GB2417366A - Carrier for array of optical emitters - Google Patents

Abstract

A carrier for multiple monolithic semiconductor optical components facilitates precision mounting of the components so as to achieve a two-dimensional array of optical output beams. The carrier includes a first planar substrate region for positioning and supporting a first one of said optical components in a first plane and a second planar substrate region for positioning and supporting a second one of said optical components in a second plane different from the first plane. When optical components are mounted, a resulting device comprises a first and a second monolithic laser array, each monolithic laser array comprising a plurality of laser elements each extending generally in an x-z plane, the z-axis being defined substantially parallel to the optical axes of the laser elements. The first and second laser arrays are separated along the y-axis by being disposed on different regions of the carrier substrate in different x-z planes.

Description

24 1 7366

CARRIER FOR ARRAY OF OPTICAL

EMITTERS

The present invention relates to semiconductor laser arrays for producing plural output beams in a precise array configuration. In particular, though not exclusively, such laser arrays have applicability in telecommunications devices, graphics devices, image transfer devices such as print heads, imaging and display technologies, solid-state laser pumping and in optical pumping of arrays of vertical cavity surface emitting laser diodes.

A number of systems in the above technical fields require the production of an array of optical spots with at least some of the following characteristics: (i) output beams in the lowest order transverse mode with a Gaussian beam profile for each spot; (ii) a well-controlled spot size and beam divergence; (iii) spots that are located in space with sub-micron precision in two dimensions (x and y) orthogonal to the direction of beam propagation (z); (iv) a well-controlled mode with respect to the divergence of each beam in the array, the positions of the beam waists and other characteristics along the axes of propagation (z); and (v) independent control of the optical power in each beam.

For convenience, throughout the present specification, we shall refer to the x-direction as that which is parallel to the plane of the semiconductor laser array substrate and orthogonal to the beam direction; the y-direction as orthogonal to the plane of the substrate and orthogonal to the beam direction; and the z-direction as the direction of beam propagation.

UK patent application number 0321145.5, filed on 10 September 2003, describes a monolithic planar laser array having a plurality of parallel or near-parallel optical outputs in which the optical outputs are coupled into a corresponding monolithic array of waveguides. The waveguide array is positioned in relation to the laser array such that each laser output from the laser array couples into an input of a respective waveguide in the waveguide array. The planar laser array and planar waveguide array generally occupy a common x-z plane.

The waveguide array is configured to control the pitch or spacing of the optical output beams in the x-direction, for example maintaining the beam pitch, decreasing the beam pitch or increasing the beam pitch in the xdirection. The waveguide array may also be configured to control the alignment of each beam (e.g. its angle relative to the z-direction of the laser array output).

The waveguide array may also be configured to combine individual optical outputs of the laser array, for increasing the intensity of each output from the waveguide array. Alternatively, the waveguide array may be configured to divide individual optical outputs of the laser array, for increasing the density of output beams in the x-direction (or, stated another way, to reduce the pitch of optical outputs).

The optical systems described in GB 0321145.5 provide an array of output beams having sub-micron alignment that are particularly useful for print heads and other applications.

It is particularly desirable for some applications to provide a twodimensional array of output beams, and in particular to provide a high degree of precision in alignment of the output beams in not only the xdirection, but also in the y-direction. Preferably, the alignment accuracy of the output beams in the y-direction should be to the sub- micron level.

One approach to the formation of two dimensional laser arrays is to fabricate a monolithic laser array with multiple levels so that two or more layers of lasers are formed within the same substrate. A potential problem with this solution is that as the number of layers to be processed during device fabrication rises, and as the number of individual devices to be fabricated on the same substrate rises, the device yields reduce and the number of arrays in which devices are non- functional increases.

It is an object of the present invention to provide a carrier for multiple monolithic semiconductor optical components that facilitates precision mounting of the components so as to achieve a two-dimensional array of optical output beams.

It is a further object of the invention to provide a carrier for precision mounting of at least one waveguide array into which the two dimensional array of optical output beams from the laser arrays may be coupled.

According to one aspect, the present invention provides a carrier for supporting at least two monolithic semiconductor optical components, the carrier including a first planar substrate region for positioning and supporting a first one of said optical components in a first plane and a second planar substrate region for positioning and supporting a second one of said optical components in a second plane different from the first plane.

