OPTICAL COMPONENTS

Abstract: An optical grating structure formed on a substrate material, comprises a plurality of grating elements (10) arranged to form a distributed series (10). Each grating element except the first in the series is spaced from the previous in the series. Each grating element defines first and second reflective discontinuities (50, 52), the first discontinuity (50) being arranged for receiving an optical signal, and for guiding that signal to the second discontinuity (52).

Description: OPTICAL COMPONENTS FIELD OF THE INVENTION The present invention relates to optical components and in particular to optical grating components. DESCRIPTION OF THE RELATED ART Optical gratings are used in a number of different applications to divide optical signals into a number of components of different respective wavelengths. Applications include sensing, spectroscopy, and other forms of optical signal processing. Another such area is in the field of optical communications networks which are making increasing use of the increased capacity and flexibility offered by wavelength division multiplexing (WDM) of signals. As is well known, each channel in a WDM optical network is assigned an optical wavelength, and so it is possible to transmit many channels having respective wavelengths along an optical link, such as an optical fibre. The separate channels can then be recovered by separating out the wavelengths concerned. However, in order that full use can be made of WDM technology, routing, switching and demultiplexing technology must be available which is robust and cost effective to produce. One conventional device is a wavelength dependent coupler, such as that described in "Strong Bragg gratings for WDM devices in nonsensitised low loss Ge doped waveguides", Electronics Letters, vol.32, no.23, p.2151, 1996. Another conventional device is a Bragg reflecting filter, such as that described in "Fabrication of wavelength tunable InGaAsP/InP grating assisted codirectional coupler filter with very narrow bandwidth" , Electronics Letters, vol.33, no.9, p773, 1997. Application of such components to multiwavelength networks has highlighted optical crosstalk as a key performance consideration for practical implementation. An additional consideration is the compactness of such components which has an important impact upon the component count per wafer, ultimate yield on manufacture and so the ultimate cost per unit component . Another device, known as a two-dimensional integrated optics (2DI0) demultiplexer, is also used to demultiplex WDM signals. Figures 1 and 2 illustrate a 2DIO single element device which is formed in an optical substrate layer 1. The device is typically fabricated on a silicon base material (not shown for clarity) and has upper and lower cladding layers 3 (shown only partially for clarity) which contain any optical signals so that they propagate in the plane of the core layer (ie. in two dimensions) . Optical signals 2 are input to the substrate 1 of the device via a single mode input optical waveguide 4. The input optical signal, containing a number of spectral channels, propagates through the input waveguide 4 and emerges into a slab region of the substrate 1 in which confinement by the upper and lower cladding regions 3 occurs only in a direction perpendicular to the plane of the device. The transversely diverging output beam from the guide is collimated by an input etched mirror structure 6 and is guided to illuminate a transmission grating structure 10. The known grating structure 10 consists of a series of triangular reflecting elements 10 x ..10 x . ,10 n arranged so as to form a distributed series of elements, with each element spaced from the next in the series. The elements are spaced so that the optical signals input to and output from the elements are not blocked by the neighbouring elements. An incident wavefront is split into multiple sections and a constant phase delay added to each section upon reflection by each transmission grating element. Each section of the wavefront emerges from the grating and interferes with the other section to produce an interference pattern in the far field. The pattern consists of a series of peaks and troughs corresponding to where constructive and destructive interference occur respectively. As such this component can be used to spatially separate a wavelength division multiplexed signal. The diffracted output from the grating structure 10 is then focussed onto a set of output waveguides by an output etched mirror structure 8. The signals carried by input signal have respective wavelengths and so have respective different angles of diffraction. Each signal is thereby focussed to a particular output waveguide 12 for output from the device. Optical crosstalk in such components imposes a major limit to the practical implementation of multi- wavelength optical cross-connects networks. Experimental studies on an established multi-wavelength transport network (MWTN) demonstrator have highlighted optical crosstalk to be one of the key performance considerations in the network design and performance specifications . Crosstalk in the 2DIO demultiplexer can be caused by component imperfections such as internal reflections, multiplexer/demultiplexer leakage and stray and scattered light. In addition, an equally important consideration is the crosstalk due to spectral overlapping between adjacent channels. This is a result of overlapping foci due to the limited wavelength resolution of the 2DIO grating. Two basic optical crosstalk mechanisms arising from this effect can be classified as either interband or intraband crosstalk, generated by non-ideal wavelength channel demultiplexing. Possible sources of both stray and scattered light and spectral overlap within the 2DI0 single element component will now be discussed. As can be seen from Figures 2 and 3 , the grating elements 10 n each act very small mirrors. As is the case for all mirrors a reversal of the image from each of these micro-mirror grating elements occurs. This reversal effect does not affect the reflection of a plane wave from the grating as such a wave has a uniform amplitude and phase across the grating. However, in the case of gaussian beam illumination it has been found that due to the rapidly varying amplitude distribution on the grating this reversal effect can result in substantial variations in the amplitude distribution of the reflected beam as illustrated in