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Diffractive infrared optical elements in chalcogenide glasses
~I~[,U~,~II]~ f O U I ~ N A L, O P
Journal of Non-Crystalline Solids 164-166 (1993) 1247-1250 North-Holland
Diffractive infrared optical elements in chalcogenide glasses P.J.S. Ewena, A. Zekak a. C.W. Slingerb, G. Dale a, D.A. Painb and A.E. Owena aDepartment of Electrical Engineering, University of Edinburgh, Edinburgh, EH9 3JL, U.K. bDefence Research Agency, RSRE, Great Malvem, Worcs., WR14 3PS, U.K.
Aspects of the fabrication and performance of infrared diffractive optical elements based on the metalphotodissolution effect in As-S glasses are investigated. It is shown that the exposure time required to produce such elements can be decreased by an order of magnitude by heating the As-S/Ag film combination to ~110°C during illumination. Bulk phase gratings made using this effect were found to have good stability at elevated temperatures: trial gratings held at 100*C in the dark for over 6 months did not deteriorate si£,nificanfly, although the Ag continued to diffuse thermally into the undoped regions. The results of a theoretical analysis based on numerical solution of the appropriate coupled-wave equations predict that >95% efficiency should be possible with suitable anti-reflection coatings and optimised structures.
1. INTRODUCTION The extension of diffractive optical teclmiques from the visible to the IR is an important goal since diffractive elements have many potential applications in this waveband, including mirrors, lenses, illters and beam combiners, and may have advantages over conventional refractive/reflective components as regards weight, cost and ease of manufacture, However, the range of materials suitable for the fabrication of IR elements is limited. Chalcogeuide glasses are well known IR transmitting materials and have been employed in producing IR elements such as filters, windows and fibres. They also exhibit a wide variety of fight-induced effects which enable them to be used as optical imaging or storage media in applications such as holography, integrated optics and VLSI lithography. These effects can be used to produce either embedded or surface-relief structures in thin chalcogenide films and hence, because of the transparency of these materials in the IR spectral region, can be used to fabricate diffractive elements which are transmissive in the IR. This paper is concerned with how one of these photo-induced effects- metal photodissolution- can be used to fabricate diffractive structures in thin As-S films. The metal photodissolution effect was Elsevier Science Publishers B.V.
selected because it yields the largest change in the properties of the chalcogeuide, particularly the etch resistance and refractive index. The techniques used to fabricate these structures have been described in previous studies [1.2]. In this paper we investigate the potential of thermaUy-enhanced photodissolution as a means of improving the exposure requirements and examine the stability of photodoped As-S grating structures at elevated temperatures. In addition, a theoretical analysis of the performance of such grathags is discussed and used to predict the effect of different profiles on the first-order diffraction efficiency.
2. EXPERIMENTAL The As-S and Ag films used were prepared by vacuum evaporation onto glass substrates, the films being deposited successively with no break in the vacuum (~10-s torr) between depositions. The evaporation sources for the As-S films were fragments of melt-quenched glasses of the appropriate composition, while 99.998% pure Ag wire was used as the source for the Ag films. Because Ag is highly reflecting in the IR, to produce transmissive elemerits the Ag film must be on top so that any remaining Ag can be etched away after exposure.
1248
P.J.S. Ewen et al. / Diffractive infrared optical dements in chalcogenide glasses
The Ag was therefore deposited above the As-S layer and illumination was through the substrate. The photodissolution kinetics were obtained by the reflectance monitoring technique [3], using an expanded, uniform 514.5nm beam from an Ar-ion laser. The rates reported here were derived from the end-point of the process and correspond to the time at which the Ag first penetrates all the way across the undoped As-S layer. To measure the temperature dependence of the rate the samples were mounted on a temperature-controlled hotplate. Bulk phase gratings were recorded holographically by interfering two equal intensity beams derived from an Ar-ion laser operating at 514.5 nm. The interbeam angle was 25 ° and the incident intensity 250 mW cm-2 per wave.