According to another aspect, the present invention provides a semiconductor laser array device comprising a first and a second monolithic laser array, each nonolithic laser array comprising a plurality of laser elements each extending generally in an x-z plane, the z-axis being defined substantially parallel to the optical axes of the laser elements, the first and second laser arrays being separated along the y-axis by being disposed on different regions of a carrier substrate in different x-z planes.

Embodiments of the present invention will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 shows a side elevation of a stepped carrier with a plurality of monolithic semiconductor lasers mounted thereon in a first orientation; Figure 2 shows a side elevation of a stepped carrier with a plurality of monolithic semiconductor lasers mounted thereon in a second orientation; Figure 3 shows a plan view of the stepped carrier of figure 1, showing possible disposition of bonding pads; Figure 4 shows a plan view of a stepped carrier similar to that of figure 2, showing possible disposition of bonding pads; Figure 5 shows a schematic view of the stepped carrier of figure 1 in conjunction with a waveguide array for providing lateral position adjustment of the laser array outputs; Figure 6 shows a schematic view of the stepped carrier of figure 2 in conjunction with a waveguide array for providing optical output at a planar output face; Figure 7 shows a schematic plan view of a prior art optical system comprising a pair of laser diode arrays and a waveguide array; and Figure 8 shows a perspective schematic view of the optical system of figure 7.

With reference to figures 7 and 8, the optical system 1 of GB 0321145.5 comprises one or more laser diode arrays 2, 3 each including a plurality of laser diodes 4 each having an optical output 5 in an output facet 6. An array lO of optical waveguides 11 fonned in a single substrate 12 provides a waveguide 11 having an input 13 and an output]4 for each laser diode output 5. The one or more laser diode arrays 2, 3 and the waveguide array are precisely positioned relative to one another on a common planar substrate 15.

The common substrate 15 is typically a packaging substrate to which the laser diode arrays 2, 3 and waveguide array 10 may be bonded using conventional die bonding techniques.

In many applications, each of the laser diodes 4 in the arrays 2, 3 is adapted to operate in a single transverse mode of operation. In this case, each of the waveguides 1 1 in the waveguide array 10 is particularly adapted to maintain, at the outputs 14 thereof, the single transverse mode of any optical beams presented at the inputs 13 thereof. A single transverse mode in the waveguides can be sustained by appropriate selection of refractive index contrast and thickness of materials.

As shown particularly in figure 8, the ease of alignment of the or each laser diode array 2 with the respective waveguide array 10 may be significantly improved by the formation of a tapered section at the waveguide input 13 such that the diameter of each waveguide input 13 is greater than the diameter of each waveguide output 14. As shown in the example, the taper may extend only a short distance into the waveguide. The taper may exist in either the x or y direction, or both for easing both lateral and vertical alignment.

Each waveguide 11 may include mode filters and beam shapers to take into account any mode instabilities and beam steer that might otherwise occur across the array during operation. Examples of such mode filters include, for example, a tapered structure to filter out any tendency to multimode operation. In another example, a beam shaper may be incorporated to s convert an elliptical optical beam output of a laser into a circular beam by providing a waveguide channel that allows one axis to expand slightly.

In one arrangement, the waveguide array 10 may include a number of photodiode structures 20 for monitoring power output of a respective waveguide 11. Each photodiode structure is coupled to a feedback loop 21 including a sensing circuit 22 for determining the power output. The sensing circuit 22 supplies a control signal 23 to a drive circuit 24 used to power the laser diode array 2. The feedback loop thereby enables the laser diode drive circuit 24 to tune the power output of each laser diode 4 in the array 2 to maintain a substantially constant power output from the or each element in the array.

The semiconductor laser arrays 2, 3 of figures 7 and 8 effectively comprise a one-dimensional array of n laser beams in which the optical package delivers a one-dimensional array of n optical beams parallel to the z-axis and in the same (x-y) plane.

With reference now to figures 1 to 6, the present invention provides for the positioning of a number m of linear one-dimensional arrays, each of n laser beam outputs, to a two dimensional array of n x m laser beam outputs on a single carrier.

