CA2331240A1 - Optical reflection grating and method for its optimization and optical spectrometer - Google Patents
Optical reflection grating and method for its optimization and optical spectrometer Download PDFInfo
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- 238000005457 optimization Methods 0.000 title description 2
- 238000001228 spectrum Methods 0.000 claims description 5
- 230000010287 polarization Effects 0.000 abstract description 12
- 230000003595 spectral effect Effects 0.000 description 4
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 description 3
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- 239000004922 lacquer Substances 0.000 description 3
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B5/00—Optical elements other than lenses
- G02B5/18—Diffraction gratings
- G02B5/1861—Reflection gratings characterised by their structure, e.g. step profile, contours of substrate or grooves, pitch variations, materials
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Abstract
In an optical reflection grating with a multitude of parallel grooves (5), for a ratio of grating period (g) to wavelength (.lambda.) of smaller than 0.9, preferably of smaller than 0.7, the groove depth is chosen so deep that the diffraction efficiencies (DE) for TE
and TM
waves of a certain wavelength (.lambda.) differ by less than 10%, preferably by less than 5%
from one another and are at least 60% respectively. That means, the diffraction efficiency (DE) is practically independent fro m the polarization direction of the incident light and is very high.
and TM
waves of a certain wavelength (.lambda.) differ by less than 10%, preferably by less than 5%
from one another and are at least 60% respectively. That means, the diffraction efficiency (DE) is practically independent fro m the polarization direction of the incident light and is very high.
Description
P7473EI' Optical reflection grating and method for its optimization and optical spectrometer The invention concerns an optical reflection grating with a multitude of parallel grooves, an optical spectrometer with such a reflection grating and a method for finding an optical reflection grating with optimum diffraction efficiency for TE and TM waves of a certain wavelength.
It is common knowledge that optical reflection gratings consist of a plane or curved carrier plate, onto which a periodic division in form of many parallel grating grooves or flutes (lines) are deposited. The surface of the reflection grating is highly reflective.
According to the shape of the grating grooves, one speaks of sinusoidal gratings, rectangular grating, saw-toothed gratings or triangular gratings. The periodic grating structures are of similar size as the wavelength of the incident light, such that the amplitude or phase of the reflected light are changed, which leads to the diffraction of light.
Nonpolarized light incident on an optical reflection grating can be separated into two polarized parts being orthogonal to each other, the E-vectors thereof are orthogonal and parallel to the plane of incidence defined by the direction of incidence and the normal direction. They are called orthogonal and parallel polarization or 'rE
(transverse electric) and TM (transverse magnetic) wavca. The diffraction efficiency depends on the wavelength and is very different for TE and TM waves, such that their intensities differ from one another. This must be taken into account in evaluating the results of the optical spectrum analysis by suitable measures, for example structural optical measures.
Indeed, optical reflection gratings exist, where both polarization directions are present with high diffraction efficiency in a, certain wavelength range. These reflection gratings posses only a small line density in relation to the wavelength (for a wavelength of 1550 nm about 600 lines/mm, i.e. a grating period (g) / wavelength (~,) ratio of about 1.1;).
These known reflection gratings have the disadvantage that they split the incident wavelength range only into a very small angular range and thus are disadvantageous for the spectral resolution and accuracy. A higher spectral resolution and accuracy results from a grating period (g) / wavelength (7~) ratio of 0.7.
Hence, it is the object of the invention, to improve an optical grating of the aforementioned type such that a dii:fraction efficiency as equal as possible can be reached for TE and TM waves with, at the same time, a diffraction efficiency as high as possible.
According to the invention this object is solved for an optical reflection grating in that for a ratio of the grating period to the wavelength of smaller than 0.9, preferably smaller than 0.7, the groove depth is chosen so deep that the diffraction efficiencies for TE and TM
waves of a certain wavelength diffc,r by less than 10%, preferably by less than 5%, from one another and is at least 60% respectively.
For such inventive optical reflection gratings it was found that the diffraction efficiency for TE and TM waves is almost equal and is over 60%. That means that the diffraction efficiency is practically independent from the polarization direction of the incident light and is very high.
Preferably, the ratio of groove depth to grating period is larger than 0.7, particularly preferred about 0.8 to 2Ø
The optical reflector grating is particularly a sinusoidal grating, a rectangular grating, a saw-toothed grating or a triangular grating, but all other grating shapes are also possible.
