Patent Application
20060001972
The present invention relates to a diffraction element and an optical device, in particular, to a spectrometry device, and in more detail, to a diffraction element for changing the output direction of light depending on the wavelength, which is to be used for optical multiplex communication or spectrophotometry, and to a spectrometry device employing the diffraction element.
There is a method of diffracting light having various wavelengths into different directions to separate the light depending on the respective wavelengths, and measuring the intensity of each of the separated light to determine the intensity of light of various wavelengths contained in the incident light. Here, in order to utilize the separated light with good efficiency, it is preferred that a diffracted light is efficiently concentrated on a specific diffraction order, and as the method for the wavelength separation, a method of employing a diffraction element having a saw-tooth-like cross section is known.
In order to increase the angle between the incident light and the diffracted light to increase the flexibility in disposing optical elements, it is preferred to use a transmission type diffraction grating.
Further,
Here, an arrow mark of solid line shows incident light, an arrow mark of dashed line shows transmission light, and an arrow mark of broken line shows the −1st order diffraction light.
In the above conventional example, in order to obtain a high diffraction efficiency with a transmission type saw-tooth-like diffraction grating, it is necessary that the phase difference defined as a product of the refractive index difference between the grating material and output-side medium such as an air, multiplied by the depth of the grating, is determined to be about the wavelength of light. Meanwhile, the period of the grating is preferably small to increase the separation angle depending on wavelength. For these reasons, the shape of the saw-tooth-form diffraction grating has to be such that the period of grating is small and the aspect ratio (D/T) defined as the ratio of the grating depth D to the grating period T, is large.
However, a saw-tooth-form diffraction grating having a large aspect ratio is difficult to produce, and when it is produced with a resin, there are problems such as limitation in producing a mold, transferring capability to the resin or separation capability of the resin from the mold. Thus, there has been a problem that a diffraction grating having a large separation performance depending on the wavelength required and having a high utilization efficiency of light, can not be obtained with high productivity at low cost.
Further, when a resin material is employed, there has been a problem of reliability such as deterioration of the element under high temperature or high temperature and high humidity, and thus there has been a problem that the element can be used only under limited environmental conditions.
On the other hand, with respect to the pseudo-saw-tooth form diffraction grating, although the grating excellent in reliability and productivity at low cost can be produced in a large quantity, it is necessary to form such a pseudo-saw-tooth-like diffraction grating to have further fine steps in the period. Accordingly, there has been a problem that it is difficult to produce an element having a short period. Further, the variation of shape due to the error in production process significantly deteriorates the property, and thus, there has been a problem that a diffraction grating having a high light-utilization efficiency can not be produced at high yield.
In both of these conventional examples, in a case where the grating is used for 0 degree incidence (incidence perpendicular to the element surface), it is known that the diffraction efficiency significantly decreases when the period of the grating is about twice as large as the wavelength. Accordingly, there has also been a problem that it is difficult to satisfy high diffraction efficiency and large separation of light depending on wavelength by shortening the period (such as about twice) at the same time.
The present invention has been made under the above circumstances, and it is an object of the present invention to provide a diffraction element having high utilization efficiency of light, providing large separation depending on wavelength, being excellent in flexibility of disposing optical elements, suitable for mass production and excellent in reliability and temperature property.
The present invention provides a diffraction element comprising a grating formed in a substrate surface or a layer formed on a substrate, the grating having a concave/convex-like shape in cross section, the upper surface of a convex portion being substantially flat and the convex portion being symmetrical, wherein the grating of the diffraction element is formed with a period substantially equal to the wavelength of incident light, and the diffraction element is adapted so that light is incident obliquely to its surface where the grating is formed.
Further, the present invention provides the diffraction element, wherein in the proportion of a convex portion to a concave portion in one period in the grating, the proportion of the convex portion is equal to or larger than the proportion of the concave portion.
Further, the present invention provides the diffraction element, wherein the wall surfaces of the convex portion of the grating are inclined, and the proportion of the upper end of the convex portion of the grating in one period is equal to or smaller than the proportion of the bottom portion of the convex portion.
Further, the present invention provides the diffraction element, wherein at least one layer of an optical material different from the material for the convex portion is laminated on at least the convex portion.
Further, the present invention provides the diffraction element, wherein the optical material is one selected from TiO2, SiO2, Ta2O5 or Al2O3.
Further, the present invention provides the diffraction element, wherein a transparent substrate is used for the substrate; convex portions of the grating are formed in a layer formed on the transparent substrate, and the convex portions of the grating have a higher refractive index than the transparent substrate.