With reference to figure 1, a stepped carrier 30 provides a 'staircase' of substrate regions 31...34 onto each of which is bonded a respective one of a plurality of monolithic semiconductor laser arrays 41...44. Each laser array 41...44 comprises a plurality of laser elements (e.g. 50, 51). The laser arrays 41...44 are bonded onto the stepped carrier 30 using conventional die bond techniques, such as conventional gold / tin solder teclmiques, in such a manner as to control parallelism with an appropriate alignment accuracy.

The substrate regions 31...34 of the stepped carrier 30 are preferably fabricated from conventional semiconductor chip carrier materials, such as ceramic, aluminium nitride, or metal (e.g. copper-tungsten).

In the arrangement of figure 1, the laser arrays 41...44 are oriented on the stepped carrier 30 such that their optical output axes (z) extend out from the carrier on axes that are orthogonal to the axis of the staircase (x).

Stated more generally, the embodiment of figure l represents an example of a laser array device having a plurality of monolithic laser arrays mounted thereon, in which each laser array comprises a plurality of laser elements each extending generally in an x-z plane, the z-axis being defined substantially parallel to the optical axes of the laser elements, and in which the plurality of laser elements are disposed on separate regions of the carrier substrate, which regions are separated on the x- and y-axes but substantially coincident on the z-axis. This provides an optical output comprising a two dimensional array of spots distributed in x and y.

With reference to figure 2, a stepped carrier 50 provides a 'staircase' of substrate regions 51...54 onto each of which is bonded a respective one of a plurality of monolithic semiconductor laser arrays 61...64. Each laser array 61...64 comprises a plurality of laser elements. The laser arrays 61...64 are bonded onto the stepped carrier 50 using conventional die bond techniques, such as conventional gold / tin solder techniques, in such a manner as to control parallelism with an appropriate alignment accuracy. The substrate regions 51...54 of the stepped carrier 50 are preferably fabricated from conventional semiconductor chip carrier materials, such as ceramic, aluminium nitride, or metal (e.g. copper-tungsten).

In the arrangement of figure 2, the laser arrays are oriented on the stepped carrier 50 such that their optical output axes (z) extend out from the carrier on axes that are parallel to the axis of the staircase (z).

Stated more generally, the embodiment of figure 2 represents an example of a laser array device having a plurality of monolithic laser arrays mounted thereon, in which each laser array comprises a plurality of laser elements each extending generally in an x-z plane, the z-axis being defined substantially parallel to the optical axes of the laser elements, and in which the plurality of laser elements are disposed on separate regions of the carrier substrate, which regions are separated on the y- and z-axes but substantially coincident on the x-axis. This again provides an optical output comprising a two dimensional array of spots in x and y.

To assist in accurate positioning of each laser array within the x-z plane, the carrier substrate regions 31...34 and 51...54 may be provided with alignment fiducials or other markings or positioning features.

The periphery of the stepped carriers 30, 50 may be provided with wire bond pads for suitable wire bonding to bond pads on the laser arrays.

Figure 3 shows schematically a plan view of an exemplary layout for the stepped carrier of figure 1. Bond pads 70 may occupy outboard rows 71, 72 that are parallel to the optical axes of the laser arrays and along a lateral edge of the outermost arrays. Bond pads 70 may further occupy inboard rows 73, 74, 75 that are also parallel to the optical axes of the laser arrays and along a lateral edge of inside ones of the arrays. Alternatively, or in addition, bond pads 70 may occupy a back row 76 that is orthogonal to the optical output axes, but 'behind' the laser arrays so as not to interfere with the optical output.

As with conventional carrier substrates, the wire bond pads 70 may occupy ledges that stand proud of the substrate regions 31...34 of the carrier 30 upon which the laser array is bonded. In other words, the wire bond attachment pads 70 may occupy different, and preferably higher, planes than the plane of substrate region of the laser array to which they relate. The wire bond pads 70 may occupy double rows, with one row (usually an outer row) being slightly higher than the other (inner) row, to allow for 'reach-over' wire bonding.

Figure 4 shows schematically a plan view of an exemplary layout for a stepped carrier similar to the stepped carrier of figure 2. In this layout, it will be noted that multiple laser arrays (e.g. 61a...61d) may be used side by side on each stepped region 51...54 of the carrier 50. This is, in principle, equivalent to providing a single long laser array on each stepped region of the carrier, but offers advantages in yield by using smaller laser arrays.