It was found for a sinusoidal grating, that the ratio of groove depth to grating period is optimal at about 0.9, 1.25, 1.9, or preferably about 1.8. The ratio of groove depth to grating period for saw-toothed gratings is optimal at about 1.25 or 1.7, whereas the ratio of groove depth to grating period i;> optimal at about 1.0, 1.25, or 1.85 for triangular gratings.
It was found for a rectangular grating that the optimal ratio of graove depth to grating period strongly depends on the mark-space-ratio of the ridge width to the grating period and that only for a mark-space-ratio below 0.35 useful reflection gratings are possible.
In preferred embodiments of the invention, the groove depth is about 0.6 to 1.4 times the wavelength of the incident light, where the groove depth is preferably 0.65 times, 0.9 times, 1.35 times or particularly preferred 1.25 times the wavelength of the incident light.
Depending on the application, an optical reflection grating tuned to the desired wavelength must be used. For the use in optical communications, the optical reflection grating is preferably designed for the diffraction of light with a wavelength of about 1500 to 1600nm, particularly of about 1:p50nm.
The surface of the optical reflection grating can be plane or curved, in particular concave.
The invention also concerns an optical spectrometer, in particular an optical spectrum analyzer with an optical reflection grating as described above. The advantage of such a spectrometer consists in that an almost equally high diffraction efficiency can be obtained for the incident light independent from its particular polarization direction.
Finally, the invention also concerns a method for finding an optical reflection grating with almost equally high diffraction efficiency for TE and TM waves of a certain wavelength with given grating period and given shape of the grating grooves.
According to the invention the groove depth is increased until the diffraction efficiencies for TE and TM waves differ by less than 10% from one another and are at least 60%
respectively.
Generally it must be pointed out that for the groove depths given above a tolerance of +/-10% must always be applied.
Additional advantages of the invention can be gathered from the description and the drawing. Also, the previously mentioned and the following characteristics can be used according to the invention each individually or collectively in any combination. The embodiments shown and described are not to be taken as a conclusive enumeration, but have exemplary character for the description of the invention.
Fig. 1 shows the diffraction efficiency of an inventive sinusoidal grating as a function of its groove depth for the TE and the; 'TM waves of the incident light.
Fig. 2 shows the diffraction efficiency of an inventive sinusoidal grating with a groove depth of 1.95 pm as a function of the wavelength for the TE and TM waves of the incident light.
Fig. 3 shows the diffraction efficiency of an inventive rectangular grating with a groove depth of 1.05 pm and a mark-space-ratio of 0.25 as a function of the wavelength for the TE and TM waves of the incident :Light.
Fig. 4 shows the diffraction efficiency of an inventive triangular grating with a groove depth of 2.05 ~m as a function of the wavelength for the TE and TM waves of the incident light.
Fig. 5 shows various kinds of optical reflection gratings.
Fig. 6 shows the typical construction of an optical spectrum analyzer with an optical reflection grating.
Fig. 5 shows an overview of the various optical reflection gratings in a basic presentation.
In Fig. Sa a sinusoidal grating 1 is shown, in Fig. Sb a rectangular grating 2 is shown, in Fig. Sc a saw-toothed grating 3 is shown, and in Fig. Sd a triangular grating 4 is shown.
In all the figures Sa through Sd, the grooves of the gratings axe denoted with 5, the period of the gratings with g and the groove depths with T. For the rectangular grating 2 the ridge is additionally denoted with ,3. The surface of the individual reflection gratings is coated with a reflecting layer, for evxample, aluminum or gold, as indicated by the hatching.
These optical gratings are produced in the traditional way by scribing grooves in a glass substrate having a low thermal expansion or into a film of aluminum or gold on glass with a diamond. This original is first coated with a layer of non-sticking material which can later again be separated from t:he original. Aluminum evaporation follows this step. A
lacquer layer follows over this layf;r combination, onto which lacquer layer is placed the future copy in the still uncured stage. The copy can be separated from the original when the artificial resin lacquer is cured. Naturally, the optical reflection grating can be produced by any other known method, in particular by a holographic production method.
The nonpolarized light incident on an optical reflection grating can be separated into two polarized parts being orthogonal to one another, the E-vectors thereof are orthogonal and parallel to the plane of incidence defined by the direction of incidence and the normal direction. They are called orthogonal and parallel polarization or TE
(transverse electric) and TM (transverse magnetic) waves.
Optical reflection gratings exist, far which the two polarization directions are present in a certain wavelength range with high efficiency. These reflection gratings have, however, a small line density in relation to their wavelength (for a wavelength of 1550 nm about 600 lines/mm, i.e. a grating period (g) / wavelength (7~) ratio of about 1.1 ).