Further, the present invention provides the diffraction element, wherein the layer formed on the transparent substrate is the layer comprising one selected from the group consisting of SiO2, TiO2, Ta2O5 Si3N4 and Si as the major component or a mixture thereof.
Further, the present invention provides the diffraction element comprising another optical element laminated integrally on a surface of the diffraction element.
Further, the present invention provides a method of use of a diffraction element which comprises a grating formed in a substrate surface or a layer formed on a substrate, the grating having a concave/convex-like shape in cross section, the upper surface of a convex portion being substantially flat and the convex portion being symmetrical, wherein the grating of the diffraction element is formed with a period substantially equal to the wavelength of incident light, and the diffraction element is adapted so that light is incident obliquely to its surface where the grating is formed.
Further, the present invention provides the method of use of a diffraction element, wherein the incident angle of obliquely incident light is in a range of from 15 to 80° with respect to the normal set on the surface of the diffraction element.
Further, the present invention provides an optical device in which the diffraction element is used.
Further, the present invention provides a spectrometry device comprising the optical device wherein a transparent substrate is used as the substrate of the diffraction element and the diffraction element is used as a transmission type element.
The present invention relates to a diffraction element formed in a surface of a substrate and having a cross-section of concave-convex form whose convex portions are symmetrical, wherein the period of the grating is equal to or smaller than the wavelength of incident light. Further, the diffraction element is adapted to be used so that light is incident obliquely to its surface where the grating is formed.
Further, the grating having a cross-section of concave-convex form may be formed in a layer formed on the substrate. The material for the layer formed on the substrate is preferably one excellent in etching property, and such material containing SiO2, TiO2, Ta2O5, Si3N4, Si or the like as the main component, or a mixed film of these materials may be used. By thus constructing the diffraction element, a diffraction element excellent in mass-productivity having a high diffraction efficiency and a large wavelength-separation effect can be realized with simple process. Since the diffraction element has a large wavelength-separation effect, it is preferably used as a diffraction grating for spectrometry.
Here, “the period of a grating is substantially equal to the wavelength of incident light” means that the period is within ±45% with respect to the wavelength of incident light. Namely, the period is within a range of from 0.55 to 1.45 times as large as the wavelength. When the period is within ±45%, only a specific diffraction light is generated by appropriately selecting the incident angle, and the effect of the present invention is not deteriorated. Further, “light is incident obliquely to its surface where the grating is formed” means that the angle is from 15 to 80° with respect to a normal line set on the surface of the element, and in particular, the angle of from 25 to 65° provides sufficiently the effect of the present invention.
The symmetrical cross-sectional shape of the convex portion of the concave-convex portion of the diffraction grating, may be rectangular, trapezoidal or the like, and any of these cross-sectional shape of the convex portion provides the effect of the present invention. However, one having a shape close to a rectangle is preferred from the viewpoint of production.
With respect to the substrate to be employed for the diffraction element, a substrate of quartz glass, glass, silicon, polycarbonate or the like may be employed as a transparent substrate. Among these, a substrate of quartz glass or glass is preferred from a viewpoint of e.g. durability. Further, when the diffraction element is constituted as a reflection type diffraction element, it is preferred to apply a reflective coating comprising a metal or a multi-layer film on the diffraction grating, and a non-transparent substrate such as a crystallized glass or ceramics may also be employed besides the above transparent substrates.
In the following, under the assumption that the shape of the convex portion is rectangular, the present invention will be described with reference to the drawings.
sin(θ)+sin (i)=m·λ/P Formula 1
By utilizing this effect by the oblique incidence, sufficiently high diffraction efficiency and large diffraction angle can be obtained even with a symmetrical rectangular diffraction grating which is easily producible.
In order to further improve the diffraction efficiency and to decrease the wavelength dependence, it is effective to form an appropriate multilayer structure on the convex portions of the grating.
Namely, the structure that at least one layer of optical material different from the convex portions is laminated on the convex portions, is preferred since it improves the diffraction efficiency. The optical material may, for example, be Ta2O5, Al2O3 or the like besides the above-mentioned TiO2 and SiO2.
In the example shown in
In order to reduce the difference between the diffraction efficiencies of linearly polarized lights polarized perpendicularly to each other, it is effective to maintain the proportion of the convex portion forming the top of the grating to the concave portion to be about 1 and to adjust the angle (hereinafter referred to as taper angle) of the wall surface of the convex portion of the grating to the normal line of the substrate surface.