Bond pads 70 may occupy outboard rows 81, 82 that are parallel to the optical axes of the laser arrays and along a lateral edge of the outermost arrays. Bond pads 70 may further occupy inboard rows 83, 84, 85 that are also parallel to the optical axes of the laser arrays and along a lateral edge of inside ones of the arrays. Alternatively, or in addition, bond pads 70 may occupy a back row 86 that is orthogonal to the optical output axes, but behind' the laser arrays so as not to interfere with the optical output.

As with conventional carrier substrates, the wire bond pads 70 may occupy ledges that stand proud of the substrate regions 51...54 of the carrier 50 upon which the laser array is bonded. The wire bond pads 70 may occupy double rows, with one row (usually an outer row) being slightly higher than the other (inner) row, to allow for 'reach-over' wire bonding.

The wire bond pads 70 may be corrected to any suitable arrangement of external connectors on the carrier, such as edge connectors, pin or ball grid array connectors and the like.

Preferably, two further issues are addressed in connection with positioning laser arrays 41...44 and 61...64 on a carrier substrate as discussed above.

With reference to figure 1, it will be noted that the two dimensional array of laser optical outputs are distributed in x and y, but are not in a regular 'square grid' array, i.e. one in which each monolithic laser array occupies coextensive 'x-space' to other ones of the monolithic laser arrays.

Frequently, it is desirable to provide a regular square grid or rectangular grid array of optical outputs. This can be achieved in connection with the array orientation of figure 1 by the use of an appropriate waveguide array as shown in figure 5.

A monolithic waveguide array 90 is used to provide selective displacement of the optical beams in the x-direction so as to converge the optical outputs of the various laser arrays 41, 42, 43 into a regular two dimensional array 91 at the output of the waveguide 90. The individual laser element outputs may be coupled into the monolithic waveguide 90 in similar manner to that described in connection with figure 8, and as generally described in GB 0321 145.5.

Preferably, the monolithic waveguide array 90 is mounted on the same carrier 30 substrate as the laser arrays 41...43, although in a region of the carrier substrate without the 'staircase'. In a preferred arrangement, the planar substrate region that carries the waveguide array is adjacent to each of the steps of 'staircase' as shown in figure 5, and at approximately the same level as the lowest level of the 'staircase'.

By contrast, the array configuration of figures 2 and 4 can, of course, already provide a regular square grid or rectangular grid array of laser outputs distributed in x and y. However, in this embodiment it will be noted that the distance from the laser element output facets to a target plane 89 (figure 4) is different for each laser array 61...64. If appropriate control of beam shape and spot intensity distribution at the target plane 89 cannot be achieved by design of the individual laser arrays 61 64, or by electronic conko1 of the individual laser arrays 61... 64, beam shape and/or mode control can be effected by a waveguide array such as shown in figure 6.

With reference to figure 6, a waveguide array 100 comprises a stack of planar waveguide arrays 101 104 of varying length each adapted to 'reach through' to a respective one or more of the laser arrays 61...64. Thus, each planar waveguide array 101...104 extends from its input end or facet lOla...104a approximately at or adjacent to the corresponding laser array 61...64 output to a substantially planar output facet 106. Of course, output facet 106 need not be planar if required to control output beam distribution differently.

Preferably, the waveguide array 100 is mounted on the same carrier SO substrate as the laser arrays 61...64, although in a region of the carrier substrate without the 'staircase'. In a preferred arrangement, the planar substrate region that carries the waveguide array is adjacent to the lowest step of the 'staircase', and at approximately the same level as the lowest level ofthe 'staircase'.

The waveguide arrays 90, lOO may be fabricated substantially in ferroelectric materials such as lithium niobate or lithium tantalate using conventional techniques including epitaxy, diffusion of ions including Ti or protons to modify the refractive index, etching etc. The waveguide arrays 90, 100 may be fabricated substantially in silicon, silicon dioxide, silicon nitride, polymer or other dielectric materials.

Standard photolithography and etch techniques may be used to define waveguide channels in one dimension. The waveguide arrays 90, 100 may be fabricated in III-V materials or other semiconductors. The waveguide arrays 90, 100 may be fabricated with hollow waveguides, clad with metal or other materials, in which the light is confined substantially to the hollow region.