These known reflection gratings have the disadvantage of splitting up the incident wavelength range into only a very small angular range, and are thus disadvantageous for the spectral resolution and accuracy. A higher spectral resolution and accuracy is obtained for a grating period (g) / wavelength (~,) ratio of 0.7. This value is to be seen as a compromise between many parameters and is also used here.
The optical reflection gratings described in the following are the gratings shown in Fig. 5 with a grating density of 900 lines per mm, which corresponds to a grating period of 1.11 pm. The angle of incidence of the light was 55° and the angle of reflection 35°, i.e. an angular difference of 20°. This angular difference was chosen for mechanical reasons.
Generally, this angular difference can be slightly changed without changing much in the reflection efficiency or optimal depth. For the reflection gratings used in the following, which are relatively dense in relation to the wavelength, the reflection efficiency generally increases, particularly for the TE-wave, if the angular difference between incident and reflected light rays is reduced (quasi Littrow). The corresponding change of the optimum groove depth should be relatively small.
In Fig. 1 the diffraction efficiency DE (in percent) of a sinusoidal grating 1 is shown as a function of its groove depth T (in pm) for the TE and TM waves of the incident light (wavelength ~, = 1550 nm). It was found that the diffraction efficiency DE for the 'TE
wave rises continually with increasing groove depth T to about 88% at a groove depth of 2.80 p.m, whereas in this range the diffraction efficiency DE changes periodically between 2% and 96% with increasing groove depth T for the TM wave. The diffraction efficiency DE is equal for both waves in the points of intersection A, B, C, and D of the two TE and TM curves, i.e. at groove depths of about 1.95 pm, about 1.0 Vim, about 1.4 pm and about 2.1 Vim. Generally, this corresponds to a groove depth (T) /
wavelength (~,) ratio of 1.26, 0.65, 0.9, and 1.35, where among these values the ratio of 1.26 is the optimum value. The diffraction efjficiency DE for TE and TM waves is equally high in the points of intersection A through D, i.e. independent from the polarizing direction of the incident light. In particular, in the point of intersection A the diffraction efficiency DE
is over 85%.
In Fig. 2 the diffraction efficiency DE of a sinusoidal grating 1 with a groove depth of 1.95 pm is plotted as a function of the wavelength n, for the TE and TM waves of the incident light. It was found that in a larger wavelength range of about 1420 nm to about 1570 nm the diffraction efficiency DE is almost the same for both waves and is almost 90%. In this wavelength range the diffraction efficiency is almost independent of the polarizing direction of the incident light.
The calculated values of the diffraction efficiency DE are summarized in table I for different groove depths T at differe;nt wavelengths ~,, and that for TE and TM
waves.
From table I it is clear that for a sinusoidal grating with a groove depth between 1.95 ~m and 2.1 pm a very high diffraction efficiency with minimal dependence on polarization is found for both waves. This optimal range is highlighted with bold numbers in table I.
In Fig. 3 the diffraction efficiency of an inventive rectangular grating 2 with a groove depth of 1.05 ~m and a mark-space-ratio of 0.25 is plotted for the TE and TM
waves of the incident light (grating period: 1.11 p,m; angle of incidence: 55°;
angle of reflection:
35°). It was found that the diffraction efficiency DE is almost the same for both waves in a larger wavelength range of about 1530nm to about 1620nm and is over 85%. The diffraction efficiency of the inventiive rectangular grating 2 is almost independent of the polarization direction of the incident light in this wavelength range.
It was found for rectangular gratin;;s 4 that the optimum ratio of groove depth T to grating period g strongly depends on the mark-space-ratio of the ridge width S
to the grating period g and that useful diffraction gratings are only possible for mark-space ratios S below 0.35. In table II the optimum ratio of groove depth T to grating period g is summarized for rectangular gratinc;s with different mark-space-ratios.
In Fig. 4 the diffraction efficiency of an inventive triangular grating 4 with a groove depth of 2.05 ~m is plotted for the TE and TM waves of the incident light (grating period: 1.11 pm; angle of incidence: 55°; angle of reflection:
35°). It was found that the diffraction efficiency DE is almost the same for both waves in a larger wavelength range of about 1550 nm to about 1610 nrn and is over 85%. The diffraction efficiency of the inventive triangular grating 4 is almost independent of the polarization direction of the incident light in this wavelength range.
Such a reflection grating which is :independent of polarization in a certain wavelength range, is preferably finds application in optical spectrometers like for example spectrum analyzers. The typical construction of such a spectrum analyzer 10 is shown in Fig. 6.