As evident from
By reducing the polarization dependence by these adjustments, variation of the diffraction property depending on the grating depth is also reduced, whereby the yield at a time of producing the grating is also increased.
Namely, in the proportion of the convex portion to the concave portion in a period of the diffraction grating, when the proportion of the convex portion is larger than the proportion of the concave portion, namely when the proportion of the convex portion is at least 0.5, the difference between diffraction efficiencies of the two polarized lights polarized perpendicularly to each other can be reduced, such being preferred. Further, by inclining the wall surface forming the convex portion of the diffraction grating and making the proportion of the convex portion at the top of the diffraction grating in a period to be at most the proportion of the convex portion forming the bottom of the grating, the difference between diffraction efficiencies of the two polarized lights polarized perpendicularly to each other, can be reduced, such being preferred.
Further, it is effective to improve the wavelength resolution by shortening the period of the diffraction grating to increase the change of the separation angle depending on the wavelength. However, if the grating period is shortened from 1,500 nm to 1,000 nm as in the above example, the difference of the diffraction efficiency by the polarization direction of incident light tends to be significant.
Therefore, even if the proportion of the convex portion of the grating to the grating period is adjusted, the effect is not sufficient although a certain effect can be obtained. In order to reduce the difference of the diffraction efficiency due to the difference of polarization directions of incident light when the grating period is 1,000 nm, it is effective to employ a material having higher refractive index than the transparent substrate, as the material constituting the convex portion of the grating formed on the transparent substrate.
On the other hand, in
In the same manner as the example shown in
Further, from all of the above examples, it is understandable that high diffraction efficiency is obtained in a case where the incident light beam and a diffracted light beam are in a relation of approximately a mirror image to each other with respect to the grating plane of the transmission type diffraction grating, namely, in a case where the incident angle and the diffraction angle are approximately equal. Such an arrangement is called as Littrow arrangement, which is found in a transmission type diffraction grating in which incident light beam and a diffracted light beam overlap each other.
Further, in general, optical parts are coated with optical multi-layer films to reduce reflection at the interface. In a case of a diffraction grating having a large aspect ratio as in the above example, there are many problems such as deterioration of the shape due to the difference of deposition properties between the concave portions and the convex portions, and as a result, the coating of the optical multi-layer film decreases utilization efficiency of light in most cases. By forming a diffraction grating on a substrate on which a low reflection coating is applied, it is possible to improve the transmittance. In this construction, there is no low-reflective coating on the side and the bottom of the grating, but there is no deterioration in the shape of the grating and only the top of the grating is provided with the low-reflective coating.
By employing the construction of the present invention, a spectral diffraction grating having a large wavelength-separation effect and capable of obtaining high light-utilization efficiency can be realized, by a simple production process, as a transmission type diffraction grating capable of sufficiently separating incident light as diffraction lights. By this construction, a spectral diffraction element excellent in e.g. reliability and mass-productivity can be realized, and a low cost spectral system can be realized. Further, since such an element can be used in a transmission arrangement, there is a large flexibility in designing a spectral system.
The diffraction grating pattern in the diffraction element of the present invention, can be produced using e.g. a photomask. Accordingly, the diffraction grating pattern is not limited to a straight shape but it can be, for example, a curved shape. The curved diffraction grating pattern allows the addition of a lens function for converging diffraction light on a photodetector. Further, by employing a large area wafer process, an optical element having another function such as a phase plate can be laminated on the diffraction element, whereby higher functionality, complex system or the like can be achieved.
The optical element having another function to be laminated may, for example, be an optical element having a function of aperture limitation, aberration correction, lens function, beam shaping, polarization transformation, phase adjustment, intensity adjustment or the like. The optical element to be laminated is preferably one to which a large area wafer process can be applied such as a diffraction element, a liquid crystal element, a multi-layer film element or the like.
Namely, the construction that on one surface of the above diffraction element, another optical element is laminated and integrated, is preferred since the diffraction element is added with another optical function to realize high functionality.
The diffraction grating of the present invention is produced by processing, for example, a glass substrate as it is or an inorganic material film formed on the glass substrate. In particular, a grating formed by directly processing a glass substrate is preferred for the reasons that the film-forming process can be omitted and there is no interface between the formed film and the substrate. Namely, it is extremely preferred to directly process a quartz type glass substrate having etching properties of high rate and high uniformity. Further, from the viewpoints of reliability, mass-productivity and the like, a quartz type glass substrate is preferred.
