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Spectrographic performance of holographically made diffraction gratings
Volume
1, number
1
OPTICS COMMUNICATIONS
SPECTROGRAPHIC
PERFORMANCE DIFFRACTION
MADE
Antoine Observatoire
de Paris
April
1969
OF HOLOGRAPHICALLY GRATINGS
LABEYRIE -Meudon,
92, Meudon,
France
and Jean FLAMAND Soci&t& Jobin
et Yvon,
26, rue Berthollet,
Received
22 November
94, Arcueil,
France
1968
Diffraction gratings for spectrographic applications have been produced using holographic techniques. From the standpoint of efficiency, aberrations and stray light these gratings prove comparable or superior to conventionnal ruled gratings.
1. INTRODUCTION It appears that the theoretical possibility of obtaining diffraction gratings as material recordings of interference fringes was first mentioned, with a view to spectrographic applications, by Denisyuk [l] at the very beginning of the laser era (1962). This possibility was subsequently verified by a number of authors, most of them working in the field of holography [2,3,12]. Typically, such “holographic gratings” were obtained by illuminating high resolution photographic plates with two coherent light-waves from a laser. Large gratings could be obtained by this method within a few minutes, however these gratings were not suitable for spectrographic applications because of their poor optical quality and low diffracting efficiency. The optical properties of such holographic gratings have been studied in detail by one of us (A.L.), then at the University of Michigan [3]: it was concluded that conventional photographic emulsions as a recording medium had to be replaced by very thin photosensitive layers such as the Daguerre silver-iodide layer and dichromated gelatin or photoresist processes. It was also indicated that a sine-wave groove profile for gratings is not incompatible with high blaze efficiency. We have now been able to verify both assumptions. Good quality gratings bearing up to 3000 lines per mm were obtained in sizes up to 150 mm X 110 mm, and larger dimensions are expected to be possible.
We report here on the properties and performance of these holographic gratings, as made by us in the Jobin-Yvon ruling engine laboratory at Longjumeau (France): Glass blanks were polished to quarterwave flatness and coated with a thin photoresist film of controlled uniform thickness, then exposed to the interference field of two coherent plane waves of laser light. The plane waves were obtained through the use of good quality, diffraction limited, collimators. Special care had to be taken in order to avoid fringe drift and vibration during exposures (caused by thermal or acoustical disturbances as well as by atmospheric pressure variations). After exposure, the blanks were immersed in a suitable solvent, and rinsed in such a way as to obtain on their surface a periodic groove profile of optimum amplitude. The holographic gratings were subsequently aluminized for maximum efficiency. Replication was successfully achieved, using the same technique as for conventional ruled gratings [4].
2. BLAZE As predicted, it was observed that high blaze efficiencies are also attained with gratings featuring a sine-wave groove profile, as obtained by the holographic method (echelette groove profiles may also be produced holographically in the case of low spatial frequency gratings [5]). 5
Volume
1, number
1
OPTICS COMMUNICATIONS
This is especially the case when the groove spacing is comparable to or smaller than the wavelength. For example, a holographic grating bearing 2000 lines/mm was found to diffract in the first order 68% of an incident 5460 A polarized light-beam (which corresponds to about 75% true efficiency if the absorption of aluminium is taken into account). Another 3000 lines/mm grating gave 50% of a 6328 A light-beam in the first order. Probably because sine-wave profiles are not encountered on ruled gratings, little has been published on the electromagnetic theory of their blaze properties whereas saw-tooth (echelette) profiles have been studied in much detail, following initial work by Mare&al and Stroke [6,7,8]. We found experimentally that the blaze efficiency of sine-wave gratings obtained by holography can be equal to or higher than that of ruled echelette gratings when the groove spacing is smaller than approximately twice the wavelength. Thus, holographic gratings featuring either echelette or sine-wave groove profile may attain a high blaze efficiency when used in the visible spectral region. In view of space applications, it would be of interest to compare the blaze properties of both types of gratings in the vacuum ultra-violet region. 3. ABERRATIONS As shown by the elementary theory of holograms [9,10], aberration-free plane gratings are obtained only if the recording surface is optically flat and if both incident laser waves are plane within a fraction of a wavelength. Also, the recording medium should feature a high degree of rigidity during and after the processing phase
131.