3. RESULTS AND DISCUSSION 3.1 Influence of temperature on Ag photodissolution kinetics
Several previous studies of the photodissolution effect have observed that the rate of the process can be increased by heating the Ag/As-S film combination during exposure [4]. In the case of Ag/As4oS6o an increase by a factor of about 3.5 in the linear photodissolution rate has been reported [5] for an increase in temperature from ~200C" to ~ 1 0 0 * C . These, experiments were performed in thin As-S films (~350rim) using broadband mercury lamp illumination. A comparable increase has been observed in similar experiments on As33567, reported ill these proceedings [6], although these results show that the kinetics of the process are more complicated than previously supposed and the assumption of a simple linear time dependence is not valid, There is, however, a need to obtain kinetics data under conditions which match those likely to be used in making actual diffractive elements. For grathag fabrication using Ag photodissolution in As-S films, the optimum exposure wavelength is in the green region of the specmma [1,2]. As holographic exposure requires monochromatic light, the most convenient source is the 514.5 nm line of the At-ion laser. Figure 1 plots, as a function of temperature,
~ 800 [ ~u |I ,~600 nI
t
400
t t |
200
0!
0
,
25
50
75 100 TEMPERATURE/*C Figure 1. Photodissolution rate as a function of temperature in Ag/As40Sro film combinations. The full line is a guide for the eye. the photodissolution rates measured for 1.84gan As4oSro / 0.46 gm Ag samples using 250 mW cm-2 of 514.5nm illumination. Since the kinetics has a root-time dependence for these thicker films the rates in Figxtre 1 are in A/s°s. Whereas these rates increase by a factor of 2.7 over the range 20-100°C, the simple linear rate calculated from the end-points increases by a factor of ~7.5, which is significantly higher than suggested by other studies [5,6]. This is probably due to the difference in the wavelength and intensity of the illumination used in the present case. In particular, the intensity was ~80 times higher than that used by Plocharski et al. [5], so no induction period was observed in the present experiments. These results suggest that the exposure times required to produce grating structures using photodissolution could be reduced by an order of magnitude by heating the films during illumination. Although holographic exposure at elevated temperatures may be problematic because of, for example, the creation of air currents, mask exposure should not be much more difficult than at room temperature. It is important to note that at 85°C the thermal doping rate (i.e. in the dark) was found to be ~25 times slower than the photodissolution rate, so there should be no sideways diffusion of the Ag, and consequently no significant smearing of the grating pattern in the f i l l as a result of heating.
1249
P.J.S. Ewen et al. / Diffractive infrared optical elements in chalcogenide glasses
3.2 Grating stability
~-~ 2.s ~ ~ ] ....._
An important technological aspect of grating devices is the stability of their performance at elevated temperatures. In the case of gratings produced by the metal photodissolution effect it is possible that, due to thermal diffusion, over time there may be some redistribution of the metal ions photodoped into the chalcogenide. Although this may have little or no effect in the case of surface relief devices, it might degrade the performance of bulk phase grat-
o.i 2.0 ~ tu~ 1.s z _o~.1.0 o< 0.5 "
.
• ,;,t.""
The distance of 0.43pm diffused by the Ag over this period is approaching half the grating period (1.2/an), yet there is no evidence of the grating
" "" "
"
0 So
[figs.
Bulk phase gratings were recorded holographically in a 1.37/an AS33567 / 0.8 ,tim mg combination, the Ag being on the top surface and removed after exposure. The gratings were placed in an oven and kept at 100+_5 °C for a period of over 200 days, the heating being carded out in the dark in an air atmosphere. Measurements of the transmitted +1 order diffraction efficiency were made periodically by removing the grating from the oven, allowing it to cool to room temperature and probing it with a low power (95 pW) 632.8 nm beam. Figure 2 is a plot of the efficiency as a function of time at 100°C for a grating made using an exposure time of 480s. The efficiency oscillates about a mean value but there is no general trend except at the very end. The form of the curve is similar to that observed for the reflectance as a function of time during photodissolution kinetics experiments [3], which suggests that the oscillations in Figure 2 arise from the thermal diffusion of Ag from the grating strncture into the undoped layer below it. The upturn in the efficiency towards the end is probably due to the Ag reaching the substrate/As-S interface. From previous measurements of the kinetics, the depth of the photodoped grating is estimated to be ~l.25/an and since about one quarter of this extends into the Ag layer there will be ~0.43 pm of undoped material below the grating. These results imply that the diffusion coefficient for Ag in As33567 at 100°C is ~lxl0 -16 cm2s-l. which is comparable to the value of ~8x10-Is cm2s-] reported [5] for Ag diffusion in As4oSto at this temperature,
.