The light entering through an entr,mce gap 11 is collimated via a lense 12 onto an optical reflection grating 13, which diffracts the wavelengths present in light to different extents.
The light passing through an exit f;ap 16 hits a detector 14 (for example a photo diode), such that the light intensity can be measured for a certain wavelength. The respective wavelength measured can be changed by turning the reflection grating 13 in direction of the double arrow 15. An almost equally high diffraction efficiency can be obtained with the inventive reflection gratings described earlier for the incident light independent of its polarization direction.
Table I
DE DE DE DE DE
[o] [o] [o] [o] [o]
~ [nm]T=1, T=1 ~ T=2 T=2 95 , =1 , , ~Lm 8 , 0 1 ~m 9 ~m ~,m ~.m TE TM TE TM TE TM TE TM TE TM
1280 21.2 37.2 53.6 26.3 63.5 16.6 57.5 8.6 37.4 61.0 1300 31.7 47.5 58.1 36.9 66.1 26.7 52.4 17.4 31.1 61.6 1350 58.1 70.0 66.9 62.3 76.6 53.8 59.0 44.9 25.3 71.7 1400 77.0 83.7 69.5 79.5 83.2 74.4 74.5 68.4 35.9 82.9 1450 86.9 89:3 68.0 87.9 84.7 85.8 85_8 82.9 57.7 88.0 1500 89_6 89.1 64.0 89_5 83.3 89.4 90.4 88.8 77.0 88.7 1520 89.1 87.8 61.8 88.8 82.3 89.3 91_0 89.3 82.2 88.3 1545 87.7 85.6 58.5 87_1 80.8 88_2 91.1 88_8 86_8 87.5 i 1570 85_4186_3 82.8 54.5 84.6 78.9 86_1 90_8 87_3 89_8 1600 82.0;84.7 78.9 48.8 81.0 76.3 82.9 90.0 84.4 91.8 1650 74.9 71.5 36.7 73.8 170.676.0 88.0 77.9 93.0 81.1 Table II
Mark-Space-ratio optimal ratio of T
to g 0.05 0.81; 1.5; 1.8 0.1 0.81; 1.5 0.15 0.81 0.2 0.81; 2.2 0.25 c).54 through 1.3; 2.0 through 2.4 0,3 0.63; 0.9; 1.6 0.33 1.0; 1.5; 1.7; 2.3; 2.4 0.35 0.81; 1.2; 1.5; 1,8; 2.2; 2.5
It is common knowledge that optical reflection gratings consist of a plane or curved carrier plate, onto which a periodic division in form of many parallel grating grooves or flutes (lines) are deposited. The surface of the reflection grating is highly reflective.
According to the shape of the grating grooves, one speaks of sinusoidal gratings, rectangular grating, saw-toothed gratings or triangular gratings. The periodic grating structures are of similar size as the wavelength of the incident light, such that the amplitude or phase of the reflected light are changed, which leads to the diffraction of light.
Nonpolarized light incident on an optical reflection grating can be separated into two polarized parts being orthogonal to each other, the E-vectors thereof are orthogonal and parallel to the plane of incidence defined by the direction of incidence and the normal direction. They are called orthogonal and parallel polarization or 'rE
(transverse electric) and TM (transverse magnetic) wavca. The diffraction efficiency depends on the wavelength and is very different for TE and TM waves, such that their intensities differ from one another. This must be taken into account in evaluating the results of the optical spectrum analysis by suitable measures, for example structural optical measures.
Indeed, optical reflection gratings exist, where both polarization directions are present with high diffraction efficiency in a, certain wavelength range. These reflection gratings posses only a small line density in relation to the wavelength (for a wavelength of 1550 nm about 600 lines/mm, i.e. a grating period (g) / wavelength (~,) ratio of about 1.1;).
These known reflection gratings have the disadvantage that they split the incident wavelength range only into a very small angular range and thus are disadvantageous for the spectral resolution and accuracy. A higher spectral resolution and accuracy results from a grating period (g) / wavelength (7~) ratio of 0.7.
Hence, it is the object of the invention, to improve an optical grating of the aforementioned type such that a dii:fraction efficiency as equal as possible can be reached for TE and TM waves with, at the same time, a diffraction efficiency as high as possible.
According to the invention this object is solved for an optical reflection grating in that for a ratio of the grating period to the wavelength of smaller than 0.9, preferably smaller than 0.7, the groove depth is chosen so deep that the diffraction efficiencies for TE and TM
waves of a certain wavelength diffc,r by less than 10%, preferably by less than 5%, from one another and is at least 60% respectively.