It appears that these conditions were not satisfied in previously published experiments dealing with holography and holographic gratings. For example, as mentioned above, early holographic gratings recorded on photographic emulsions (such as Kodak 649 Lippmann-type plates) had very important aberrations because a) the emulsion thickness was not uniform [ll], b) the emulsion drifted, relative to the substrate, under the influence of stresses which appeared during the developing, fixing and drying phases of the processing [3]. The resulting aberrations of the reconstructed wavefronts were tolerable for most holographic, spatial filtering or holographic in-
6
April
1969
Fig. 1. Michelson interferogram of holographic grating (size 75 mm, 2000 grooves/mm, first order diffracted wavefront). The residual aberrations result from substrate imperfections.
terferometric applications, but certainly not for diffraction gratings. It was realized that these dimensional instability effects could be avoided by using a recording layer much thinner than the conventional 10 microns thick emulsion. This layer was also required to be homogeneous with a continuous, rather than grainy, response to illumination. Among the wide range of existing photosensitive organic compounds [13], photoresists were selected because of their excellent chemical stability which allows direct vacuum aluminizing of the grating surface. Fig. 1 shows the diffracted wavefront of a grating recorded on photoresist, as examined by the Michelson-Twyman method [18]. It should be noted that the residual aberrations which are seen here result from imperfections of the substrate. Resolution tests with the 5461 A line of mercury have shown that the theoretical resolving power is attained. 4. STRAY LIGHT Diffused light in ruled gratings consists of the well-known Rowland and Lyman ghost patterns and of a more or less uniform background. It originates in the rapidly varying phase defects
Volume
1, number 1
OPTICS COMMUMCATIONS
April
1969
c)
Fig. 2. Stray light patterns of gratings: a) ruled grating (120 X 140 mm, 305 grooves/mm); b) ruled grating from a different manufacturer (254 X 127 mm, 300 grooves/mm); c) holographic grating (75 X 75 mm, 2000 grooves/mm). The single horizontal bright line observed in the spectra of all ruled gratings result from systematic ruling errors. In b, the multiple horizontal lines result from accidental ruling defects (see fig. 3).
of the diffracted wavefront, these defects being themselves produced by systematic and accidental ruling irregularities, as well as by cristallographic graininess of the thick aluminium layer in which the grooves are ruled. Because holographic gratings are produced by a process which is essentially static, we were anxious to compare the stray light level and distribution in holographic and ruled gratings. This comparison was achieved using a lo-meter spectrograph whose entrance slit was replaced by a pin-hole illuminated with 6328 A laser light (fig. 2). The level of stray light was observed to be lower in the case of holographic gratings than for the best ruled ones. This was explained by the fact that 1) accidental ruling defects are suppressed and 2) systematic random errors on the groove position are lower in the case of holographic gratings. As seen on the Schlieren photograph (fig. 3), the wavy grooves which are observed on one of the ruled gratings are responsible for the multiple horizontal lines of diffused light seen in the corresponding spectrum (fig. 2b), while the single horizontal line seen in the case of the other ruled grating (and in all others) results from the above mentioned systematic random error of ruling engines. Stray light, in the case of one holographic grating, was seen to originate mainly from a finger-print which occured on the grating once completed: this gives an idea of the optical quality attainable with holographic gratings.
Fig. 3. Schlieren aspect of ruled grating with accidental ruling defects. The oscillating grooves seen in several locations are responsible for the multiple horizontal lines in fig. 2b.
5. APPLICATIONS We believe that the most interesting applications for this technique will be in the production
Volume 1, number 1
OPTICS COMMUNICATIONS
of large gratings, plane or concave, featuring a high spatial frequency (2000 to 4000 lines/mm) with corresponding advantages regarding the free spectral range. Of particular interest are the new possibilities for making stigmatic concave gratings of high aperture as well as objective gratings for space experiments.
ACKNOWLEDGEMENT This work was supported by the French National Center for Space Studies (C. N. E.S.).
April 1969
REFERENCES [I] Y.Denysiuk, Dokl. Akad. Nauk SSSR, 144 (1962) 6. [2] Communications by K. Stetson and by A. K. Rigler and T. P. Vogl, Fiftieth Anniversary Meeting, Opt. Sot. Am. (1966). [3] A. Labeyrie, Thesis, Orsay (1966). [4] R. F. Jarrel and G. W. Stroke, Appl. Optics 3 (1964) [5] ?.K.Sheridon, Appl. Phys. Letters 12 (1966) 3 [6] G. W. Stroke, Rev. Optique 39 (1960) 291. [7j R. Petit, Thesis, Orsay (1966). [8] A. Wirgin, Thesis, Orsay (1967). [9] D.Gabor, Nature, No. 4098 (1948). [lo] G. W. Stroke, in: An introduction to coherent optics and holography (Academic Press, New York, 1966). [ll] A.L.Ingalls. Phot. Sci. Eng. 4 (1960) 3. [12] Rosen, Rev. Sci. In&r. 38 (1967) 3. [13] A.T.Shankoff, Appl. Optics 7 (1968) 10.
