WO2002095457A2

WO2002095457A2 - Method of defining grating patterns for optical waveguide devices

Info

Publication number: WO2002095457A2
Application number: PCT/GB2002/002422
Authority: WO
Inventors: Douglas Charles John Reid; Elizabeth May Ward; Neil William Forbes
Original assignee: Bookham Technology Plc
Priority date: 2001-05-22
Filing date: 2002-05-22
Publication date: 2002-11-28
Also published as: WO2002095457A3

Abstract

The method comprises exposing a photoresist coated wafer (8) twice to a source of coherent light adapted to imprint a grid into the photoresist. In between the two exposures, the wafer is rotated through a predetermined angle ζ whereby two interesting grids (30, 32) are formed. Development of the result produces a plurality of points (33) where the grid intersections occur. These points form a number of Bragg gratings, the pitch (Μv, Μh) of which is determined by the angle ζ. Hence the process may be used to produce a wide variety of Bragg gratings simply by rotating the substrate wafer through a predetermined angle, thereby enabling a much faster procedure.

Description

METHOD OF DEFINING GRATING PATTERNS FOR OPTICAL WAVEGUIDE DEVICES

TECHNICAL FIELD

The present invention relates to a method of defining grating patterns for^' optical waveguide devices. More particularly, but not exclusively, it relates to a holographic process for creating a plurality of Bragg gratings within an optical waveguide device. The gratings thus formed may be part of a DFB laser and tuneable DBR laser for use with a wavelength division multiplex (WDM) optical communication system.

BACKGROUND ART

In order to increase the information carrying capability of optical fibre communication systems, wavelength division (WDM) systems are being developed in which a number of different wavelengths, often termed , wavelength channels, may be carried over a single optical fibre. As is known there are two principal communications wavelength bands, at 1300nm, and 1550nm, of which the latter band is receiving the greater commercial exploitation because of its suitability for a variety of different generic communication applications. Currently 1550nm WDM communication systems are evolving into architectures comprising eighty 2.5Gb/s modulated wavelength channels on a single fibre. The same 1550nm band is also being proposed for forty 10Gb/s modulated channels thereby doubling the fibre information capacity.

To provide forty or eighty individual wavelength circuits within the 1550nm band requires a light source that can be accurately set to specific wavelengths and maintained at these wavelengths, within specified limits, over the equipment operating life. Such light sources historically comprise a plurality of distributed Bragg reflector (DBR) lasers, each of which is operated to produce light of one of the wavelength channels and optical switching means for selecting a required wavelength channel. Whilst such an arrangement is effective it is impracticable for more than ten or so wavelength channels.

To overcome this problem wide tuning range lasers are being developed, such as sampled grating distributed Bragg reflector (SGDBR) lasers as described in A M Gulisano, D J Robbins, P J Williams, and P Verhoeve (1997) "Widely Tuneable Sampled Grating DBR Lasers to address 100 channels over 40nm for WDM applications", Proc. Euro. Conference on Integrated Optics. The tuning mechanism of the SGDBR laser is by differential current steering of the operating wavelength by means of the currents injected into the front and rear grating Bragg reflector sections, with fine tuning (trimming) being possible by means of current control in the phase section in combination with ganged current injection in the front and rear Bragg grating sections.

An explanation of the operation of a Bragg grating can be found in "Optical Fiber Communications", J M Senior, Prentice Hall International Series in Optoelectronics ISBN 0-13-635426-2.

Figure 1 illustrates the prior art and shows an optical waveguide 330 being part of a semiconductor laser 300. A grating 320 is illuminated with light of wavelength λi that propagates along the waveguide 330. Such a structure acts as a diffraction grating, and light that satisfies the Bragg condition may be reflected back along the original path. The wavelength λi of light that will be reflected back along its original path is given by: -

where n_e is the effective refractive index of the waveguide material 330, Λ is the pitch of the grating , and m is the number of wavelengths λi in 2 x Λ, i.e. 1 , 2, 3, 4 etc. m may sometimes be referred to as the order of the grating and for many applications m is unity and thus equation (1) simplifies to λi = 2 x n_e x Λ. A necessary condition for reliable lasing is that a single mode of light travels through the waveguide.

Thus the total reflection wavelength is directly proportional to the pitch or periodicity Λ of the grating. At this wavelength, for a given set of structure conditions, substantially all the incident light power is totally reflected. Bragg gratings are therefore widely used as wavelength selective filters and in particular as mirrors in semiconductor DBR and wide bandwidth tuneable lasers.

An SGDBR laser has two grating structures: a front grating, and a rear grating. The fabrication of semiconductor lasers uses wafers of Group III - V semiconductor substrate, in particular Indium Phosphide (InP). The specific laser structures can be grown onto the wafers using Low Pressure Metal Organic Vapour Epitaxy or other processes. Various dopant materials may be grown into the substrate, and various materials are grown onto the substrate. These materials and dopants typically comprise Gallium and Arsenic. For the purposes of this patent specification the term substrate means the base semiconductor material and such growth layers as pertain to the device in fabrication, depending upon the stage reached in its fabrication process. Typical laser devices are indium Gallium Arsenide Phosphide structures. It is not necessary for an understanding of this patent specification to detail the precise architecture of various optical waveguide devices; these will be known to those skilled in the art. The monolithic structures are fabricated in many hundreds on circular semiconductor substrates typically of 50, 100 or 155 mm diameter, commonly known as wafers. Typically laser devices are grown in bars across the wafer, which is then cleaved into individual devices. The pitch of gratings for photonic applications are sub-micron, for example 200 - 300nm, and are therefore not readily amenable to being defined using standard photolithography techniques. Electron beam techniques can be used to write a grating directly onto a photoresist coating covering the wafer, but this is very costly as a production process. Alternative production means are required for producing gratings which are to be manufactured in large quantities.

A process is known form a paper published in 1982 by The Aerospace Corporation - Electronics Research Laboratory, (Optical Engineering, SPIE, May/June 1982, volume 21 No. 3 "Sub-micron grating fabrication on GaAs by holographic exposure"), which involves cleaning the wafer substrate, applying a high definition photoresist coating (positive or negative), making a holographic exposure in a special precision aligned rig, developing the resist to leave the desired mask pattern and then transferring the grating to the wafer by means of dry or wet etching.