For such inventive optical reflection gratings it was found that the diffraction efficiency for TE and TM waves is almost equal and is over 60%. That means that the diffraction efficiency is practically independent from the polarization direction of the incident light and is very high.
Preferably, the ratio of groove depth to grating period is larger than 0.7, particularly preferred about 0.8 to 2Ø
The optical reflector grating is particularly a sinusoidal grating, a rectangular grating, a saw-toothed grating or a triangular grating, but all other grating shapes are also possible.
It was found for a sinusoidal grating, that the ratio of groove depth to grating period is optimal at about 0.9, 1.25, 1.9, or preferably about 1.8. The ratio of groove depth to grating period for saw-toothed gratings is optimal at about 1.25 or 1.7, whereas the ratio of groove depth to grating period i;> optimal at about 1.0, 1.25, or 1.85 for triangular gratings.
It was found for a rectangular grating that the optimal ratio of graove depth to grating period strongly depends on the mark-space-ratio of the ridge width to the grating period and that only for a mark-space-ratio below 0.35 useful reflection gratings are possible.
In preferred embodiments of the invention, the groove depth is about 0.6 to 1.4 times the wavelength of the incident light, where the groove depth is preferably 0.65 times, 0.9 times, 1.35 times or particularly preferred 1.25 times the wavelength of the incident light.
Depending on the application, an optical reflection grating tuned to the desired wavelength must be used. For the use in optical communications, the optical reflection grating is preferably designed for the diffraction of light with a wavelength of about 1500 to 1600nm, particularly of about 1:p50nm.
The surface of the optical reflection grating can be plane or curved, in particular concave.
The invention also concerns an optical spectrometer, in particular an optical spectrum analyzer with an optical reflection grating as described above. The advantage of such a spectrometer consists in that an almost equally high diffraction efficiency can be obtained for the incident light independent from its particular polarization direction.
Finally, the invention also concerns a method for finding an optical reflection grating with almost equally high diffraction efficiency for TE and TM waves of a certain wavelength with given grating period and given shape of the grating grooves.
According to the invention the groove depth is increased until the diffraction efficiencies for TE and TM waves differ by less than 10% from one another and are at least 60%
respectively.
Generally it must be pointed out that for the groove depths given above a tolerance of +/-10% must always be applied.
Additional advantages of the invention can be gathered from the description and the drawing. Also, the previously mentioned and the following characteristics can be used according to the invention each individually or collectively in any combination. The embodiments shown and described are not to be taken as a conclusive enumeration, but have exemplary character for the description of the invention.
Fig. 1 shows the diffraction efficiency of an inventive sinusoidal grating as a function of its groove depth for the TE and the; 'TM waves of the incident light.
Fig. 2 shows the diffraction efficiency of an inventive sinusoidal grating with a groove depth of 1.95 pm as a function of the wavelength for the TE and TM waves of the incident light.
Fig. 3 shows the diffraction efficiency of an inventive rectangular grating with a groove depth of 1.05 pm and a mark-space-ratio of 0.25 as a function of the wavelength for the TE and TM waves of the incident :Light.
Fig. 4 shows the diffraction efficiency of an inventive triangular grating with a groove depth of 2.05 ~m as a function of the wavelength for the TE and TM waves of the incident light.
Fig. 5 shows various kinds of optical reflection gratings.
Fig. 6 shows the typical construction of an optical spectrum analyzer with an optical reflection grating.
Fig. 5 shows an overview of the various optical reflection gratings in a basic presentation.
In Fig. Sa a sinusoidal grating 1 is shown, in Fig. Sb a rectangular grating 2 is shown, in Fig. Sc a saw-toothed grating 3 is shown, and in Fig. Sd a triangular grating 4 is shown.
In all the figures Sa through Sd, the grooves of the gratings axe denoted with 5, the period of the gratings with g and the groove depths with T. For the rectangular grating 2 the ridge is additionally denoted with ,3. The surface of the individual reflection gratings is coated with a reflecting layer, for evxample, aluminum or gold, as indicated by the hatching.
These optical gratings are produced in the traditional way by scribing grooves in a glass substrate having a low thermal expansion or into a film of aluminum or gold on glass with a diamond. This original is first coated with a layer of non-sticking material which can later again be separated from t:he original. Aluminum evaporation follows this step. A
lacquer layer follows over this layf;r combination, onto which lacquer layer is placed the future copy in the still uncured stage. The copy can be separated from the original when the artificial resin lacquer is cured. Naturally, the optical reflection grating can be produced by any other known method, in particular by a holographic production method.