100
15o
200
TIME / DAYS
Figure 2. Diffraction efficiency as a function of time for an Ag/As33567 bulk phase grating held at 100°C. structure being "washed out". This suggests that the thermal diffusion in this case is not isotropic but mainly in the direction normal to the film. Since it is known that the photodissolution rate depends on the illumination history of the As-S film, this anisotropic diffusion may be due to the fact that the undoped As-S has been exposed to a non-uniform (sinusoidal) intensity pattern.
3.3 Theoretical analysis In principle rigorous diffraction theory can now be used to predict how light will be diffracted by any spatially periodic dielectric medium [7]. However, the use of the rigorous theories in grating design is at present limited by the complexity of the analysis coupled with the need for high computing power to implement it. In the present work first-order multiwave coupled-wave theory has been used to analyse the type of gratings that may be produced by metalphotodopingofchalcogenides. This theory hasbeen found to give a good approximation of diffraction provided that the modulation of the grating is not too high [2]. Thus for example this theory would not be sufficiently accurate for modelling of surface-relief (etched) gratings produced by Agphotodoping. An Ag-photodoped grating that has not been etched on the other hand can be considered as a surface-relief structure immersed in a dielectric fluid. In this case the variation in the index of refraction between undoped and doped material is small and the accuracy of first-order theory is greatly increased.
1250
P.J.S. Ewen et al. / Diffractive infrared optical elements in chalcogenide glasses
1.0)O Z _kl 0.8_O h h to Z 0.6O
In practice it is not feasible to produce Agphotodoped gratings with an arbitrary profile, grating period and depth. In general it is difficult to produce deep photodoped structures (for IR operation) or structures which have a large normalised depth (d/A). The most readily available profile at the required resolution is the parabolic which can be obtained by holographic recording. Yet unless deep photodoping can be obtained it would appear that the square profile is to be preferred. The practical difficulties in producing different grating structures are the subject of current investigations.
~ ~
O < tr 0.40
ACKNOWLEDGEMENTS 0.2
I 2
I 3
I 4
I 5
6
NORMALISED DEPTH
Figure 3. Theoretical first-order diffraction efiiciencies of bulk gratings with various profiles: [] square; o - sinusoidal; A - triangular; ° - parabolic.
This work was carded out with the support of the Procurement Executive, Ministry of Defence. GD is grateful to the Science and Engineering Research Council and Pilkington plc for provision of a CASE studentship. REFERENCES
Four different grating profiles have been analysed for their peak first-order diffraction efficiency at various depths normalised to the grating period (A). For each profile (square, sinusoidal, triangular and parabolic) an anti-reflection coating has been simulated by setting the index of refraction of the external medium to the bulk (mean) index of the grating. In each case it has been assumed that the doped and undoped material have refractive indices of 2.65 and 2.3 respectively and that A=0.79;t, where 2 is the replay wavelength. Although this grating period may not be the optimum, high diffraction efficiencies (>90%) are observed for each profile. The results of this analysis, shown in Figure 3, agree with the previous observations inasmuch as the maximum diffraction efficiency increases for sharper, more pointed profiles, i.e. from square to sinusoidal to triangular. This general observation is also borne out by consideration of the parabolic and triangular gratings which have very similar behaviour except that the triangular grating has an increased efficiency resulting from its sharper profile.
1.
2. 3. 4. 5. 6. 7.
P.J.S. Ewen, C.W. Slinger, A. Zakery, A. Zekak and A.E. Owen, Prec. ECO4 Int. Congress on Optical Science and Engineering, The Hague, SPIE Proc. 1512 (1991) 101. C.W. Slinger, A. Zakery, P.J.S. Ewen and A.E. Owen, Appl. Optics 31 (1992) 2490. P.J.S. Ewen, A. Zakery, A.P. Firth and A.E. Owen, Phil. Mag. B57 (1988) 1. A.V. Kolobov and S.R. Elliott, Adv. in Physics 40 (1991)625. J. Plocharski, J. Pryzluski and M. Teodorczyk, J. Non-crystalline Solids 93 (1987)303. T. Wagner, M. Vlcek, V. Smrcka, P.J.S. Ewen and A.E. Owen, these proceedings. M.G. Moharam and T.K. Gaylord, Proc. IE-~.E 73 (1985) 894.