The waveguide arrays 90, 100 may include passive waveguide platforms in which optical elements such as lenses, mode filters and photodiodes are located within the passive waveguides.

The waveguide array l OO may be fabricated as independent planar waveguide arrays 101 104 which are then bonded together, or it may be fabricated as a unitary multilayer structure using known deposition, etch and planarization techniques.

The laser arrays 41...44 and 61...64 and the waveguide arrays 90, 100 can be positioned onto their respective substrate regions of the carriers 30, 50 with submicron accuracy in x and z position, to give the desired accuracy of alignment of laser beam with waveguide, using conventional die attach processes. Alignment fiducials or other reference marks or structures may be used. More particularly, the carrier substrate regions 31...34 and 51...54 can readily be machined or otherwise fabricated to provide the sub-micron accuracy in y position required and with the required co-planarity to achieve correct alignment of the beam angles with respect to the x-z plane.

Thus, using the techniques as described above, it is possible to achieve a laser output spot distribution in an x-y plane in which the positional accuracy of the spots, in the x-y plane, can be achieved to sub-micron precision. More particularly, the variability in pitch, spot-to-spot, across the array in both the x and y directions can be controlled to less than 1 micron.

In some embodiments, this accuracy may be achieved directly with the laser arrays; in other embodiments, this accuracy may be achieved using at least one waveguide array in conjunction with the laser arrays.

Although in most embodiments, it is important to achieve a high degree of co-planarity of the planar substrate regions upon which the individual laser arrays will be mounted, it will be understood that there could be applications in which the substrate regions are deliberately formed to have divergent or convergent planarity relative to the z-axis so as to effect divergence or convergence of the laser beams propagating generally in the z-direction.

For alternative beam array configurations (e.g. non-rectangular grid arrays), the planar substrate regions may be canted at one or more angles relative to the base of the carrier.

A particularly important application of the present invention is in the formation of print heads for optical / thermal printers. It has been recognised that it would be desirable to provide print heads with arrays of laser beam outputs of different optical characteristics, in particular different wavelength or colour.

In preferred embodiments, the each of the laser arrays 41...44 and 61... 64 provides a different wavelength output. Thus, with reference to figure 5, laser array 41 provides output beams of a first colour output, laser array 42 provides output beams of a second colour output, and laser array 43 provides output beams of a third colour output. With reference to figure 6, laser array 61 provides output beams of a first colour output (74), laser array 62 provides output beams of a second colour output (73), laser array 63 provides output beams of a third colour output (72), and laser array 64 provides output beams of a fourth colour output (Al).

State of the art ink technology can provide primary colour inks that are sensitive to different wavelengths of optical radiation. The primary colour inks can be deposited in multiple layers onto a print surface, such as paper, and activated by laser radiation. A laser beam of wavelength, il. will activate the dye and produce a primary colour dot. A laser beam of wavelength, 72, will activate a second dye and produce a second primary colour and so on.

Raster controlled switching of each laser defines the image. The size and density of the dots defines the 'quality' of the image. The carrier and waveguide arrays described above can position each laser beam to submicron accuracy and hence full colour, very high dpi resolution can be obtained with a print engine that is low cost, robust, easy to mass produce and that has small form-factor.

Such arrays can also be used in WDM telecom applications.

The dimensions of the optical systems described above, the number of laser elements and the performance of the laser array can readily be varied according to the optical system application.

The waveguide arrays 90, 100 may also incorporate various beam array pitch control and intensity conko1 techniques as described in GB 0321145. 5.

For example, the waveguide array may be configured to divide each or some of the laser diode outputs into a plurality of distinct output beams to increase array spot density.

The waveguide array may be configured to reduce the pitch of the output beams from the laser diode arrays by configuring each of the waveguides so that they extend generally in the beam propagation direction (z) but at slightly varying angles to the indirection in order to provide a degree of convergence of the varying beams passing therethrough. In this way, the waveguide array acts as a pitch reduction waveguide. The waveguides can be straight, curved or angled to provide the pitch change.