The nonpolarized light incident on an optical reflection grating can be separated into two polarized parts being orthogonal to one another, the E-vectors thereof are orthogonal and parallel to the plane of incidence defined by the direction of incidence and the normal direction. They are called orthogonal and parallel polarization or TE
(transverse electric) and TM (transverse magnetic) waves.
Optical reflection gratings exist, far which the two polarization directions are present in a certain wavelength range with high efficiency. These reflection gratings have, however, a small line density in relation to their wavelength (for a wavelength of 1550 nm about 600 lines/mm, i.e. a grating period (g) / wavelength (7~) ratio of about 1.1 ).
These known reflection gratings have the disadvantage of splitting up the incident wavelength range into only a very small angular range, and are thus disadvantageous for the spectral resolution and accuracy. A higher spectral resolution and accuracy is obtained for a grating period (g) / wavelength (~,) ratio of 0.7. This value is to be seen as a compromise between many parameters and is also used here.
The optical reflection gratings described in the following are the gratings shown in Fig. 5 with a grating density of 900 lines per mm, which corresponds to a grating period of 1.11 pm. The angle of incidence of the light was 55° and the angle of reflection 35°, i.e. an angular difference of 20°. This angular difference was chosen for mechanical reasons.
Generally, this angular difference can be slightly changed without changing much in the reflection efficiency or optimal depth. For the reflection gratings used in the following, which are relatively dense in relation to the wavelength, the reflection efficiency generally increases, particularly for the TE-wave, if the angular difference between incident and reflected light rays is reduced (quasi Littrow). The corresponding change of the optimum groove depth should be relatively small.
In Fig. 1 the diffraction efficiency DE (in percent) of a sinusoidal grating 1 is shown as a function of its groove depth T (in pm) for the TE and TM waves of the incident light (wavelength ~, = 1550 nm). It was found that the diffraction efficiency DE for the 'TE
wave rises continually with increasing groove depth T to about 88% at a groove depth of 2.80 p.m, whereas in this range the diffraction efficiency DE changes periodically between 2% and 96% with increasing groove depth T for the TM wave. The diffraction efficiency DE is equal for both waves in the points of intersection A, B, C, and D of the two TE and TM curves, i.e. at groove depths of about 1.95 pm, about 1.0 Vim, about 1.4 pm and about 2.1 Vim. Generally, this corresponds to a groove depth (T) /
wavelength (~,) ratio of 1.26, 0.65, 0.9, and 1.35, where among these values the ratio of 1.26 is the optimum value. The diffraction efjficiency DE for TE and TM waves is equally high in the points of intersection A through D, i.e. independent from the polarizing direction of the incident light. In particular, in the point of intersection A the diffraction efficiency DE
is over 85%.
In Fig. 2 the diffraction efficiency DE of a sinusoidal grating 1 with a groove depth of 1.95 pm is plotted as a function of the wavelength n, for the TE and TM waves of the incident light. It was found that in a larger wavelength range of about 1420 nm to about 1570 nm the diffraction efficiency DE is almost the same for both waves and is almost 90%. In this wavelength range the diffraction efficiency is almost independent of the polarizing direction of the incident light.
The calculated values of the diffraction efficiency DE are summarized in table I for different groove depths T at differe;nt wavelengths ~,, and that for TE and TM
waves.
From table I it is clear that for a sinusoidal grating with a groove depth between 1.95 ~m and 2.1 pm a very high diffraction efficiency with minimal dependence on polarization is found for both waves. This optimal range is highlighted with bold numbers in table I.
In Fig. 3 the diffraction efficiency of an inventive rectangular grating 2 with a groove depth of 1.05 ~m and a mark-space-ratio of 0.25 is plotted for the TE and TM
waves of the incident light (grating period: 1.11 p,m; angle of incidence: 55°;
angle of reflection:
35°). It was found that the diffraction efficiency DE is almost the same for both waves in a larger wavelength range of about 1530nm to about 1620nm and is over 85%. The diffraction efficiency of the inventiive rectangular grating 2 is almost independent of the polarization direction of the incident light in this wavelength range.
It was found for rectangular gratin;;s 4 that the optimum ratio of groove depth T to grating period g strongly depends on the mark-space-ratio of the ridge width S
to the grating period g and that useful diffraction gratings are only possible for mark-space ratios S below 0.35. In table II the optimum ratio of groove depth T to grating period g is summarized for rectangular gratinc;s with different mark-space-ratios.