© British Crown Copyright / DRA 1993
Journal of Non-Crystalline Solids 164-166 (1993) 1247-1250 North-Holland
Diffractive infrared optical elements in chalcogenide glasses P.J.S. Ewena, A. Zekak a. C.W. Slingerb, G. Dale a, D.A. Painb and A.E. Owena aDepartment of Electrical Engineering, University of Edinburgh, Edinburgh, EH9 3JL, U.K. bDefence Research Agency, RSRE, Great Malvem, Worcs., WR14 3PS, U.K.
Aspects of the fabrication and performance of infrared diffractive optical elements based on the metalphotodissolution effect in As-S glasses are investigated. It is shown that the exposure time required to produce such elements can be decreased by an order of magnitude by heating the As-S/Ag film combination to ~110°C during illumination. Bulk phase gratings made using this effect were found to have good stability at elevated temperatures: trial gratings held at 100*C in the dark for over 6 months did not deteriorate si£,nificanfly, although the Ag continued to diffuse thermally into the undoped regions. The results of a theoretical analysis based on numerical solution of the appropriate coupled-wave equations predict that >95% efficiency should be possible with suitable anti-reflection coatings and optimised structures.
1. INTRODUCTION The extension of diffractive optical teclmiques from the visible to the IR is an important goal since diffractive elements have many potential applications in this waveband, including mirrors, lenses, illters and beam combiners, and may have advantages over conventional refractive/reflective components as regards weight, cost and ease of manufacture, However, the range of materials suitable for the fabrication of IR elements is limited. Chalcogeuide glasses are well known IR transmitting materials and have been employed in producing IR elements such as filters, windows and fibres. They also exhibit a wide variety of fight-induced effects which enable them to be used as optical imaging or storage media in applications such as holography, integrated optics and VLSI lithography. These effects can be used to produce either embedded or surface-relief structures in thin chalcogenide films and hence, because of the transparency of these materials in the IR spectral region, can be used to fabricate diffractive elements which are transmissive in the IR. This paper is concerned with how one of these photo-induced effects- metal photodissolution- can be used to fabricate diffractive structures in thin As-S films. The metal photodissolution effect was Elsevier Science Publishers B.V.
selected because it yields the largest change in the properties of the chalcogeuide, particularly the etch resistance and refractive index. The techniques used to fabricate these structures have been described in previous studies [1.2]. In this paper we investigate the potential of thermaUy-enhanced photodissolution as a means of improving the exposure requirements and examine the stability of photodoped As-S grating structures at elevated temperatures. In addition, a theoretical analysis of the performance of such grathags is discussed and used to predict the effect of different profiles on the first-order diffraction efficiency.
2. EXPERIMENTAL The As-S and Ag films used were prepared by vacuum evaporation onto glass substrates, the films being deposited successively with no break in the vacuum (~10-s torr) between depositions. The evaporation sources for the As-S films were fragments of melt-quenched glasses of the appropriate composition, while 99.998% pure Ag wire was used as the source for the Ag films. Because Ag is highly reflecting in the IR, to produce transmissive elemerits the Ag film must be on top so that any remaining Ag can be etched away after exposure.
1248
P.J.S. Ewen et al. / Diffractive infrared optical dements in chalcogenide glasses
The Ag was therefore deposited above the As-S layer and illumination was through the substrate. The photodissolution kinetics were obtained by the reflectance monitoring technique [3], using an expanded, uniform 514.5nm beam from an Ar-ion laser. The rates reported here were derived from the end-point of the process and correspond to the time at which the Ag first penetrates all the way across the undoped As-S layer. To measure the temperature dependence of the rate the samples were mounted on a temperature-controlled hotplate. Bulk phase gratings were recorded holographically by interfering two equal intensity beams derived from an Ar-ion laser operating at 514.5 nm. The interbeam angle was 25 ° and the incident intensity 250 mW cm-2 per wave.