Figure 2 shows diagrammatically the principle, which is known in the art, that two intersecting coherent plane wavefront beams of optical energy 2 and 4 interfere to produce fringes. In the area of intersection the wavefronts interfere to produce a sinusoidal pattern of light and dark bands 6. The pitch Λ_O of the sinusoidal pattern is given by: -

Λ₀ = λ₀/ (2 x Sin θ/2) (2)

where λo is the wavelength of the optical energy, and θ is the angle between the two beams of optical energy, Λ_O is hereinafter, referred to as the native pitch.

From equation (2) it can be seen that the pitch ΛO of the sinusoidal pattern of light and dark bands (fringes) is inversely proportional to the angle θ and thus as the angle increases so the pitch decreases, and conversely as the angle decreases the pitch increases.

From equation (1) it can be seen that by changing the pitch Λ of the gratings, the Bragg wavelength of the fringes may be varied.

Exposure to electromagnetic radiation in the manner described above, defines a grating pattern in the photoresist coating. A physical realisation of this pattern is not apparent until the photoresist is developed. Until then, it exists as a latent pattern of areas whose properties have been altered by the electromagnetic irradiation. The term "pattern" used herein is to be understood as including such latent patterns.

As described above, interference fringes can thus be used to produce sub- micron gratings on a wafer that has been coated with photoresist. Figure 3 illustrates diagrammatically the prior art effect on a substrate wafer 8a prepared with a surface layer of photoresist which may be introduced into the interference fringe domain. The resultant interference grating 6 is shown on a plan view of the wafer 8b, after the photoresist has been developed and the grating etched. The wafer 8a and 8b may, for some applications, be a dielectric material. Clearly, when fabricating monolithic laser devices a grating over the whole area of the wafer is not usually required. Those areas of the wafer not requiring a grating can be created either by secondary etching or spatial masking.

An existing practical arrangement for producing gratings on a substrate wafer is shown in Figure 4. This is a known holographic interferometer arrangement that will be explained by way of an. example to produce a 240nm pitch grating in Indium Gallium Arsenide Phosphide, for use with a target semiconductor laser operating in the 1550nm band. With reference to Figure 4, 10 is an Argon Ion Ultra Violet laser emitting laser light at 351.1 nm. The emitted beam 11 is reflected by a high efficiency mirror 12 to produce beam 13 which is half reflected by 50:50 beam splitter 14 to form beam 15, while the balance of the light forms beam 19. Beam 15 illuminates a focusing lens 22 which passes the collimated light through a pin hole 24. Together 22 and 24 form a spatial filter. The resultant divergent beam 17 is reflected by a high efficiency mirror 18 to illuminate the photoresist coated target wafer 8a. Similarly, beam 19 enters an equivalent spatial filter comprising a focusing lens 26, and a pin hole 28, and the resultant divergent beam 21 illuminates the target wafer 8a after reflection by high efficiency mirror 16. The angle between the two light beams 17 and 21 is 0. The resultant sinusoidal interference grating 6 is shown for clarity of explanation.

The process relies upon just one time controlled exposure of the target wafer to give the required degree of photoresist hardness for the development and etch time. As a consequence of the beams 21 and 17 being divergent wavefronts the pitch of the interference grating progressively increases away from the centre of the target wafer. However, by appropriate design of the optical system the magnitude of this increase is very small compared to the pitch of the sinusoidal interference grating being recorded, and as such is within the tolerances required for all practical purposes.

Putting Λ_O as 240nm, and λo as 351.1nm, into equation (2) and solving for θ gives a result of θ = 94°.

Semiconductor lasers are monolithic devices grown with epitaxial layer deposition. Laser wafer gratings need to be orthogonal to the direction of optical power propagation. Thus wafers for optical device manufacture must have a known orientation. Figure 5 shows a wafer 8b whose orientation is known by reference to quadrature major 7, and minor 9, flats on the circumference. As a rule the major flat 7 is substantially parallel to the crystallographic structure of the wafer. For optimum performance Bragg grating rulings (fringes) need to be substantially parallel to the crystallographic structure of the wafer. Therefore, in one convention, optical grating grooves 6 are fabricated to be parallel with the major flat 7 on the wafer, and the laser bars 23, each containing a plurality of lasers 27, are parallel to the major flat. For the purposes of clarity, the plurality of bars 23 across the wafer is not shown.

The holographic arrangement as described with reference to Figure 4 needs to be precision aligned on an optical bench. The process of getting the right interference gratings typically takes many days of set up and producing test grating samples for inspection using an electron microscope.

Once set up, the holographic interferometer process for producing optical wavelength Bragg gratings works well, but it is not possible to make a quick change over if another grating pitch is required. For example with the arrangement of Figure 4., to produce a grating for use in the 1300nm band requires θ to be approximately 120° to give a grating pitch of nominally 200nm for use with a typical indium Gallium Arsenide Phosphide laser structure. This complex alignment situation is exacerbated by the need normally for the mark space ratio of the grating to be one to one, which requires multiple trials of exposure time, photoresist development, etch times, and electron microscope measurement to achieve the desired result.

It is an object of the present invention to provide a process for creating holographic interference gratings in which changing the resultant Bragg grating wavelength is straightforward and rapid. The process should be ideal for large scale volume production of optical waveguide devices incorporating Bragg gratings. SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided a method of defining grating patterns for an optical waveguide device comprising the steps of providing a target substrate having a photosensitive coating, providing means to irradiate said coating with two coherent electromagnetic beams, exposing the coating to such irradiation to define a first a grating pattern therein, rotating said target substrate surface through a predetermined angular offset and exposing said coating a second time to define therein a second grating pattern which intersects said first grating pattern so as to form a Bragg lattice grating pattern.

The photosensitive coating is preferably photoresist.

The target substrate may be a dielectric material or preferably may be a semiconductor material, such as layers of Indium Gallium Arsenide Phosphide.

The photosensitive coating may be developed to establish in the substrate a pianar grating structure derived from the lattice grating pattern, and the exposure times and/or development time of the coating may be varied to give a planar pattern of the resultant grating structure which is substantially an array of rectangular forms, circular forms, elliptical forms or square forms.