Figure 4. Modelling shows that a doubling of the higher order interference mode profile is observed due to the perturbation of the amplitude profile of the reflected gaussian distribution caused by image reversal on reflection from the grating elements. This ultimately leads to a reduction in the power contained within the designed diffraction mode of the component and a subsequent increase in the crosstalk floor of the resultant component. One of the major sources of stray or scattered light within previous 2DIO demultiplexers is due to imperfect grating element fabrication as illustrated in Figure 3. The quality of the vertices of the triangular grating elements 10 n is highly dependent upon the fabrication process employed to manufacture the device . Due to the limited resolution of the mask and photolithographic process usually used to produce such a device, the vertices 16 of the elements suffer from rounding. This results in a periodic modulation in the amplitude and phase of the reflected beam. Such rounding of the vertices also results in the effective shortening of the reflective surface (L nom to L eff ) and causes gaps in the phase distribution as shown in Figure 4. The result is the superposition of amplitude and phase gratings with identical periods. As is well known, the power content of the main mode falls with a reduction of the reflector width and the power content of the first and second higher order modes increases almost linearly drawing power away from the main mode with reflector width reduction. Thus, rounding of the grating element vertices results in a degradation of grating efficiency and significantly increases power coupling into higher order diffraction modes . This leads to a reduction in power within the designed diffraction mode of the component and a subsequent increase in the crosstalk floor of the resultant component . As mentioned above, such a grating structure can be used for many applications, and it will be appreciated that the example described is purely exemplary. SUMMARY OF THE PRESENT INVENTION It is therefore desirable to provide an optical grating structure which can overcome the disadvantages of the prior art structures. It is also desirable that such a device is cost effective to produce. According to one aspect of the invention, there is provided an optical grating structure formed on a substrate material, the grating structure comprising a plurality of grating elements arranged to form a distributed series, each grating element except the first in the series being spaced from the previous in the series, wherein each grating element defines first and second reflective discontinuities, the first discontinuity being arranged for receiving an optical signal, and for guiding that signal to the second discontinuity. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic view of a previously considered demultiplexer device; Figure 2 is a plan view of a grating structure of the device of Figure 1; Figure 3 is a detailed view of part of the grating structure of Figure 2; Figure 4 illustrates input and output fields of the device of Figures 1 , 2 and 3 ; Figure 5 is a plan view of part of an optical grating structure embodying the present invention; Figures 6a to 6d are scanning electron microscope images of an optical grating structure embodying the present invention; Figure 7 is a schematic view of an etching process for use in producing the optical grating structure of Figures 5 and 6; Figure 8 illustrates a mask used in the process of Figure 7; and Figure 9 is a graph showing the wavelength response of the optical grating structure of Figures 5 to 8. DESCRIPTION OF THE PREFERRED EMBODIMENT Figure 5 shows an optical grating structure embodying the present invention. Similar to the known structure described above, a distributed series of grating elements is provided on a substrate material . For a two dimensional device, the substrate can carry cladding layers which confine propagation of optical signals to be within the plane of the substrate layer. Each grating element, except the first in the series, is spaced from the previous element structure in the series . The grating structure is arranged to receive an input optical signal and to output a diffracted output optical signal. By way of example only, a demultiplexer device incorporating a grating structure embodying the present invention includes input and output ports through which respective input and output signals are transferred. In a demultiplexer device incorporating such a grating structure, and as in the previously described demultiplexer device, use is made of slab waveguide propagation to guide an input multiplexed optical signal from an input waveguide via a collimating reflector onto the grating structure. A reflector focuses the signals from the elements onto an array of output guides of the device. Each output waveguide receives an output signal of a predetermined wavelength. A grating structure embodying the present invention includes grating elements which each provide two reflective regions, an input region 50 and an output region 52. The regions are provided by respective reflective discontinuities in the substrate. The discontinuities can be provided by any suitable means. For example, the refractive index of the substrate can be adjusted, for example by doping, to give a step change. Alternatively, a cavity can be formed, for example by etching, in the substrate. The boundary surface of the cavity can then define a reflective surface. The cavities could be filled with material different to that of the substrate. The skilled person will be able to provide the required reflective regions in a number of ways which are covered by the scope of this invention. In one particular embodiment, the surfaces 50 and 52 are provided by edges of substantially triangular structures etched into the core layer. The ends of the two triangular structures preferably meet to provide a single continuous etched structure, as shown in the drawings . The grating element may be provided by a single continuous element which defines the two reflective regions, or may be provided by two or more elements which may be separated from one another. The term "grating element" in the context of the present invention is intended to cover both of these possibilities, such that the present invention is not simply limited to the specific embodiment shown in the drawings which uses a single composite etched