3. RESULTS AND DISCUSSION 3.1 Influence of temperature on Ag photodissolution kinetics
Several previous studies of the photodissolution effect have observed that the rate of the process can be increased by heating the Ag/As-S film combination during exposure [4]. In the case of Ag/As4oS6o an increase by a factor of about 3.5 in the linear photodissolution rate has been reported [5] for an increase in temperature from ~200C" to ~ 1 0 0 * C . These, experiments were performed in thin As-S films (~350rim) using broadband mercury lamp illumination. A comparable increase has been observed in similar experiments on As33567, reported ill these proceedings [6], although these results show that the kinetics of the process are more complicated than previously supposed and the assumption of a simple linear time dependence is not valid, There is, however, a need to obtain kinetics data under conditions which match those likely to be used in making actual diffractive elements. For grathag fabrication using Ag photodissolution in As-S films, the optimum exposure wavelength is in the green region of the specmma [1,2]. As holographic exposure requires monochromatic light, the most convenient source is the 514.5 nm line of the At-ion laser. Figure 1 plots, as a function of temperature,
~ 800 [ ~u |I ,~600 nI
t
400
t t |
200
0!
0
,
25
50
75 100 TEMPERATURE/*C Figure 1. Photodissolution rate as a function of temperature in Ag/As40Sro film combinations. The full line is a guide for the eye. the photodissolution rates measured for 1.84gan As4oSro / 0.46 gm Ag samples using 250 mW cm-2 of 514.5nm illumination. Since the kinetics has a root-time dependence for these thicker films the rates in Figxtre 1 are in A/s°s. Whereas these rates increase by a factor of 2.7 over the range 20-100°C, the simple linear rate calculated from the end-points increases by a factor of ~7.5, which is significantly higher than suggested by other studies [5,6]. This is probably due to the difference in the wavelength and intensity of the illumination used in the present case. In particular, the intensity was ~80 times higher than that used by Plocharski et al. [5], so no induction period was observed in the present experiments. These results suggest that the exposure times required to produce grating structures using photodissolution could be reduced by an order of magnitude by heating the films during illumination. Although holographic exposure at elevated temperatures may be problematic because of, for example, the creation of air currents, mask exposure should not be much more difficult than at room temperature. It is important to note that at 85°C the thermal doping rate (i.e. in the dark) was found to be ~25 times slower than the photodissolution rate, so there should be no sideways diffusion of the Ag, and consequently no significant smearing of the grating pattern in the f i l l as a result of heating.
1249
P.J.S. Ewen et al. / Diffractive infrared optical elements in chalcogenide glasses
3.2 Grating stability
~-~ 2.s ~ ~ ] ....._
An important technological aspect of grating devices is the stability of their performance at elevated temperatures. In the case of gratings produced by the metal photodissolution effect it is possible that, due to thermal diffusion, over time there may be some redistribution of the metal ions photodoped into the chalcogenide. Although this may have little or no effect in the case of surface relief devices, it might degrade the performance of bulk phase grat-
o.i 2.0 ~ tu~ 1.s z _o~.1.0 o< 0.5 "
.
• ,;,t.""
The distance of 0.43pm diffused by the Ag over this period is approaching half the grating period (1.2/an), yet there is no evidence of the grating
" "" "
"
0 So
[figs.
Bulk phase gratings were recorded holographically in a 1.37/an AS33567 / 0.8 ,tim mg combination, the Ag being on the top surface and removed after exposure. The gratings were placed in an oven and kept at 100+_5 °C for a period of over 200 days, the heating being carded out in the dark in an air atmosphere. Measurements of the transmitted +1 order diffraction efficiency were made periodically by removing the grating from the oven, allowing it to cool to room temperature and probing it with a low power (95 pW) 632.8 nm beam. Figure 2 is a plot of the efficiency as a function of time at 100°C for a grating made using an exposure time of 480s. The efficiency oscillates about a mean value but there is no general trend except at the very end. The form of the curve is similar to that observed for the reflectance as a function of time during photodissolution kinetics experiments [3], which suggests that the oscillations in Figure 2 arise from the thermal diffusion of Ag from the grating strncture into the undoped layer below it. The upturn in the efficiency towards the end is probably due to the Ag reaching the substrate/As-S interface. From previous measurements of the kinetics, the depth of the photodoped grating is estimated to be ~l.25/an and since about one quarter of this extends into the Ag layer there will be ~0.43 pm of undoped material below the grating. These results imply that the diffusion coefficient for Ag in As33567 at 100°C is ~lxl0 -16 cm2s-l. which is comparable to the value of ~8x10-Is cm2s-] reported [5] for Ag diffusion in As4oSto at this temperature,
.