Between the first and second exposures, in some embodiments, measures may be taken to make the grating pitch created by the second exposure dissimilar to the pitch created by the first exposure, such measures comprising, for example:

- displacement of the target substrate along the axis of the bisector of the two coherent beams; and/or

- non-orthogonal rotation of the target substrate; and/or

- tilt of the target substrate, and/or - shift of the target substrate in the plane of the target substrate surface.

The duration times of the first and second exposures may be dissimilar whilst keeping the overall exposure time constant, whereby there is a blazing of the defined lattice grating.

According to a second aspect of the present invention, there is provided a Bragg lattice grating definedby a method as described above wherein the pitch of the grating depends upon the wavelength of the two interfering electromagnetic beams, upon the angle between the two interfering electromagnetic beams and upon the angular offset, between the first and second exposure, of the target substrate surface.

According to a third aspect of the present invention, there is provided a method of defining grating patterns for an optical waveguide device comprising the steps of providing a target substrate, providing means to irradiate said substrate with two coherent electromagnetic beams, exposing the substrate to such irradiation to define a first a grating pattern therein, rotating said target substrate through a predetermined angular offset and exposing said substrate a second time to define therein a second grating pattern which intersects said first grating pattern so as to form a Bragg lattice grating pattern.

According to a fourth aspect of the present invention, there is provided a Bragg lattice grating created by the above method wherein the pitch of the grating depends upon the wavelength of the two interfering electromagnetic beams, upon the angle between the two interfering electromagnetic beams and upon the angular offset, between the first and second exposure, of the target substrate. In this case, the pitch ΛM₍I -_{1 O)} of the grating M(1 -1 0) may be given by the equation. ^■-

Λ (I -_{1 O}) = Λ_O / ( 2 x Sin φ/2)

where Λ_O is the native pitch of the holographic interference fringe and φ is the angular offset of the target substrate surface between the first and second exposures.

Alternatively, the pitch ΛM(I _{1 O}) of the grating M(1 1 0) may be given by the equation:- M(I 1 o) = ΛO / ( 2 x Cos φ/2).

Where Λ₀ is the native pitch of the holographic interference fringe and φ is the angular offset of the target substrate surface between the first and second exposures.

Other possibilities include the pitches Λ (-21 _O) and ΛM(I -2 o) of the gratings M(-2 1 0) and M(1 -2 0) being given by the equations:-

ΛM(-21 o) = Λ (I -20) = ΛO / ( 1 + 8 x Sin φ/2 )

where ΛO is the native pitch of the holographic interference fringe and φ is the angular offset of the target substrate surface between the first and second exposures, and

the pitches ΛM(2 1 o) and ΛM ₂ o) of the gratings M(2 1 0) and M(1 2 0) being given by the equations:-

Λ_M(2 1 0) = Λ_M(1 20) = Λ₀ / ( 1 + 8 X CθS²φ/2 )^{1 2} where Λ_O is the native pitch of the holographic interference fringe and φ is the angular offset of the target substrate surface between the first and second exposures.

The defined grating structure may be etched into the target substrate surface at such a depth as to strengthen the effect of the grating upon incident waveguided electromagnetic energy.

The grating rulings, or fringes, are preferably substantially parallel to the crystallographic structure of the target substrate.

Other Bragg gratings may be created in which:

the pitch ΛM₍M _O) of the grating M(1 -1 0) is given by the equation:-

Λ_M(M o) = 1 / ( ( 1/ΛO' f + ( 1/ΛO" f - ( 2 x Sin ψ )/ ( Λ₀' X Λ₀" ) )^{1 2}

where ΛO is the native pitch of the first grating, Λ₀" is the native pitch of the second grating, and φ is the angular offset of the target substrate between the first and second exposures;

the pitch ΛM₍I _{1 O)} of the grating M(1 1 0) is given by the equation:-

Λ_M(1 ι o) = 1 / ( ( 1/ΛO' )² + ( 1/ΛO" f - ( 2 x Cos φ )/ ( Λ₀' x Λ₀" ) )^{1 2}

where Λ_O' is the native pitch of the first grating Λ₀" is the native pitch of the second grating, and φ is the angular offset of the target substrate between the first and second exposures;

the pitch A _@ -I _O) of the grating M(2 -1 0) is given by the equation:- Λ_M(2 -1 o) = 1 / ( (1/ΛO' f + ( 2/ΛO")² + ( 4 x Cos φ )/ ( ΛO' X ΛO" ) )^1/2

where Λ₀' is the native pitch of the first grating, Λ₀" is the native pitch of the second grating, and φ is the angular offset of the target substrate between the first and second exposures;

the pitch Λ ₍-I ₂₀₎ of the grating M(-120) is given by the equation: -

Λ_M(-120)= 1 / ( ( 2/ΛO' )²+ ( 1/ΛO" f + ( 4 x Cos φ )/ ( Λ₀' X ΛO" ) )¹²

where Λ_O' is the native pitch of the first grating, Λ₀" is the native pitch of the second grating, and φ is the angular offset of the target substrate between the first and second exposures;

the pitch ΛM_{@ 1} o₎ of the M(210) is given by the equation:-

Λ_M(21O)= 1 /((1/ΛO')²+(2/Λ₀")²- (4xCosφ)/(Λ₀ ^,x Λ₀"))¹²

where Λ_O' is the native pitch of the first grating, Λ₀" is the native pitch of the second grating, and φ is the anguiar offset of the target substrate between the first and second exposures; or

the pitch ΛM₍I ₂ o) of the grating M(120) is given by the equation: -

Λ_M(120)= 1 /((2/ΛO')²+( 1/Λ₀")²- (4xCosφ)/(Λ₀'x Λ₀"))^1/2

where Λ₀' is the native pitch of the first grating, Λ₀" is the native pitch of the second grating, and φ is the angular offset of the target substrate between the first and second exposures.