structure, but can be provided by respective distinct configurations . The provision of two reflecting regions corrects the inversion of the optical signal that occurs with a single surface, thereby overcoming one of the major disadvantages of the prior art device. The angle of incidence upon the individual grating elements is preferably the same as in the previous design. However, the angle of incidence of the input optical signals upon the complete grating (with respect to the grating normal) is decreased because of the double reflective region (mirror) configuration. With the double mirror configuration, when the input beam is gaussian, then the focussed spot at the output will also be gaussian. In the example shown, the grating elements are arranged such that the input light beam 54 to one of the elements 10 n is effectively limited by an outer edge of the next element 10 n+1 . In this configuration, only a portion of the reflective regions reflect the optical signal. In the embodiment shown in Figure 5, each reflective region extends past the extent of the incoming optical signal (i.e. the reflective regions incorporate extensions past the minimum length required for reflection) . This construction removes any rounding effect of one end of the reflecting region, by effectively removing that end. By extending the grating element reflecting region beyond the point where light is incident upon each element facet, the corner will appear to all incoming light to be perfectly formed. This naturally gives enhanced performance, since the rounding losses can be effectively reduced. In this example, the other end of the grating element, the outer edge or vertex, is still be defined by the end point of the reflecting region, since the element concerned must not block or interfere with the input signal of the next element in the series . In other applications, the cavity that provides the reflecting region may be of another shape such that some of the incident wavefield is blocked and perhaps reflected by the cavity . Rounding effects of the outer vertices of the elements can be further reduced or even eliminated by the use of a double masking technique which is the subject of a co-pending UK patent application. Briefly, such a technique uses two mask layers and a intermediate sacrificial layer to define accurately the end vertices . Although the results presented here are based upon the fabrication of a the double reflective grating (DRG) using standard single mask photolithography ( such that rounding of the more outer corner is still present ) initial fabrication results of the double mask technique for highly defined "outer' grating vertices may be observed in figure 6 along with an illustration of the double mask etch window configuration in figure 7. In the example, standard diffraction theory was used to design the grating structure, with adjustments being made to account for the double reflective surface configuration. The derivation will be assumed here and the new grating equations may be summarised as: Grating pitch d = Grating element height h = neff cos(#<) 1 Half mirror aperture width Wm = -Nd 4 Grating Element Width w = d fmg f dλ Channel spatial separation dX = neffd sin(#) Channel spatial separation diffraction order N number of grating elements d grating pitch neff effective refractive index f parabolic mirror focal length g f group refractive index dλ Channel wavelength separation λc Centre wavelength Fabrication and Testing A 4 channel polymeric wavelength division multiplexer based upon a slab waveguide and the double reflective transmission grating was designed and fabricated. The channel spacing was 200 GHz (l.βnm) and was designed and realised for operation within the erbium doped fibre amplifier (EDFA) window. The double reflective grating (DRG) demultiplexer was designed to operate around 1538nm with a diffraction order of 48 and an effective refractive index of 1.51. The DRG has input and output waveguides, parabolic mirrors with a focal length of 2318mm and a transmission grating which consists of 71 double reflective elements with a pitch of 28.4mm. The input and output waveguides are separated by 250mm at each facet for fibre coupling. The device was fabricated on a silicon substrate utilising the standard commercial polymers of PMMA and PMGI as cladding and optical core layers respectively. Multiple polymer layers were applied using spin coating techniques yielding a refractive index step of 0.02. The formation of input/output waveguides, parabolic mirrors and DRG elements was achieved in a single deep reactive ion etch step with silicone based photoresist as a mask layer patterned by standard photolithography. The component was diced out of the wafer yielding a die that is only 6x3.5mm. Transfer loss associated with butt coupling optical fibres to the waveguides is of the order of 4dB per endface, and the planar waveguide loss was measured using the cutback method and was found to be 3dB/cm at 1540nm The demultiplexing performance of the device was tested using a wavelength tunable laser source. The transmission of all four outputs in figure 6 are shown against wavelength as measured with unpolarised light injected into the device. The insertion loss of the device in the passbands varies between 20-22dB of which 8dB can be attributed to waveguide propagation loss, 8dB to fibre coupling loss and 4dB to loss from the reflective elements . The background crosstalk level is <15dB and the usable passband width is 1.4nm. The double reflector grating (DRG) demultiplexer embodying the invention is compact and results in a compression of the overall width of the component due to the decrease in the angle of incidence allowed (wrt grating normal) , whilst maintaining an actual angle of incidence upon the grating facet that is within the critical angle for total internal reflection. The crosstalk was found to be <=15dB. It will be readily appreciated that embodiments of the present invention can provide improved grating structures that are suitable for use in a number of different applications . The WDM signal demultiplexer described above is purely used as an example of one such application. A device utilising a grating embodying the invention can provide improved performance and reliability over previously considered devices . In addition, since the device is relatively easily fabricated from polymer materials, the cost of manufacturing the device can be lowered.

