OPTICAL FILTER AND OPTICAL DEVICE UTILIZING SAME
Provided is an optical filter including first and second reflection layers separated from each other, a dielectric region interposed between the first and second reflection layers and in which two materials of which refractive indexes are different are alternately disposed, and a buffer layer disposed between the dielectric region and at least one of the first and second reflection layers, wherein there are at least two filter regions in which relative volume ratios of the two materials alternately disposed are different.
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This U.S. non-provisional patent application claims priority under 35 U.S.C. § 119 of Korean Patent Application No. 10-2016-0106416, filed on Aug. 22, 2016, the entire contents of which are hereby incorporated by reference.
TECHNICAL FIELDThe present invention disclosed herein an optical filter, and particularly to am optical filter for controlling a transmission central wavelength with a relatively simple and small structure and an optical device using the same.
BACKGROUND ARTThe linear variable filter (LVF) kind of optical filter having a Fabry-Perot resonator structure, and has a structure of which the thickness of a cavity is linearly variable in the length direction. In the LVF, a lower mirror layer and an upper mirror layer are disposed with the dielectric cavity interposed therebetween.
Such an LVF has a limitation in process reproducibility because of a linear structure in which the thickness varies in the length direction. Also, the resolution of spectrometer using a typical LVF is determined by a height-to-length ratio of the LVF and thus there is difficulty in minimizing a spectrometer element. In particular, due to the linear structure, it is disadvantageous in productivity, resulting from poor process compatibility with a two-dimensional imaging sensor technology.
Since a per-position transmission spectrum of an LVF is formed from an overlap of consecutive spectrums, and integration between the LVF and a photodetector is not monolithic, the LVF is spaced apart from a photodetector array, and due to a stray light effect according thereto, a filter performance is lowered.
In addition, U.S. Pat. No. 5,726,805 discloses a planer optical filter including a dielectric layer. According to the patent, the optical filter includes a reflection layer and a dielectric layer, and the dielectric layer has a period structure formed therein and the period structure is provided with trenches or grooves.
However, according to this scheme, substrates are attached to each other, and thus a manufacturing process is difficult to be understood and may be costly. A central wavelength may not also be easily adjusted.
DISCLOSURE OF THE INVENTION TECHNICAL PROBLEMThe present invention provides an optical filter with a miniaturized structure.
The present invention also provides an optical filter structure capable of increasing productivity by enhancing process reproducibility.
The present invention also provides am optical filter structure for making monolithicity and integration with a photodetector easy, preventing a stray light effect by minimizing a distance between the filter and detector, and enhancing a wavelength variable range and performance such as out-of-band rejection performance.
Technical SolutionAn embodiment of the present invention is to provide an optical filter including: first and second reflection layers separated from each other; a dielectric region interposed between the first and second reflection layers and in which two materials of which refractive indexes are different are alternately disposed; and a buffer layer disposed between the dielectric region and at least one of the first and second reflection layers, wherein there are at least two filter regions in which relative volume ratios of the two materials alternately disposed are different.
In an embodiment, the two materials may be alternately disposed, and there may be at least two filter regions in which relative width ratios of the two materials are different. A width of a pair of the two materials may be smaller than a wavelength of a light passing through the filter.
In an embodiment, a pair of the two adjacent materials may be allowed to respectively have same widths as a pair of two adjacent other materials in one direction, or the filter regions, in which relative volume ratios of the two materials are different from each other, may be in two or more directions in a plane.
In an embodiment, an intermediate reflection layer parallel to the reflection layers may be further added to a central region of the dielectric region. In this case, an optical filter has a structure provided with two double resonance cavities of an upper structure and a lower structure on the basis of the intermediate reflection layer. In this case, each of the upper structure and lower structure is possible or is not possible to include a buffer layer. Namely, the buffer layer may be further included between the dielectric region and at least one of the first reflection layer, the second reflection layer, and the intermediate reflection layer.
An embodiment of the present invention is to provide an optical device including: first a second reflection layers separated from each other; a dielectric region interposed between the first and second reflection layers and at least two materials, of which refractive indexes are different from each other, are alternately disposed; and a buffer layer disposed between the dielectric region and at least one of the first and second reflection layers, wherein at least two filter regions in which relative volume ratios of the two materials alternately disposed are different from each other, and photodetectors are respectively provided in correspondence to the filter regions. The optical device may be a spectroscope, a CMOS image sensor, or a hyper-spectra imaging device.