Further, in order to reduce the change of the diffraction direction due to temperature change, control to the linear expansion coefficient of the substrate to be used is important, and it is preferred to employ a transparent substrate having an optimized linear expansion coefficient, in order to obtain an element excellent in thermal properties. In this case, the grating may be formed by directly processing the surface of a transparent substrate having an optimized linear expansion coefficient, or the grating may be formed in a processed layer made of an inorganic material excellent in etching properties, deposited on a transparent substrate having an optimized linear expansion coefficient.
As the glass substrate material whose linear expansion coefficient is optimized, a quartz glass doped with Ti, a glass containing e.g. Al2O3—LiO3—SiO2 as the main component or the like may be employed. Among these, a quartz glass doped with Ti unsusceptible to the thermal history of the substrate during the process and excellent in etching properties, is particularly preferred for the direct process.
Further, it is preferred that the surrounding area of the element is maintained unprocessed at a time of forming a grating having a large aspect ratio as in the above example, in order to prevent breakage of photoresist mask in the process or the grating material.
When the diffraction element of the present invention is a diffraction grating having a small period for improving wavelength resolution by particularly increasing the diffraction angle, the effect is remarkable. Particularly, when it is a diffraction grating whose period is approximately equal to the central wavelength or a diffraction grating whose period (pitch) is within a small range as compared with the central wavelength, the effect is significant.
Various optical devices are constituted by employing the above diffraction element of the present invention. Any optical device may be constituted so long as it uses the properties of the diffraction element of the present invention such as spectral properties.
Further, the construction that a transparent substrate is employed as the substrate of the above diffraction element and the diffraction element is used as a transmission type element to constitute a spectral device, is preferred since the diffraction element of the present invention has a large wavelength-separation effect and enables to increase the separation angle of transmission diffraction light to the incident light, whereby there is no limitation in the arrangement.
An example of the spectral device of the present invention is a detector for wavelength-multiplex communication employed for optical communication. When signal lights having different wavelengths from 1,520 nm to 1,620 nm are transmitted through a single transmission fiber, it is necessary to separate them into signal lights of different wavelengths and to measure the intensity of each of the signals. The lights output from the transmission fiber are diffracted and transmitted in different directions depending on the wavelength by the diffraction element of the present invention, and are incident into different photodetectors, whereby the signal intensities of the respective wavelengths can be measured. Moreover, since the transmission path is different depending on the wavelength, it is also possible to independently adjust the intensity or the phase of each of the wavelengths. In a case of employing a reflection type element, an arrangement called as Littrow arrangement is generally employed in which incident light and reflected diffraction light are close to each other, and there is a limitation in the arrangement of incident fiber and a detector. However, the transmission type diffraction element of the present invention is excellent in that there is little limitation in the arrangement since the angle between the incident light and the diffraction light is large while the diffraction element maintains a high utilization efficiency of light equivalent to the reflection Littrow arrangement.
Examples are shown below.
Then, on the surface of the transparent substrate 101 opposite from the surface on which the rectangular diffraction grating 103 was formed, an antireflective film 104 as a four-layer film for incident light of 50° was provided in the same manner as the above, and then, the transparent substrate 101 was cut into a rectangular shape of 15 mm×10 mm by a dicing saw to form a diffraction element 10.
When lights having wavelengths 1,520, 1,570 and 1,620 nm polarized in a direction in parallel with the grating were incident into the diffraction grating 10 from the side of the rectangular diffraction grating 103 at an external incident angle of 50°, the diffraction element 10 showed high diffraction efficiencies of 92, 93 and 91% respectively to the wavelengths. Further, the output directions of the diffraction light corresponding to the wavelengths of the incident light were 49, 54 and 59° respectively, and thus, three lights having different wavelengths can be sufficiently separated. In
Then, on the surface of the transparent substrate 201 opposite from the surface on which the rectangular diffraction grating 202 was formed, an antireflective film 203 adapted for incident light of 30° was provided. Then, the transparent substrate 201 was cut into a rectangular shape of 10 mm×6 mm by a dicing saw to form a diffraction element 20.
