccess tolerance of bar pitch. These would be select-on-test components.

Alternatively, fixed pitch arrays can be formed which achieve a target divergence fcr a laser array by combining the divergence due to the performance of an individual lens with an increase in overall divergence caused by misalignment between the lens array and the laser array, where this misalignment may be due random or systematic variation in bar pitch variation. . However, in a preferred embodiment the micro-optical element 100 is a fused silica monolithic array of biconvex fast-axis collimator lenses with acylinder entrance and exit surfaces, and pitch matched to diode laser bar pitch. Collimation need not be diffraction-limited as the key objectives are maximising NA (which is a challenge for a low-index material) and achieving a predetermined divergence.

Lenses are designed to generate a beam of predetermined divergence, which may be defocussed, in order to produce a beam of well-defined divergence properties. The reason for using biconvex acylinder is to get good a numerical aperture in low-index material. The conic constant of the entrance surface is chosen to optimise the numerical aperture. The radius of curvature and the conic constant of the exit surface are chosen to achieve a specific divergence angle and profile.

Other dimensions may be calculated to provide further properties to the element 100. For example, collimation may be achieved across the slow-axis. Corrective measures may also be incorporated such as bow-tie correction as is known in the art. Beam shaping properties may also be incorporated such as producing a fast-axis flat-top function in the far-field of each emitter.

Reference is now made to Figure 9 of the drawings which illustrates the components of a laser micro-machining process, generally indicated by reference numeral 8C, for creating a micro-optical element 100 for use with a laser diode bar stack 12. An RE excited CC2 laser 82 is arranged before an acousto-optic modulator (ACM) 84. Various optical elements (not shown) direct the output beam 86 to a silica substrate 88 upon which the lens shapes 22 will be machined.

The fused silica substrate 88 (typically a piece of flat, parallel-sided fused silica 1mm thick) is mounted upon an XY translation stage 90, which is computer 42 controlled to move in steps of lOOnm in the two dimensions. A focussing lens 94 mounted on a computer controlled Z stage 96, focuses the beam 86 onto the substrate, a required depth to ablate the silica.

The computer 92 moves the stages 90,96 in a raster configuration so that controlled ablation, by shot-by-shot laser writing, of the silica 88 is achieved to create the required lens shapes 22 to form the array of fast-axis collimators on the exit surface 120.

Typically the spot on the substrate 88 corresponds to a Gaussian beam waist such that the spot profile at the surface to be machined is circular Gaussian. The beam radius may be on the order of approximately 25 pm.

The time needed to machine each lens shape 22 is approximately minutes. The entire element 11 can thus be manufactured in a relative short amount of time providing the ability to undertake rapid prototyping.

The as-machined surface 120 of the element 100 is then subjected to a raster scan of the laser beam in a near CW operating regime. A melt zone of diameter approximately 50 to 200jm is thus created which removes the residuai pattern of the raster and smoothes or polishes the surface 120. This can be carried out by the same system, as described with reference to Figure 8, as performs the smoothing, under a near-OW mode of operation, in which case the substrate 88 does not require to be moved between the machining and polishing steps.

Alternatively, the process can be carried out by a separate system that is optimised for the laser smoothing process.

Once the exit surface 120 has been completed, the substrate 88 is turned over to expose the entrance surface 120 to the laser. An alignment stage 98 achieves the micron-scale back-front alignment required. The above-described process is repeated to create the lens forms 222 in the surface 120.

These are also smoothed in the same way as those of the exit surface 118.

The principle advantage of the present invention is that it provides a monolithic biconvex fast-axis collimator array for use with a high power laser diode with stacked bars which maximises the numerical aperture for low-index materials.

A further advantage of the present invention is that it provides a monolithic biconvex fast-axis collimator array for use with a high power laser diode with stacked bars which by varying the conic constant of the entrance and exit surfaces can either modify the beam profile at the output or optimise efficiency given some process constraint. Additionally adding acylinder terms can optimise output collimation or control the output beam profile.

A still further advantage of the present invention is that it provides a monolithic double-sided micro-optical element by laser micro-machining. This approach to fabrication avoids sharp grinding errors. Though it is generally assumed that such a biconvex array could not be achievable due to the requirements of micron-scale back-front alignment, aoylinder or freeform surface generation and concave features (valleys/troughs between lenses), the present invention shows that by combining laser micro-machining with melting to smooth a low-index material such as fused silica, high tolerance elements without sharp grinding errors are realised.

It will be appreciated by those skilled in the art that modifications may be made to invention herein described without departing from the scope thereof. For example, not all emitter positions need to be measured to provide a tailored element, and the exact form of the lens array may be varied in order to trade off optical performance against ease of fabrication and test or availability of measurement data.