BRIEF DESCRIPTION OF DRAWINGS Embodiments of the invention will now be more particularly described by way of example and with reference to Figures 6 to 15 of the accompanying drawings, in which:

Figure 1 is a schematic representation of a prior art Bragg grating function in a laser context;

Figure 2 is a schematic representation of holographic beam interference and the resultant grating, according to prior art;

Figure 3 is a schematic representation of holographic beam interference applied to a semiconductor wafer for grating creation, according to the prior art;

Figure 4 is a schematic representation of an arrangement for holographic Bragg grating creation, according to the prior art;

Figure 5 is a schematic representation of an example of orientation of Bragg gratings in semiconductor laser wafer fabrication process, according to the prior art;

Figure 6 is a schematic representation of an arrangement for creation by holographic means of a Bragg grating embodying the invention;

Figure 7 is a schematic representation of intersecting gratings creating a lattice structure;

Figure 8 is a schematic representation to an enlarged scale of the lattice gratings shown in Figure 7, identifying new grating structures;

Figure 9 is a geometric representation of the lattice gratings showing the derivation of the pitch relationship of the new gratings; Figures 10a, 10b, 10c and 10d are schematic representations of the Moire Indices description of individual lattice gratings;

Figures 11a, 11b, 11c and 11d are schematic representations of a variety of etched planar patterned lattice gratings;

Figures 12a and 12b are schematic representations of one means of producing two dissimilar native pitch gratings;

Figures 13a and 13b are schematic representations of the geometry of first order lattice gratings produced with dissimilar native pitch gratings;

Figure 14. is a schematic representation of the geometry of second order lattice gratings produced with dissimilar native pitch gratings; and

Figure 15. is a schematic representation of a blazed second order Moire grating.

Figure 1 to 5 are acknowledged to show the prior art and are discussed in some detail above.

BEST MODE OF THE INVENTION

The invention relates to a new process for creating holographic gratings. The invention relies upon using a fixed geometry holographic interferometer arrangement as shown in Figure 6 that upon exposure of the target wafer 8a creates a native pitch ΛO grating. The target wafer 8a is then rotated through an angle φ about a central axis 23 substantially perpendicular to the wafer plane and a second exposure made which produces a second grating with a native pitch of ΛO. If desired, alternative orthogonal centres of rotation on the target surface may be used. As a consequence of the beams 21 and 17 being divergent wavefronts the pitch of both of the interference gratings progressively increases away from the centre of the target wafer. However, by appropriate design of the optical system the magnitude of this increase is very small compared to the pitch of each of the sinusoidal interference gratings being recorded, and as such is within the tolerances required for all practical purposes. The wafer 8a now has two sinusoidal gratings 30 and 32 at an angle φ one to the other as shown in Figure 7 in its plan form 8b, where φ is the angle of rotation between the first and second exposure of the target wafer 8a. With typical positive photoresist material, where the gratings 30 and 32 overlap the photoresist becomes more soluble in the developer and a lattice structure in photoresist is defined. By suitable developing of the photoresist the lattice intersections may be made dominant as shown in Figure 8, where each of the intersecting peaks of the sinusoidal gratings is identified with a black dot 33.

Thus the invention is a holographic process for the creation of Bragg gratings in optical semiconductor waveguide devices comprising a photosensitive recording medium attached to a target substrate surface, and an optical arrangement to create two interfering coherent electromagnetic beams whereby the recording medium is irradiated using a first and second controlled exposure in which the first and second exposure are angularly offset by means of rotating the target surface about a substantially orthogonal axis to create two intersecting gratings on the recording medium, wherein the resultant grating structure creates planar lattice Bragg gratings.

The target substrate surface material is a semiconductor, such as Indium Gallium Arsenide Phosphide, or a dielectric material. Figure 8 shows the doubly exposed wafer 8b with a section 40 of the lattice intersection grating enlarged. The enlargement shows the original gratings 30 and 32 both with native pitch Λ₀ at angle φ. The intersections of the gratings 30 and 32 are marked with dots 33 showing the areas of photoresist corresponding to lattice peaks.

As my be seen from Figure 8 these dots 33 defined not only points along the written gratings but also define alternative lattice gratings such as for example the orghogonal gratings of pitch Λ_Π and Λ_V.

Referring now to Figure 9 (which is an enlargement of part of Figure 8) the geometry of the relationship between the original cross gratings 30 and 32 of native pitch Λ_O is shown in relation to prime implicit lattice gratings 34 and 36 of pitches respectively Λ_V and . In order to illustrate this relationship with more clarity the angle φ in Figure 9 is shown greater than it was in Figure 8. From the geometry of the lattice the following relationships can be derived: -

Λ_V = Λ_O / ( 2 x Sin φ/2) (3)

Λ_h = Λ_O / (2 x Cos φ/2) (4)

The two prime implicit lattice gratings are orthogonal and the Bragg condition for reflecting light back along its path of origin is that the reflecting means is orthogonal to the light direction of travel. Thus only one of the identified prime implicit gratings can operate upon the light for a specific orientation.

In an optical waveguide a grating is created by the periodic variation produced in the refractive index of the composite material. The periodicity is determined by the lattice grating structure and is maximised by the choice of fill factor (the planar profile dimensions) selected at the time of lattice creation. For example, first order gratings require circular or elliptical dots of fill factors of substantially 25% or 75%; and with second order gratings square or rectangular profiles of fill factors of substantially 50% are required.

There are more lattice gratings than the prime implicit gratings identified in Figures 8 and 9. The act of producing intersecting native gratings immediately produces a finite but extremely large number of interference or lattice gratings. Whichever Bragg lattice grating is chosen for use, for optimum performance, the grating rulings, or fringes, need to be substantially parallel to the crystallographic structure of the target wafer as indicated by the major flat discussed in relation to the prior art Figure 5. A lattice grating can be seen to exist wherever a straight line can be drawn between any two intersection points of the superposed original ΛO holographic gratings. The closer together are adjacent intersection points the stronger will be the lattice grating if illuminated with orthogonally directed light at an appropriate wavelength. In practice the dominance of a lattice grating is also influenced by the exposure, development and etching processes used.