When lights having wavelengths 1,520, 1,570 and 1,620 nm were incident into the diffraction element 20 from the side of the rectangular diffraction grating 202 at an external incident angle of 30°, diffraction efficiencies to the wavelengths were 92, 92 and 92% respectively for polarized lights polarized in parallel with the grating. On the other hand, with respect to polarized lights polarized perpendicularly to the grating, they were 95, 92 and 89% respectively, and the diffraction element 20 showed high diffraction efficiency to either the change of the wavelength of incident light or the change of the polarization direction of the incident light. Further, the output directions of the diffracted lights corresponding to the wavelengths of the incident light were about 31, 33 and 35° respectively, and three lights having different wavelengths can be sufficiently separated. In
Then, on the other surface of the transparent substrate 901 opposite from the surface on which the rectangular diffraction grating 904 was formed, an antireflective film 905 for incident light of 50° was provided in the same manner as the above. Then, the transparent substrate 901 was cut into a rectangle of 15 mm×10 mm by a dicing saw to form a diffraction element 90.
When lights having wavelengths 1,520, 1,570 and 1,620 nm polarized in parallel with the grating were incident at an external incident angle of 50° to the diffraction element 90 from the side of the rectangular diffraction grating 904, the diffraction element 90 showed high diffraction efficiencies of 93, 94 and 92% with respect to the respective wavelengths. Further, with respect to lights of wavelengths 1,520, 1,570 and 1,620 nm polarized perpendicularly to the grating, the diffraction element 90 showed high diffraction efficiencies of 91, 92 and 91% respectively.
Further, the output directions of the diffracted lights were 49, 54 and 59° respectively depending on the wavelengths of the incident light, whereby light of three wavelengths can be sufficiently separated. In
Then, on the surface of the transparent substrate 1101 opposite from the surface on which the diffraction grating 1104 was formed, an antireflective film 1105 for incident light of 50° was provided in the same manner as the above. Then, the transparent substrate was cut into a rectangle of 15 mm×10 mm by a dicing saw to form a diffraction element 110.
When lights having wavelengths 1,520, 1,545 and 1,570 nm polarized in parallel with the grating were incident at an external incident angle of 50° to the diffraction element 110 from the side of the rectangular diffraction grating 1104, the diffraction element 110 showed high diffraction efficiencies of 94, 95 and 95% with respect to the respective wavelengths. Further, with respect to lights having wavelengths 1,520, 1,545 and 1,570 nm polarized perpendicularly to the grating, the diffraction element 110 also showed high diffraction efficiencies of 95, 95 and 95% respectively.
In this Example, a quartz glass substrate of 2.0 mm thick was used as a transparent substrate 1201. The transparent substrate 1201 was coated with a photoresist, and applied with a photolithography method using a photomask, not shown, to form a photoresist mask. Then, a diffraction grating 1202 having a depth of 3,600 nm and a grating period of 1,500 nm was formed by using a dry etching technique. In this step, the wall surfaces forming the convex portions of the diffraction grating were inclined by optimizing the opening of the photomask, not shown, and the etching conditions, to form a tapered grating 1202. The proportion of the width of the convex portion to the concave portion at the grating top of the diffraction grating produced, was approximately 1:1, and the taper angle θ of the wall surfaces of the grating convex portions was about 4°.
Then, on the surface of the transparent substrate 1201 opposite from the surface on which the diffraction grating 1202 was formed, an antireflective film 1203 for incident light of about 30° was provided. Then, the transparent substrate 1201 was cut into a rectangle of 10 mm×6 mm by a dicing saw to form a diffraction element 120.
When lights having wavelengths 1,520, 1,545 and 1,570 nm polarized in parallel with the grating were incident at an external incident angle of 30° to the diffraction element 120 from the side of the rectangular diffraction grating 1202, the diffraction element 120 showed high diffraction efficiencies of 95, 96 and 95% with respect to the respective wavelengths. Further, with respect to lights having wavelengths 1,520, 1,545 and 1,570 nm polarized perpendicularly to the grating, the diffraction element 120 also showed high diffraction efficiencies of 95, 95 and 94% respectively.
As described above, the diffraction element of the present invention functions as a spectral diffraction element having high diffraction efficiency and a large wavelength separation effect, which is excellent in mass-productivity by a simple process. Further, the diffraction element of the present invention realizes a diffraction element excellent in e.g. reliability and polarization properties.
The entire disclosures of Japanese Patent Application No. 2003-068214 filed on Mar. 13, 2003 and Japanese Patent Application No. 2003-078133 filed on Mar. 20, 2003 including specifications, claims, drawings and summaries are incorporated herein by reference in their entireties.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2003-068214 | Mar 2003 | JP | national |
| 2003-078133 | Mar 2003 | JP | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP04/03305 | Mar 2004 | US |
| Child | 11223936 | Sep 2005 | US |