In Fig. 4 the diffraction efficiency of an inventive triangular grating 4 with a groove depth of 2.05 ~m is plotted for the TE and TM waves of the incident light (grating period: 1.11 pm; angle of incidence: 55°; angle of reflection:
35°). It was found that the diffraction efficiency DE is almost the same for both waves in a larger wavelength range of about 1550 nm to about 1610 nrn and is over 85%. The diffraction efficiency of the inventive triangular grating 4 is almost independent of the polarization direction of the incident light in this wavelength range.
Such a reflection grating which is :independent of polarization in a certain wavelength range, is preferably finds application in optical spectrometers like for example spectrum analyzers. The typical construction of such a spectrum analyzer 10 is shown in Fig. 6.
The light entering through an entr,mce gap 11 is collimated via a lense 12 onto an optical reflection grating 13, which diffracts the wavelengths present in light to different extents.
The light passing through an exit f;ap 16 hits a detector 14 (for example a photo diode), such that the light intensity can be measured for a certain wavelength. The respective wavelength measured can be changed by turning the reflection grating 13 in direction of the double arrow 15. An almost equally high diffraction efficiency can be obtained with the inventive reflection gratings described earlier for the incident light independent of its polarization direction.
Table I
DE DE DE DE DE
[o] [o] [o] [o] [o]
~ [nm]T=1, T=1 ~ T=2 T=2 95 , =1 , , ~Lm 8 , 0 1 ~m 9 ~m ~,m ~.m TE TM TE TM TE TM TE TM TE TM
1280 21.2 37.2 53.6 26.3 63.5 16.6 57.5 8.6 37.4 61.0 1300 31.7 47.5 58.1 36.9 66.1 26.7 52.4 17.4 31.1 61.6 1350 58.1 70.0 66.9 62.3 76.6 53.8 59.0 44.9 25.3 71.7 1400 77.0 83.7 69.5 79.5 83.2 74.4 74.5 68.4 35.9 82.9 1450 86.9 89:3 68.0 87.9 84.7 85.8 85_8 82.9 57.7 88.0 1500 89_6 89.1 64.0 89_5 83.3 89.4 90.4 88.8 77.0 88.7 1520 89.1 87.8 61.8 88.8 82.3 89.3 91_0 89.3 82.2 88.3 1545 87.7 85.6 58.5 87_1 80.8 88_2 91.1 88_8 86_8 87.5 i 1570 85_4186_3 82.8 54.5 84.6 78.9 86_1 90_8 87_3 89_8 1600 82.0;84.7 78.9 48.8 81.0 76.3 82.9 90.0 84.4 91.8 1650 74.9 71.5 36.7 73.8 170.676.0 88.0 77.9 93.0 81.1 Table II
Mark-Space-ratio optimal ratio of T
to g 0.05 0.81; 1.5; 1.8 0.1 0.81; 1.5 0.15 0.81 0.2 0.81; 2.2 0.25 c).54 through 1.3; 2.0 through 2.4 0,3 0.63; 0.9; 1.6 0.33 1.0; 1.5; 1.7; 2.3; 2.4 0.35 0.81; 1.2; 1.5; 1,8; 2.2; 2.5
Claims (15)
1. Optical reflection grating with a multitude of parallel grooves (5), characterized in that for a ratio of grating period (g) to wavelength (.lambda.) of smaller than 0.9, preferably of smaller than 0.7, the groove depth (T) is chosen so deep that the diffraction efficiencies (DE) for TE and TM waves of a certain wavelength (.lambda.) differ by less than 10%, preferably by less than 5 % from one another and are at least 60%
respectively.
respectively.
2. Optical reflection grating, in particular according to claim 1, characterized in that the ratio of groove depth (T) to grating period (g) is larger than 0.7.
3. Optical reflection grating according to claim 2, characterized in that the ratio of groove depth (T) to grating period (g) is about 0.8 to 2Ø
4. Optical reflection grating according to any of the preceding claims, characterized in that the optical reflection grating is a sinusoidal grating (1), a rectangular grating (2), a saw-toothed grating (3), or a triangular grating (4).
5. Optical reflection grating according to claim 4, characterized in that for a sinusoidal grating (1) the ratio of groove depth (T) to grating period (g) is about 0.9, 1.25, 1.9, or preferably about 1.8.
6. Optical reflection grating according to claim 4, characterized in that for a rectangular grating (2) the mark-space-ratio of ridge width (S) to grating period (g) is smaller than about 0.35.
7. Optical reflection grating according to claim 4, characterized in that for a saw-toothed grating (3) the ratio of grating depth (T) to grating period (g) is about 1.25 or 1.7.