In order to describe which lattice grating is intended for optical use, there may be used the Miiier indices used for crystaiiography in which the three mutually perpendicular axes may be defined as x, y and z. Using Miller notation a specific direction can be defined by [ Xi yi zi ] where x-i, yi and z-i are whole number values relative to an arbitrarily defined origin, corresponding to their corresponding locations on the x, y and z axes. Using Miller notation a plane is defined as ( X Y Z ) where X ,Y and Z are the reciprocal of the intercept points on the corresponding x, y and z axes, scaled to be the lowest whole number. The whole number values X, Y and Z are known as the Miller Indices.

Unlike three dimensional crystallography, holographic lattice gratings are constrained principally to two axes, and in the general case the holographic axes are not mutually perpendicular. Using Miller notation rules a terminology proposed for holographic lattice gratings is called Moire Indices, in which the third axis is always perpendicular to the first and second axes and its corresponding Moire Index is always 0 i.e. an intercept at infinity. The first and second axis can be at any angle relative one to the other but both are perpendicular to the third axis. As with the third axis the Moire Indices for the first and second axis are the reciprocal of the intercept points on the respective axis.

The Moire Indices description of a holographic lattice grating will be explained by reference to Figures 10a, b, c and d in which two native pitch Λ_O holographic gratings 30 and 32 are at an included angle φ. Grating 30 is arbitrarily selected as the a axis, and grating 32 is selected as the b axis. The c axis is perpendicular to the plane of Figures 10a, b, c and d wherein Figure 10a, b, c and d are drawn intersecting the virtual c axis at its origin. The Moire position of each intersection, according to the co-ordinate system a, b, c is marked, for example position 35 corresponds to a, b, c position 4 3 0 indicating that it is 4 units along a, 3 units along b, and at the 0 c plane. In Figure 10a, a lattice, or Moire, grating clearly exists shown as 37a, 37b, 37c. The gratings 37 continue throughout the lattice, or Moire, holographic grating but for ciarity of the drawing, just three are shown. Applying the Moire Indices rules, analogous to the Milier indices ruies, the Moire indices for 37a are calculated as: -

( 1/a 1/b 1/c ) where a is 1 b is 1 and c is oo (infinity)

Thus the Moire Indices are 1 1 0, and the Moire grating is defined as ( 1 1 0 ) the first order, or prime, lattice grating. Applying the same analysis process to 37b, 37c and so on produces the same Moire grating definition.

In Figure 10b, applying the same rules to Moire grating 39a gives the Moire grating definition ( 2 1 0 ), which is also the result for 39b, 39c and so on. This is a second order Moire grating.

In Figure 10c, applying the same rules to Moire grating 41a gives the Moire grating definition ( 3 1 0 ), which is also the result for 41b, 41c and so on. This is a third order Moire grating.

In Figure 10d, applying the same rules to Moire grating 42a gives the Moire grating definition ( 1 -1 0 ), which is also the result for 42b and so on. This is again a first order Moire grating, but orthogonal to the ( 1 1 0 ) Moire grating. As with Miller indices so Moire indices with negative terms may also be written with a negative sign printed above the corresponding index. For convenience they are written herein as negative quantities.

The nomenclature for such grating pitches is Λ (β) where (β) is the Moire Indices of the grating, and M is indicative that it is a Moire pitch. Similarly the corresponding Bragg wavelength associated with a (β) grating is referred to as λ_M(β) wherein, using equation (1) the Bragg wavelength is given by: -

λ_M(β) = ( 2 x n_e x Λ_M(_β)) / m (5)

where n_e is the effective refractive index of the substrate or waveguide, and m is the number of wavelengths λ (_β)/n_e inside the material in terms of 2 Λ _(β) ; often m is unity and for this particular case λ (_β) = 2 x n_e x ΛM^₎. Hereinafter, the description refers to deriving the Moire grating pitch Λ Φ). However, by substituting for ΛM ₎ in equation (5), the Bragg wavelength λ (β) may be derived.

In deriving the Moire Indices of a grating, the intercept point on a chosen axis may be negative i.e. a Moire Index of -1 or -2 etc.

First order Moire gratings are always in pairs e.g. M (1 1 0) and M(1 -1 0) (M(1 -1 0) is equivalent to M(-1 1 0)), whereas there are always four gratings of a chosen higher order e.g. M(1 2 0), M(2 1 0), M(-1 2 0) and M(2 -1 0).

The diminution of the strength of the higher order Moire gratings can be in part compensated by invoking the third axial dimension c, which in practical terms means that the grating is etched deeper into the structure semiconductor material.

By this means the Bragg grating pitch is in dependence upon the wavelength of the two interfering electromagnetic beams, the angle between the two interfering electromagnetic beams and the anguiar offset of the target substrate surface between the first and second exposure, wherein the depth of etching of the defined grating structure into the target substrate surface is used to strengthen the effect of the grating upon the incident waveguided electromagnetic energy.

By comparing Figure 10a and Figure 9, and using equations (3) and (4) the pitch of the first order Moire gratings ΛMΠ -_{1 O)} and ΛM₍I ₁ O) are given by:-

ΛM(I -ι o₎ = Λ_O / ( 2 x Sin φ/2)and

ΛM(I ι o) = Λ_O / ( 2 x Cos φ/2). The first order grating pitch Λ ₍I _{1 O)} is shown on Figure 10a, and the first order grating pitch ΛM(I -₁ O₎ is shown on Figure 10d. Only first order Moire gratings are orthogonal, higher order gratings are not.

Substituting for ΛO from equation (2) gives: -

Λ_M(i -ι o) = (λ₀) / ( ( 2 x Sinθ/2 ) x ( 2 x Sin φ/2 ) ) (6) and

ΛM(I ι o) = (λo) / ( ( 2 x Sinθ/2 ) x ( 2 x Cosφ/2 ) ) (7)

It can also be shown using trigonometry and equation (2) the pitch ΛMH 20) or Λ (-2 1 o) and ΛM(I ₂ o) or Λ (21 0) of the second order Moire gratings are given u. ,. uy.-

Λ_M(-2 ₁ o) = Λ_M(-I ₂0) = Λ_O / ( 1 + 8 x Sin²φ/2 )^{1 2} (8) and

Λ_M(2_{1 0}) = Λ_M(_{1 20}) = Λ0 / ( 1 + 8 X CθS²φ/2 f'² (9).