Claims (11)

1. CLAIMS1. A micro-optical element for use with an edge-emitting laser diode bar stack, the element comprising a plurality of biconvex fast-axis collimators formed as a monolithic array, providing an array of inlet lens forms on a first surface of the element and an array of outlet lens forms on a second surface, opposite the first surface, of the element.
2. 2. A micro-optical element according to claim 1 wherein the monolithic array is two-dimensional.
3. 3. A micro-optical element according to claim 1 wherein the monolithic array is one-dimensional.LU0
4. 4. A micro-optical element according to any preceding claim co wherein each lens form describes a lens shape, with each inlet lens form having a matching outlet lens form.
5. 5. A micro-optical element according to any preceding claim wherein each inlet lens form matches an emitter in the laser diode bar stack.
6. 6. A micro-optical element according to any one of claims 4 or 5 wherein the lens shape of each inlet lens form is acylindric.
7. 7. A micro-optical element according to any one of claims 4 or 6 wherein the lens shape of each outlet lens form is acylindric.
8. 8. A micro-optical element according to any preceding claim wherein the spacing between the lens forms in the fast-axis varies across the micro-optic element.
9. 9. A micro-optical element according to claim 8 wherein the spacing on the y-axis provides a pitch in the range 100 microns to 2mm.
10. 10. A micro-optical element according to any preceding claim wherein each lens form has an equivalent focal length between 200 microns and 1000 microns.
11. 11. A micro-optical element according to any preceding claim wherein each inlet lens form has a conic constant selected to optimise NA.LU12. A micro-optical element according to any preceding claim wherein each outlet lens form has a conic constant and a r radius of curvature selected to provide a desired divergence angle and a desired divergence profile.13. A micro-optical element according to claim 12 wherein the inlet lens has a conic constant selected to provide the desired divergence angle and the desired divergence profile.14. A micro-optical element according to any one of claim 12 or 13 wherein the desired divergence angle is near diffraction limit.15. A micro-optical element according to any one of claim 12 or 13 wherein the desired divergence angle is defocusseci.16. A micro-optical element according to claim 15 wherein the defocussed divergence angle is selected from a group comprising 3 degrees, 6 degrees and 10 degrees FWHM.17. A micro-optical element according to any one of claims 12 to 16 wherein the desired divergence profile is near Gaussian.18. A micro-optical element according to any one of claims 12 or 13 wherein the desired divergence profile is squeezed C, such that the profile is more flat-top than Gaussian.TI.-15 19. A micro-optical element according to any one of claims 12 or 13 wherein the desired divergence profile is stretched so as to be more peaky than Gaussian.20. A micro-optical element according to any one of claims 4 to 19 wherein a depth of the lens shape is in the range SObm to SOObm.21. A micro-optical element according to any preceding claim wherein the first and second surfaces of the element are smoothed and polished.22. A micro-optical element according to any preceding claim wherein each fast-axis collimator has characteristics determined by a corresponding individual emitter location in a measured laser diode bar stack.23. A micro-optical element according to any one of claims 4 to 22 wherein the lens shape is interpolated along the slow-axis to give a continuous transition between lens forms.24. A micro-optical element according to any preceding claim wherein the monolithic element is made of a low-index material.25. A micro-optical element according to any preceding claim wherein the monolithic element is made from fused silica.26. A micro-optical element according to any one of claims 1 to 23 wherein the monolithic element may be formed from a U) high-index material.27. A method of manufacturing a micro-optical element for use with a laser diode bar stack, comprising the steps: (a) providing a wavelength stabilized 002 laser with stable laser power operating on a laser line selected from the 002 spectrum; (b) providing a computer controlled acousto-optic modulator to give temporal control on laser pulses from the 002 laser; (c) providing a computer controlled X-Y translation stage; (d) locating a monolithic substrate on the translation stage; (e) operating the laser, acousto-optic modulator and the translation stage to ablate portions of the substrate in a shot-by-shot raster scan regime and form an array of predetermined lens forms on a first surface cf the substrate; (f) turning the substrate over; and (g) operating the laser, acousto-optic modulator and the translation stage to ablate portions of the substrate in a shot-by-shot raster scan regime and form an array of predetermined lens forms on a second surface of the substrate, opposite the first surface.28. A method of manufacturing a micro-optical element according to claim 27 wherein the method includes the step of aligning the first and second surfaces when the substrate is turned over.29. A method of manufacturing a micro-optical element according to any one of claim 27 or 28 wherein the substrate is fused silica.30. A method of manufacturing a micro-optical element according to any one of claims 27 to 29 wherein the r method includes the additional step of operating a laser, aoousto-optic modulator and the translation stage to melt the silica in a raster scan regime to laser micro-polish the surface of the micro-optical element.31. A method of manufacturing a micro-optical element according to any one of claims 27 to 30 wherein the method includes the step of analysing the emitter geometries of the laser diode bar stack to determine shapes for the lens forms reguired.