8. Optical reflection grating according to claim 4, characterized in that for a triangular grating (4) the ratio of groove depth (T) to grating period (g) is about 1.0, 1.25, or 1.85.
9. Optical reflection grating according to any of the preceding claims, characterized in that the groove depth (T) is about 0.6 to 1.4 times the wavelength (.lambda.) of the incident light.
10. Optical reflection grating according to claim 9, characterized in that the groove depth (T) is about 0.65 times, 0.9 tunes, 1.35 times, or preferably 1.25 times the wavelength (.lambda.) of the incident light.
11. Optical reflection grating according to any of the preceding claims, characterized in that the optical reflection grating is designed for the diffraction of light with a wavelength of about 1500 to 1600 nm, preferably of about 1550 nm.
12. Optical reflection grating according to any of the previous claims, characterized in that the surface of the optical reflection grating is flat.
13. Optical reflection grating according to any of the claims 1 through 11, characterized in that the surface of the optical reflection grating is curved.
14. Optical spectrometer, particularly optical spectrum analyzer (10), with an optical reflection grating according to any of the claims 1 through 13.
15. Method for finding an optical reflection grating with almost equally high diffraction efficiency (DE) for TE and TM waves of a certain wavelength (.lambda.) with given grating period (g) and given shape of the grating grooves (5), characterized in that the groove depth (T) is increased until the diffraction efficiencies (DE) for TE and TM
waves differ by less than 10% from one another and are at least 60% respectively.
waves differ by less than 10% from one another and are at least 60% respectively.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| EP00100864.8 | 2000-01-18 | ||
| EP20000100864 EP1130422B1 (en) | 2000-01-18 | 2000-01-18 | Optical reflection grating, method of optimizing such a grating and optical spectrometer |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2331240A1 true CA2331240A1 (en) | 2001-07-18 |
Family
ID=8167644
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2331240 Abandoned CA2331240A1 (en) | 2000-01-18 | 2001-01-17 | Optical reflection grating and method for its optimization and optical spectrometer |
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| Country | Link |
|---|---|
| EP (1) | EP1130422B1 (en) |
| JP (1) | JP2001242314A (en) |
| CA (1) | CA2331240A1 (en) |
| DE (1) | DE50000086D1 (en) |
Cited By (2)
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|---|---|---|---|---|
| CN102998730A (en) * | 2012-12-24 | 2013-03-27 | 四川省宜宾普什集团3D有限公司 | Stereo grating |
| EP3269291A4 (en) * | 2015-09-25 | 2018-10-24 | Olympus Corporation | Image calibration inspection tool and endoscope system |
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| US6859317B1 (en) * | 2000-06-02 | 2005-02-22 | Confluent Photonics Corporation | Diffraction grating for wavelength division multiplexing/demultiplexing devices |
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| JP6188743B2 (en) * | 2014-06-19 | 2017-08-30 | キヤノン株式会社 | Optical element having a plurality of optical functional surfaces, spectroscopic device, and manufacturing method thereof |
| JP6423056B1 (en) * | 2017-08-11 | 2018-11-14 | 冨士ダイス株式会社 | Imprint mold and method for producing imprint mold |
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Family Cites Families (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5007709A (en) * | 1987-12-28 | 1991-04-16 | Matsushita Electric Industrial Co., Ltd. | Diffraction grating and manufacturing method thereof |
-
2000
- 2000-01-18 DE DE50000086T patent/DE50000086D1/en not_active Expired - Fee Related
- 2000-01-18 EP EP20000100864 patent/EP1130422B1/en not_active Revoked
-
2001
- 2001-01-17 CA CA 2331240 patent/CA2331240A1/en not_active Abandoned
- 2001-01-18 JP JP2001010314A patent/JP2001242314A/en active Pending
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN102998730A (en) * | 2012-12-24 | 2013-03-27 | 四川省宜宾普什集团3D有限公司 | Stereo grating |
| EP3269291A4 (en) * | 2015-09-25 | 2018-10-24 | Olympus Corporation | Image calibration inspection tool and endoscope system |
| US10574971B2 (en) | 2015-09-25 | 2020-02-25 | Olympus Corporation | Image calibration inspection tool and endoscope system |
Also Published As
| Publication number | Publication date |
|---|---|
| EP1130422A1 (en) | 2001-09-05 |
| DE50000086D1 (en) | 2002-02-21 |
| EP1130422B1 (en) | 2001-11-21 |
| JP2001242314A (en) | 2001-09-07 |
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