Above a second order Moire grating the strength of the gratings become progressively weaker and use of deeper etching into the semiconductor substrate is used as a means of strengthening their effect. There is a finite, but very iarge, series of higher order Moire gratings with decreasing pitch. The native pitch used in fabricating a Moire grating would be suitably selected to accommodate a high order grating if this was required to be used.

Equations (6) and (7) show that the Bragg grating under consideration has a pitch which depends upon the wavelength of the two interfering electromagnetic beams, the angle between the two interfering electromagnetic beams and the angular offset, between the first and second exposure, of the target substrate surface. Hence, for a chosen angle θ, it is easy to produce a Bragg grating of a desired pitch simply by selecting an appropriate angle φ, through which the wafer is rotated between exposures.

The gratings have been portrayed diagrammatically as simple lines. In reality the native gratings are sinusoidal producing graded density of hardness across the pitch. Thus where the native lattice gratings intersect there is a further graded hardening of the photo-resist. By choice of photoresist, angle of holographic interference, and etch time, so the profile of the Moire gratings can be manipulated to produce a variety of cross section profiles as shown in Figures 11a, b, c and d. Thus by means of varying the exposure times and or development time of the photosensitive recording medium the planar pattern of the intersecting grating structure may be determined. The developed photoresist planar pattern has the form of peaks and wells. For a given exposure and development time a unique mark space ratio of peaks and wells will be produced in the fabricated grating.

With a first order Moire grating M (1 1 0) produced with equai native gratings in the first and second exposure, and θ (the angle between interfering common wavelength coherent energy beams) of 90°, and a 25% or 75% fill factor i.e. etching to 25% or 75% of the sinusoidal degree of hardness, or softness, depending upon whether positive or negative photoresist has been used, so substantially circular planar pattern grating structures 50 are formed as shown in Figure 11a.

In general with a first order Moire grating M (1 1 0) produced with equal native gratings in the first and second exposure, and θ (the angle between interfering common wavelength coherent energy beams) not 90° and a 25% or 75% fill factor i.e. etching to 25% or 75% of the sinusoidal degree of hardness, or softness, depending upon whether positive or negative photoresist has been used, substantially elliptical planar pattern grating structures 52 are formed, as shown in Figure 11b.

With a second order Moire grating M (2 1 0) produced with equal native gratings in the first and second exposure, and θ (the angle between interfering common wavelength coherent energy beams) of 90°, and a 50% fill factor i.e. percentage of the sinusoidal degree of hardness, or softness, depending upon whether positive or negative photoresist has been used, substantially square planar pattern grating structures 54 are formed as shown in Figure 11c.

With a second order Moire grating M (2 1 0) produced with equal native gratings in the first and second exposure, and θ (the angle between interfering common wavelength coherent energy beams) not 90°, and a 50% fill factor i.e. percentage of the sinusoidal degree of hardness, or softness, depending upon whether positive or negative photoresist has been used, substantially rectangular planar pattern grating structures 56 are formed, as shown in Figure 11d.

A variation of the invention is to make the native pitch of the first and second exposure dissimilar, as well as making an angular offset of the target substrate surface between the first and second exposure. There exist a number of measures that can be taken to achieve this: -

1. displacement of the target substrate along the axis of the bisector of the two coherent beams; and / or

2. non-orthogonal rotation of the target substrate; and / or

3. tilt of the target substrate, and / or

4. shift of the target substrate in the plane of the target substrate surface. The magnitude of any of the above variables represent small changes that are minor adjustments to the holographic equipment wafer holder.

Figure 12a schematically shows the first of these measures. The two coherent beams 17 and 21 illuminate the target substrate 8a. As a consequence of the beams 21 and 17 being divergent, the pitch of the interference grating progressively increases away from the centre of the target wafer. Normally, by appropriate design of the optical system the magnitude of this increase is very small compared to the pitch of the sinusoidal interference grating being recorded, and as such is within the tolerences required for all practical purposes. However, by displacing the target substrate 8a along the axis of the bisector of the two beams 400, a small defined distance δz to either of positions 8d, the target substrate surface can be exposed to holographic interference fringes of different native pitches as diagrammatically represented in Figure 12b, with gratings 20 a, b or c.

Tilt of the target substrate can be achieved by mounting the wafer on a wedge. Non-orthogonal rotation of the substrate surface can be achieved by rotation of the wafer on a wedge.

The purpose of creating different native pitch gratings is to enhance the effectiveness i.e. a stronger reflective or transrnissive effect, of the selected Moire grating by means of modifying the planar cross section profile of the structure making up the grating.

Irrespective of the method used to achieve dissimilar native pitches the magnitude of the required differences are small. As a consequence only minor adjustments to the holographic equipment wafer holder are required. Hence exposure calibrations are unaffected. Figures 13a and b show dissimilar native pitch Λ₀' and Λ₀" grating geometries but for clarity with grossly exaggerated pitch differences so that the first order Moire gratings M( 110) and M(1 -10) may be more clearly identified and their respective geometries appreciated. Figure 13a shows the Moire grating M(11 0) pitch Λ (I ι o) ■ Figure 13b shows the Moire grating M(1 -10) pitch Λ O -1 O).

The pitch of the M(110) Moire grating Λ (I _{1 O}) is given by: -

1 /((1/ΛO')²+(1/Λ₀" )²-(2 x Cosφ)/(Λ₀'x ΛO"))^1/2 (10)

where Λ_O" is the native pitch of the first holographic exposure, ΛO" is the native pitch of the second holographic exposure, and φ is the angular offset of the target substrate between the first and second exposures.

The pitch of the M(1 -10) Moire grating Λ O -_{1 O}) is given by: -

ΛM(I-IO)= 1 /(( 1/Λ₀')²+( 1/ΛO")²-(2 x Sinφ)/(Λ₀'x Λ₀" ) )^1/2 (11)

where Λ_O' is the native pitch of the first holographic exposure, Λ_O" is the native pitch of the second holographic exposure, and φ is the angular offset of the target substrate between the first and second exposures.

Figure 14. shows the geometry of the second order Moire gratings M(120), M(2 1 0), M(-1 2 0) and M(2 -1 0) produced with dissimilar native pitch gratings Λ₀' and Λ_O". The pitch of the gratings is defined by their corresponding shortest distance to the point O.