1, number
1
OPTICS COMMUNICATIONS
SPECTROGRAPHIC
PERFORMANCE DIFFRACTION
MADE
Antoine Observatoire
de Paris
April
1969
OF HOLOGRAPHICALLY GRATINGS
LABEYRIE -Meudon,
92, Meudon,
France
and Jean FLAMAND Soci&t& Jobin
et Yvon,
26, rue Berthollet,
Received
22 November
94, Arcueil,
France
1968
Diffraction gratings for spectrographic applications have been produced using holographic techniques. From the standpoint of efficiency, aberrations and stray light these gratings prove comparable or superior to conventionnal ruled gratings.
1. INTRODUCTION It appears that the theoretical possibility of obtaining diffraction gratings as material recordings of interference fringes was first mentioned, with a view to spectrographic applications, by Denisyuk [l] at the very beginning of the laser era (1962). This possibility was subsequently verified by a number of authors, most of them working in the field of holography [2,3,12]. Typically, such “holographic gratings” were obtained by illuminating high resolution photographic plates with two coherent light-waves from a laser. Large gratings could be obtained by this method within a few minutes, however these gratings were not suitable for spectrographic applications because of their poor optical quality and low diffracting efficiency. The optical properties of such holographic gratings have been studied in detail by one of us (A.L.), then at the University of Michigan [3]: it was concluded that conventional photographic emulsions as a recording medium had to be replaced by very thin photosensitive layers such as the Daguerre silver-iodide layer and dichromated gelatin or photoresist processes. It was also indicated that a sine-wave groove profile for gratings is not incompatible with high blaze efficiency. We have now been able to verify both assumptions. Good quality gratings bearing up to 3000 lines per mm were obtained in sizes up to 150 mm X 110 mm, and larger dimensions are expected to be possible.
We report here on the properties and performance of these holographic gratings, as made by us in the Jobin-Yvon ruling engine laboratory at Longjumeau (France): Glass blanks were polished to quarterwave flatness and coated with a thin photoresist film of controlled uniform thickness, then exposed to the interference field of two coherent plane waves of laser light. The plane waves were obtained through the use of good quality, diffraction limited, collimators. Special care had to be taken in order to avoid fringe drift and vibration during exposures (caused by thermal or acoustical disturbances as well as by atmospheric pressure variations). After exposure, the blanks were immersed in a suitable solvent, and rinsed in such a way as to obtain on their surface a periodic groove profile of optimum amplitude. The holographic gratings were subsequently aluminized for maximum efficiency. Replication was successfully achieved, using the same technique as for conventional ruled gratings [4].
2. BLAZE As predicted, it was observed that high blaze efficiencies are also attained with gratings featuring a sine-wave groove profile, as obtained by the holographic method (echelette groove profiles may also be produced holographically in the case of low spatial frequency gratings [5]). 5
Volume
1, number
1
OPTICS COMMUNICATIONS
This is especially the case when the groove spacing is comparable to or smaller than the wavelength. For example, a holographic grating bearing 2000 lines/mm was found to diffract in the first order 68% of an incident 5460 A polarized light-beam (which corresponds to about 75% true efficiency if the absorption of aluminium is taken into account). Another 3000 lines/mm grating gave 50% of a 6328 A light-beam in the first order. Probably because sine-wave profiles are not encountered on ruled gratings, little has been published on the electromagnetic theory of their blaze properties whereas saw-tooth (echelette) profiles have been studied in much detail, following initial work by Mare&al and Stroke [6,7,8]. We found experimentally that the blaze efficiency of sine-wave gratings obtained by holography can be equal to or higher than that of ruled echelette gratings when the groove spacing is smaller than approximately twice the wavelength. Thus, holographic gratings featuring either echelette or sine-wave groove profile may attain a high blaze efficiency when used in the visible spectral region. In view of space applications, it would be of interest to compare the blaze properties of both types of gratings in the vacuum ultra-violet region. 3. ABERRATIONS As shown by the elementary theory of holograms [9,10], aberration-free plane gratings are obtained only if the recording surface is optically flat and if both incident laser waves are plane within a fraction of a wavelength. Also, the recording medium should feature a high degree of rigidity during and after the processing phase
131.