Claims: CLAIMS : 1. An optical grating structure formed on a substrate material, the grating structure comprising a plurality of grating elements arranged to form a distributed series, each grating element except the first in the series being spaced from the previous in the series, wherein each grating element defines first and second reflective discontinuities, the first discontinuity being arranged for receiving an optical signal, and for guiding that signal to the second discontinuity. 2. A grating structure as claimed in claim 1, wherein the first reflective discontinuity is provided by a discontinuity in the refractive index of the substrate. 3. A grating structure as claimed in claim 1 or 2, wherein the second reflective discontinuity is provided by a discontinuity in the refractive index of the substrate. 4. A grating structure as claimed in claim 1, 2 or 3, wherein the first reflective discontinuity is provided by a boundary surface of a first cavity defined in the substrate . 5. A grating structure as claimed in claim 2, 3 or 4, wherein the second reflective discontinuity is provided by a boundary surface of a second cavity defined in the substrate. 6. A grating structure as claimed in claim 4 or 5, wherein the or each cavity is substantially triangular in plan. 7. A grating structure as claimed in claim 1, wherein the first and second reflective discontinuities are provided by respective boundary surfaces of a cavity formed in the substrate. 8. A grating structure as claimed in any one of the preceding claims, wherein the reflective discontinuities form only part of the said discontinuities. 9. A grating structure as claimed in any one of the preceding claims, wherein each grating element, except the last in the series, is arranged to guide the input optical signal to the next element in the series. 10. A grating structure as claimed in any one of the preceding claims, wherein the spacing of adjacent elements is constant over the length of the grating structure. 11. A grating structure as claimed in any one of claims 1 to 9, wherein the spacing of adjacent elements varies linearly over the length of the grating structure. 12. A grating structure as claimed in any one of claims 1 to 9, wherein the spacing of adjacent elements varies non-linearly over the length of the grating structure. 13. A grating structure as claimed in any one of the preceding claims, wherein the substrate material is substantially planar. 14. A grating structure as claimed in any one of the preceding claims, wherein the substrate material is carried by a base material . 15. A grating structure as claimed in any one of the preceding claims, wherein each grating structure, except the last in the series, is arranged to guide the output optical signal from the next structure in the series. 16. An optical grating structure substantially as hereinbefore described with reference to, and as shown in, Figures 5 to 9 of the accompanying drawings. 17. A device for demultiplexing wavelength division multiplexed optical signals comprising an input port, a series of output ports and a grating structure as claimed in any one of the preceding claims arranged so that in use a input optical signal is guided from the input port of the device to the first reflective surfaces of the grating structure, and then via the second reflective surface to the series of output ports of the structure. 18. A device as claimed in claim 9, comprising a reflective surface for guiding the input optical signal from the input port to the grating structure. 19. A device as claimed in claim 9 or 10, comprising a reflective surface for guiding the output optical signals to the output ports. 20. A demultiplexer device substantially as hereinbefore described with reference to, and as shown in, Figures 5 to 9 of the accompanying drawings.

Inventors: COWIN MICHAEL, PENTY RICHARD VINCENT, WHITE IAN HUGH, COWIN, MICHAEL, PENTY, RICHARD, VINCENT, WHITE, IAN, HUGH
Applicants: COWIN MICHAEL, PENTY RICHARD VINCENT, UNIV BRISTOL, WHITE IAN HUGH, COWIN, MICHAEL, PENTY, RICHARD, VINCENT, UNIVERSITY OF BRISTOL, WHITE, IAN, HUGH
Application year: 2000
Main classification: G02B6
IPC codes: G02B6