The pitch of the M(2 -10) Moire grating ΛM₍2-I _O) is given by: - 1 /((1/ΛO')²+(2/Λ₀")²+(4 x Cosφ)/(Λ₀'x Λ₀" ))^1/2 (12) where ΛQ' is the native pitch of the first holographic exposure, ΛO" is the native pitch of the second holographic exposure, and φ is the angular offset of the target substrate between the first and second exposures.

The pitch of the M(-120) Moire grating Λ_M(-I 20) is given by: -

Λ_M(-ι 20)= 1 / ( ( 2/Λ₀' )²+ ( 1/ΛO" f + ( 4 x Cos φ )/ ( Λ₀' x Λ₀" ) )¹² (13)

where ΛO' is the native pitch of the first holographic exposure, ΛO" is the native pitch of the second holographic exposure, and φ is the angular offset of the target substrate between the first and second exposure.

The pitch of the M(120) Moire grating Λ_M(I ₂ o) is given by. -

Λ_M(120)= 1 /((2/Λ₀')²+( 1/ΛO")²- (4xCosφ)/(Λ₀'x Λ₀"))¹² (14)

where Λ₀' is the native pitch of the first holographic exposure, ΛO" is the native pitch of the second holographic exposure, and φ is the angular offset of the target substrate between the first and second exposures.

The pitch of the M(210) Moire grating Λ_M(2₁ o) is given by: -

Λ_M(2IO)= 1 /(( 1/Λ₀')²+(2/Λ₀")²- (4xCosφ)/(Λ₀'x Λ₀"))^1/2 (15)

where ΛO' is the native pitch of the first holographic exposure, ΛO" is the native pitch of the second holographic exposure, and φ is the angular offset of the target substrate between the first and second exposures.

In equations (10) to (15), the parameters Λ₀" and ΛO" may be restated in terms of the wavelength of the laser used to produce holographic system and the angle θ between the two interfering beams of laser energy, viz: - Λo' = λo/ (2 x Sin θ72) and

Λ₀" = λ₀/ (2 x Sin θ"/2).

θ' and θ" will not be the same when the target substrate is displaced along the axis of the bisector of the two coherent beams.

A further method of strengthening Moire gratings is to use the technique of blazing. With Moire gratings this is achieved by having dissimilar exposure duration times for the first and second controlled exposures whilst keeping the overall exposure time constant. The effect of such a blazing process is that the grating laid down in the photoresist has a shorter (or longer) first exposure interference pattern relative to that of the second exposure. The choice of longer or shorter first exposure is governed by the desired blaze direction. During photoresist development the first interference grating can consequently be made to have greater or lesser dominance in the definition of the Moire Grating structure. An example of a blazed second order lattice grating is shown at 130 in Figure 15.

By making the first exposure take 35 - 45% of the overall exposure time, for example, the resultant blazing of the second order lattice grating transforms what would have been rectangular or square profiled forms into linked parallelogram shapes, and what would have been circular profiled forms become elliptical in character. By suitable manipulation of exposure time ratios, and etch times (fill factor) the modified second order lattice grating forms are made to blend in the direction of the chosen grating.

The benefit of blazing is that the chosen grating becomes strengthened. Blazing also has the advantage that the depth of etching to form the Moire grating in the waveguide structure is reduced and a corresponding reduction in the growth layer thickness. Both these benefits result in an improved production yield of the device.

The above description relates to a process in which a Bragg grating pattern is defined in a photosensitive coating and the pattern then transferred to an underlying substrate. However, a similar method may be used to create a Bragg grating pattern directly in the substrate in circumstances in which the electromagnetic radiation is able to directly affect physical properties of the substrate such as to produce periodic variations in the optical properties thereof. In such a method, the substrate would be irradiated with two coherent electromagnetic beams in a first exposure to form a first grating pattern therein, rotated through a predetermine angular offset and then exposed a second time to form a second grating pattern therein which intersects the first grating pattern so as to form a lattice grating pattern in the substrate.

Claims

1. A method of defining grating patterns for an optical waveguide device comprising the steps of providing a target substrate having a photosensitive coating, providing means to irradiate said coating with two coherent electromagnetic beams, exposing the coating to such irradiation to define a first a grating pattern therein, rotating said target substrate surface through a predetermined angular offset and exposing said coating a second time to define therein a second grating pattern which intersects said first grating pattern so as to form a Bragg lattice grating pattern.

2. A method as claimed in claim 1 , wherein the photosensitive coating is photoresist.

3. A method as claimed in either claim 1 or claim 2, wherein the target substrate is a dielectric material.

4. A method as claimed in either claim 1 or claim 2, wherein the target substrate is a semiconductor material.

5. A method as claimed in claim 4 wherein the target substrate comprises layers of Indium Gallium Arsenide Phosphide.

6. -A method as claimed in any one of the preceding claims, wherein the photosensitive coating is developed to establish in the substrate a planar grating structure derived from the lattice grating pattern.

7. A method as claimed in claim 6, wherein the exposure times and/or development time of the coating may be varied to give a planar pattern of the resultant grating structure which is substantially an array of rectangular forms.

8. A method as claimed in claim 6, wherein the exposure times and/or development time of the coating may be varied to give a planar pattern of the resultant grating structure which is substantially an array of circular forms.

9. A method as claimed in claim 6, wherein the exposure times and/or development time of the coating may be varied to give a planar pattern of the resultant grating structure which is substantially an array of elliptical forms.

10. A method as claimed in claim 6, wherein the exposure times and/or development time of the coating may be varied to give a planar pattern of the resultant grating structure which is substantially an array of square forms.

11. A method as claimed in any one of the preceding claims, wherein, between the first and second exposures measures are taken to make the grating pitch defined by the second exposure dissimilar to the pitch defined by the first exposure, said measures comprising:

- non-orthogonal rotation of the target substrate;

- tilt of the target substrate; and/or

- shift of the target substrate in the plane of the target substrate surface.

12. A method as claimed in any one of the preceding claims, wherein the duration times of the first and second exposures are dissimilar whilst keeping the overall exposure time substantially constant, whereby there is a blazing of the defined lattice grating pattern.