100
15o
200
TIME / DAYS
Figure 2. Diffraction efficiency as a function of time for an Ag/As33567 bulk phase grating held at 100°C. structure being "washed out". This suggests that the thermal diffusion in this case is not isotropic but mainly in the direction normal to the film. Since it is known that the photodissolution rate depends on the illumination history of the As-S film, this anisotropic diffusion may be due to the fact that the undoped As-S has been exposed to a non-uniform (sinusoidal) intensity pattern.
3.3 Theoretical analysis In principle rigorous diffraction theory can now be used to predict how light will be diffracted by any spatially periodic dielectric medium [7]. However, the use of the rigorous theories in grating design is at present limited by the complexity of the analysis coupled with the need for high computing power to implement it. In the present work first-order multiwave coupled-wave theory has been used to analyse the type of gratings that may be produced by metalphotodopingofchalcogenides. This theory hasbeen found to give a good approximation of diffraction provided that the modulation of the grating is not too high [2]. Thus for example this theory would not be sufficiently accurate for modelling of surface-relief (etched) gratings produced by Agphotodoping. An Ag-photodoped grating that has not been etched on the other hand can be considered as a surface-relief structure immersed in a dielectric fluid. In this case the variation in the index of refraction between undoped and doped material is small and the accuracy of first-order theory is greatly increased.
1250
P.J.S. Ewen et al. / Diffractive infrared optical elements in chalcogenide glasses
1.0)O Z _kl 0.8_O h h to Z 0.6O
In practice it is not feasible to produce Agphotodoped gratings with an arbitrary profile, grating period and depth. In general it is difficult to produce deep photodoped structures (for IR operation) or structures which have a large normalised depth (d/A). The most readily available profile at the required resolution is the parabolic which can be obtained by holographic recording. Yet unless deep photodoping can be obtained it would appear that the square profile is to be preferred. The practical difficulties in producing different grating structures are the subject of current investigations.
~ ~
O < tr 0.40
ACKNOWLEDGEMENTS 0.2
I 2
I 3
I 4
I 5
6
NORMALISED DEPTH
Figure 3. Theoretical first-order diffraction efiiciencies of bulk gratings with various profiles: [] square; o - sinusoidal; A - triangular; ° - parabolic.
This work was carded out with the support of the Procurement Executive, Ministry of Defence. GD is grateful to the Science and Engineering Research Council and Pilkington plc for provision of a CASE studentship. REFERENCES
Four different grating profiles have been analysed for their peak first-order diffraction efficiency at various depths normalised to the grating period (A). For each profile (square, sinusoidal, triangular and parabolic) an anti-reflection coating has been simulated by setting the index of refraction of the external medium to the bulk (mean) index of the grating. In each case it has been assumed that the doped and undoped material have refractive indices of 2.65 and 2.3 respectively and that A=0.79;t, where 2 is the replay wavelength. Although this grating period may not be the optimum, high diffraction efficiencies (>90%) are observed for each profile. The results of this analysis, shown in Figure 3, agree with the previous observations inasmuch as the maximum diffraction efficiency increases for sharper, more pointed profiles, i.e. from square to sinusoidal to triangular. This general observation is also borne out by consideration of the parabolic and triangular gratings which have very similar behaviour except that the triangular grating has an increased efficiency resulting from its sharper profile.
1.
2. 3. 4. 5. 6. 7.
P.J.S. Ewen, C.W. Slinger, A. Zakery, A. Zekak and A.E. Owen, Prec. ECO4 Int. Congress on Optical Science and Engineering, The Hague, SPIE Proc. 1512 (1991) 101. C.W. Slinger, A. Zakery, P.J.S. Ewen and A.E. Owen, Appl. Optics 31 (1992) 2490. P.J.S. Ewen, A. Zakery, A.P. Firth and A.E. Owen, Phil. Mag. B57 (1988) 1. A.V. Kolobov and S.R. Elliott, Adv. in Physics 40 (1991)625. J. Plocharski, J. Pryzluski and M. Teodorczyk, J. Non-crystalline Solids 93 (1987)303. T. Wagner, M. Vlcek, V. Smrcka, P.J.S. Ewen and A.E. Owen, these proceedings. M.G. Moharam and T.K. Gaylord, Proc. IE-~.E 73 (1985) 894.
© British Crown Copyright / DRA 1993