Similarly, the waveguide array can be configured to provide a pitch expansion waveguide. The waveguide array may be configured to combine the optical power of the outputs of several laser diodes into a single output beam. This is achieved by fabricating the waveguide array to include a plurality of input paths for each respective output path.

The waveguide arrays may be optically passive, i.e. not electrically driven to amplify or modulate light passing therethrough, or may include optically passive structures (e.g. for modulation).

The waveguide arrays may be fabricated such that light mainly propagates only in the lowest order or fundamental transverse mode of the waveguide.

Other embodiments are intentionally within the scope of the accompanying claims.

Claims (25)

1. A carrier for supporting at least two monolithic semiconductor optical components, the carrier including a first planar substrate region for positioning and supporting a first one of said optical components in a first plane and a second planar substrate region for positioning and supporting a second one of said optical components in a second plane different from the first plane.

2. The carrier of claim 1 including wire bond attachment pads disposed adjacent to said first and second planar substrate regions.

3. The carrier of claim 2 in which the wire bond attachment pads for the first and second planar substrate regions are positioned in different planes respectively corresponding to the first and second planes.

4. The carrier of claim 1 including three or more planar substrate regions, each in a different plane, each for positioning and supporting a respective optical component.

5. The carrier of claim 4 in which the planar substrate regions form a staircase' configuration on the carrier.

6. The carrier of claim 1, claim 2 or claim 3 in which each of the planar substrate regions occupy parallel planes.

7. The carrier of any one of claims I to 6 further including a further planar substrate region for supporting a waveguide array.

8. The carrier of claim 7 in which the further planar substrate region is adjacent to and substantially co-planar with the first planar substrate region.

9. The carrier of claim 7 in which the further planar substrate region is adjacent to and substantially co-planar with each of the other substrate regions.

10. A semiconductor laser array device comprising a first and a second monolithic laser array, each monolithic laser array comprising a plurality of laser elements each extending generally in an x-z plane, the z-axis being defined substantially parallel to the optical axes of the laser elements; the first and second laser arrays being separated along the y-axis by being disposed on different regions of a carrier substrate in different x-z planes.

I 1. The laser array device of claim 10 in which the first and second laser arrays are disposed on the substrate separated on the y- and z-axes but substantially coincident on the x-axis to produce an optical output comprising a two dimensional array of spots in x and y.

12. The laser array device of claim l O in which the first and second laser arrays are disposed separated on the x- and z-axes but substantially coincident on the y-axis to produce an optical output comprising a plurality of spots distributed in x and y.

13. The laser array device of any one of claims 10, 11 and 12 in which the laser elements of each laser array have exactly parallel optical outputs to produce a spot distribution with a pitch equal to the pitch of the array elements.

14. The laser array device of any one of claims 10, 11 and 12 in which the laser elements of each array have slightly convergent optical outputs to produce a spot distribution with a pitch smaller than the pitch of the array elements.

15. The laser array device of claim 10 further including a waveguide array having a plurality of waveguides extending between an input end and an output end, the input end adapted to receive each of the laser element outputs from the laser arrays into a waveguide.

16. The laser array device of claim 1 S in which the input end comprises a planar facet to receive optical outputs from the laser arrays distributed in x and y but in the same z-plane.

17. The laser array device of claim 15 in which the input end comprises a stepped facet to receive optical outputs from the laser arrays distributed in x and y, and also in different z-planes.

18. The laser array device of claim 16 in which the waveguide array comprises waveguides adapted to provide for beam convergence in at least the x-direction.

19. The laser array device of any one of claims 15 to 18 in which the waveguide array is mounted on the carrier substrate.

20. The laser array device of claim 19 in which the waveguide array is mounted onto the carrier substrate in a region adjacent to at least one of said different regions.

21. The laser array device of claim 20 in which the waveguide array is mounted onto the carrier substrate in a region adjacent to all of said different regions.

22. The laser array device of any one of claims 10 to 21 in which the optical output spot distribution comprises an array in an x-y plane, in which the spot pitch variability across the array in both x and y directions is less than 1 micron.

lO

23. A print head comprising the laser array device of any one of claims lOto22.

24. A carrier substantially as described herein with reference to the accompanying figures l to 6.

25. A semiconductor laser array device substantially as described herein with reference to the accompanying figures 1 to 6.