13. A method of creating grating patterns for an optical waveguide device substantially as described herein with reference to any one of Figures 6 to 15 of the accompanying drawings.

14. A Bragg lattice grating defined by a method as claimed in any one of the preceding claims wherein the pitch of the grating depends upon the wavelength of the two interfering electromagnetic beams, upon the angle between the two interfering electromagnetic beams and upon the angular offset, between the first and second exposure, of the target substrate surface.

15. A Bragg lattice grating as claimed in claim 14, wherein the pitch ΛMCI -₁ o) θf the grating M(1 -1 0) is given by the equation: -

ΛM(I -1 o) = ΛO / ( 2 x Sin φ/2 ) where Λ₀ is the native pitch of the holographic interference fringe and φ is the angular offset of the target substrate surface between the first and second exposures.

16. A Bragg lattice grating as claimed in claim 14, wherein the pitch ΛM(I 1 o) θf the grating M(1 1 0) is given by the equation: -

ΛM(I 1 0) = ΛO / ( 2 x Cosφ/2 ) where Λ₀ is the native pitch of the holographic interference fringe and φ is the angular offset of the target substrate surface between the first and second exposures.

17. A Bragg lattice grating as claimed in claim 14, wherein the pitches Λ ₍-2 _{1 O)} and Λ_M(I -2 o) of the gratings M(-2 1 0) and M(1 -2 0) are given by the equations: -

Λ (-21 o) = ΛM(I -2 o)= ΛO / ( 1 + 8 x Sin φ/2 ) where Λ₀ is the native pitch of the holographic interference fringe and φ is the angular offset of the target substrate surface between the first and second exposures.

18. A Bragg lattice grating as claimed in claim 14, wherein the pitches Λ (2 I O) and Λ_M(I ₂ o) of the gratings M(2 1 0) and M(1 2 0) are given by the equations: -

ΛM(2 1 0) = Λ_M(1 20) = Λo / ( 1 + 8 X CθS²φ/2 )^1/2 where Λ_O is the native pitch of the holographic interference fringe and φ is the angular offset of the target substrate surface between the first and second exposures.

19. A Bragg lattice grating as claimed in any one of claims 14 to 18, wherein the defined grating structure is etched into the target substrate surface at such a depth as to strengthen the effect of the grating upon incident waveguided electromagnetic energy.

20. A Bragg lattice grating as claimed in any one of claims 14 to 19, wherein the grating rulings, or fringes, are substantially parallel to the crystallographic structure of the target substrate.

21. A Bragg lattice grating created by a method as claimed in claim 11 , wherein the pitch ΛM₍I -I _O) of the grating M(1 -1 0) is given by the equation:

Λ_M(1 -ι o)= 1 / ( ( 1/ΛO' )²+ ( 1/ΛO" )² - ( 2 x Sin φ )/ ( Λ₀' X Λ₀" ) )^1/2 where Λ₀' is the native pitch of the first grating, Λ₀" is the native pitch of the second grating, and φ is the angular offset of the target substrate between the first and second exposures.

22. A Bragg lattice grating created by a method as claimed in Claim 11 , wherein the pitch ΛMO _{1 O)} of the grating M(1 1 0) is given by the equation: - Λ_M(IIO)= 1 /(( 1/ΛO')²+( 1/Λ₀")²-(2 xCosφ)/(Λ₀'x Λ₀"))¹² where Λ₀' is the native pitch of the first grating, Λ_O" is the native pitch of the second grating, and φ is the angular offset of the target substrate between the first and second exposure.

23. A Bragg lattice grating created by a method as claimed in claim 11, wherein the pitch Λ_M(2-I _O) of the grating M(2 -10) is given by the equation:

Λ_M(2-I o) = 1 / ( ( 1/ΛO' f + ( 2/ΛO" )² + ( 4 x Cos φ )/ ( Λ₀' X Λ₀" ) )¹² where ΛO' is the native pitch of the first grating, ΛO" is the native pitch of the second grating, and φ is the angular offset of the target substrate between the first and second exposures.

24. A Bragg lattice grating created by a method as claimed in claim 11, wherein the pitch Λ_M(-I ₂ o₎ of the grating M(-120) is given by the equation:

Λ_M(-120)= 1/C(2/Λo')²+(1/Λo^,f )² + (4xCθSφ)/(Λo^,X Λ₀" ) f² where Λ_O' is the native pitch of the first grating, Λ₀" is the native pitch of the second grating, and φ is the angular offset of the target substrate between the first and second exposures.

25. A Bragg lattice grating created by a method as claimed in claim 11, wherein the pitch Λ ₍2₁₀₎ of the grating M(210) is given by the equation: -

Λ_M(210)= 1 /((1/Λ₀')²+(2/Λo")²-(4xCθSφ)/(Λθ'X Λ₀" ) f where Λ₀' is the native pitch of the first grating, Λ₀" is the native pitch of the second grating, and φ is the angular offset of the target substrate between the first and second exposures.

26. A Bragg lattice grating created by a method as claimed in claim 11, wherein the pitch Λ_M(I ₂ o₎ of the grating M(120) is given by the equation: - ΛMO 20) = 1 / ( ( 2/Λ₀' )² + ( 1/ΛO" )² - ( 4 x Cos φ )/ ( Λ₀ ^, x Λ₀" ) )^{1 2} where ΛO' is the native pitch of the first grating, ΛO" is the native pitch of the second grating, and φ is the angular offset of the target substrate between the first and second exposures.

27. A Bragg lattice grating substantially as described herein with reference to any one of Figures 6 to 15 of the accompanying drawings.

28. A method of defining grating patterns for an optical waveguide device comprising the steps of providing a target substrate, providing means to irradiate said substrate with two coherent electromagnetic beams, exposing the substrate to such irradiation to define a first a grating pattern therein, rotating said target substrate through a predetermined angular offset and exposing said substrate a second time to define therein a second grating pattern which intersects said first grating pattern so as to form a Bragg lattice grating pattern.

29. A Bragg lattice grating created by a method as claimed in claim 28 wherein the pitch of the grating depends upon the wavelength of the two interfering electromagnetic beams, upon the angle between the two interfering electromagnetic beams and upon the angular offset, between the first and second exposure, of the target substrate.