It appears that these conditions were not satisfied in previously published experiments dealing with holography and holographic gratings. For example, as mentioned above, early holographic gratings recorded on photographic emulsions (such as Kodak 649 Lippmann-type plates) had very important aberrations because a) the emulsion thickness was not uniform [ll], b) the emulsion drifted, relative to the substrate, under the influence of stresses which appeared during the developing, fixing and drying phases of the processing [3]. The resulting aberrations of the reconstructed wavefronts were tolerable for most holographic, spatial filtering or holographic in-
6
April
1969
Fig. 1. Michelson interferogram of holographic grating (size 75 mm, 2000 grooves/mm, first order diffracted wavefront). The residual aberrations result from substrate imperfections.
terferometric applications, but certainly not for diffraction gratings. It was realized that these dimensional instability effects could be avoided by using a recording layer much thinner than the conventional 10 microns thick emulsion. This layer was also required to be homogeneous with a continuous, rather than grainy, response to illumination. Among the wide range of existing photosensitive organic compounds [13], photoresists were selected because of their excellent chemical stability which allows direct vacuum aluminizing of the grating surface. Fig. 1 shows the diffracted wavefront of a grating recorded on photoresist, as examined by the Michelson-Twyman method [18]. It should be noted that the residual aberrations which are seen here result from imperfections of the substrate. Resolution tests with the 5461 A line of mercury have shown that the theoretical resolving power is attained. 4. STRAY LIGHT Diffused light in ruled gratings consists of the well-known Rowland and Lyman ghost patterns and of a more or less uniform background. It originates in the rapidly varying phase defects
Volume
1, number 1
OPTICS COMMUMCATIONS
April
1969
c)
Fig. 2. Stray light patterns of gratings: a) ruled grating (120 X 140 mm, 305 grooves/mm); b) ruled grating from a different manufacturer (254 X 127 mm, 300 grooves/mm); c) holographic grating (75 X 75 mm, 2000 grooves/mm). The single horizontal bright line observed in the spectra of all ruled gratings result from systematic ruling errors. In b, the multiple horizontal lines result from accidental ruling defects (see fig. 3).
of the diffracted wavefront, these defects being themselves produced by systematic and accidental ruling irregularities, as well as by cristallographic graininess of the thick aluminium layer in which the grooves are ruled. Because holographic gratings are produced by a process which is essentially static, we were anxious to compare the stray light level and distribution in holographic and ruled gratings. This comparison was achieved using a lo-meter spectrograph whose entrance slit was replaced by a pin-hole illuminated with 6328 A laser light (fig. 2). The level of stray light was observed to be lower in the case of holographic gratings than for the best ruled ones. This was explained by the fact that 1) accidental ruling defects are suppressed and 2) systematic random errors on the groove position are lower in the case of holographic gratings. As seen on the Schlieren photograph (fig. 3), the wavy grooves which are observed on one of the ruled gratings are responsible for the multiple horizontal lines of diffused light seen in the corresponding spectrum (fig. 2b), while the single horizontal line seen in the case of the other ruled grating (and in all others) results from the above mentioned systematic random error of ruling engines. Stray light, in the case of one holographic grating, was seen to originate mainly from a finger-print which occured on the grating once completed: this gives an idea of the optical quality attainable with holographic gratings.
Fig. 3. Schlieren aspect of ruled grating with accidental ruling defects. The oscillating grooves seen in several locations are responsible for the multiple horizontal lines in fig. 2b.
5. APPLICATIONS We believe that the most interesting applications for this technique will be in the production
Volume 1, number 1
OPTICS COMMUNICATIONS
of large gratings, plane or concave, featuring a high spatial frequency (2000 to 4000 lines/mm) with corresponding advantages regarding the free spectral range. Of particular interest are the new possibilities for making stigmatic concave gratings of high aperture as well as objective gratings for space experiments.
ACKNOWLEDGEMENT This work was supported by the French National Center for Space Studies (C. N. E.S.).
April 1969
REFERENCES [I] Y.Denysiuk, Dokl. Akad. Nauk SSSR, 144 (1962) 6. [2] Communications by K. Stetson and by A. K. Rigler and T. P. Vogl, Fiftieth Anniversary Meeting, Opt. Sot. Am. (1966). [3] A. Labeyrie, Thesis, Orsay (1966). [4] R. F. Jarrel and G. W. Stroke, Appl. Optics 3 (1964) [5] ?.K.Sheridon, Appl. Phys. Letters 12 (1966) 3 [6] G. W. Stroke, Rev. Optique 39 (1960) 291. [7j R. Petit, Thesis, Orsay (1966). [8] A. Wirgin, Thesis, Orsay (1967). [9] D.Gabor, Nature, No. 4098 (1948). [lo] G. W. Stroke, in: An introduction to coherent optics and holography (Academic Press, New York, 1966). [ll] A.L.Ingalls. Phot. Sci. Eng. 4 (1960) 3. [12] Rosen, Rev. Sci. In&r. 38 (1967) 3. [13] A.T.Shankoff, Appl. Optics 7 (1968) 10.