An embodiment of present invention is to provide an optical device including: a transmissive substrate; and the optical filter of the foregoing provided on an upper part of the transmission substrate and integrated in a separate module type.
Advantageous EffectsAn optical filter structure of the present invention may be miniaturized by including a dielectric region for allowing at least two regions where relative volume ratios of two materials are different to be present.
In addition, the optical filter structure of the present invention may increase productivity by improving process reproducibility.
On the other hand, the optical filter structure of the present invention allows monolithicity and integration with a photodetector to be easy by including first and second reflection plates parallel to each other and a dielectric region interposed therebetween and including two materials, and minimizing a distance between a filter and a detector array to prevent a stray light effect and enhance performance.
Description will now be given in detail according to exemplary embodiments disclosed herein, with reference to the accompanying drawings. For the sake of brief description with reference to the drawings, the same or equivalent components may be provided with the same or similar reference numbers, and description thereof will not be repeated. In general, a suffix such as “module” and “unit” may be used to refer to elements or components. Use of such a suffix herein is merely intended to facilitate description of the specification, and the suffix itself is not intended to give any special meaning or function. In the present invention, that which is well-known to one of ordinary skill in the relevant art has generally been omitted for the sake of brevity. The accompanying drawings are used to help easily understand various technical features and it should be understood that the embodiments presented herein are not limited by the accompanying drawings. As such, the present invention should be construed to extend to any alterations, equivalents and substitutes in addition to those which are particularly set out in the accompanying drawings.
Referring to
The optical filter 100 of the present invention includes first and second reflection layers 110 and 120, a dielectric layer 130 and a buffer layer 140. The first and second reflection layers 110 and 120 may form both side surfaces of the optical filter 100. For example, the first and second reflection layers 110 and 120 may be disposed in one-dimensional type so as to be disposed parallel to one direction, and possibly form a two-dimensional optical filter 100.
The buffer layer 140 is disposed between the dielectric layer 130 and at least one of the first and second reflection layers 110 and 120. In other words, the buffer layer 140 is disposed on at least one of an upper side and a lower side of the dielectric region 130, and the dielectric region 130 and the buffer layer 140 are interposed between the first and second reflection layers 110 and 120.
The buffer layer 140 together with the dielectric region 130 operates as an optical resonance cavity. The presence of the buffer layer 140 increases the effective thickness of the optical resonance cavity, and thus a central wavelength of a transmission band of the optical filter 100 is moved to a long wavelength region while the thickness of the dielectric region layer 130 is maintained small. When the optical resonance cavity is configured only from the dielectric region layer 130, an aspect ratio is excessively increased to compensate for any one material in the dielectric region layer 130 that is composed of a combination of two materials that have different refractive indexes and a sufficiently smaller period than an operation wavelength, which results difficulty on process. Accordingly, the presence of the buffer layer 140 is advantageous in that the thickness of the dielectric region layer 130 is maintained small and a wavelength variable range is effectively increased.
The first and second reflection layers 110 and 120 may be respectively a metal thin film having semi-transmission property and a distributed Bragg reflector (DBR) formed of a periodic multilayer structure of a high refractive index dielectric region layer and a low refractive index dielectric region thin layer.
Here, the dielectric region 130 and the buffer layer 140 will be described.
The dielectric region 130 is disposed between the first and second reflection layers 110 and 120, and at least two materials 134 and 137 with different refractive indexes are disposed. There are at least two regions in which relative volume ratios of the two materials 134 and 137 forming the dielectric region 130 are different.
The two materials 134 and 137 forming the dielectric region 130 may be alternately disposed. On the other hand, the dielectric region 130 may be formed along one direction such that there are at least two regions in which relative width ratios of the two materials 134 and 137 are different. Such a structure becomes one-dimensional optical filter structure (see
Referring to
In detail, one direction of
On the other hand, the dielectric region 130 may be formed such that there are at least two regions in which relative volume ratios of the two materials 134 and 137 are different. For example, in a certain region, two materials may be alternately disposed in a pre-determined width, and in another certain region, the two materials may be alternately disposed in a differently pre-determined width. Even when the two materials have different widths in different regions, widths of the two adjacent materials in an identical filter that transmits an identical wavelength is made constant. Accordingly, a to c regions are shown, and in each region, relative volume ratios of the two materials 134 and 137 are constant. As exemplarily shown in the drawing, the relative volume ratios of the two materials 134 and 137 are different in adjacent regions and widths of the adjacent to materials 134 and 137 are constant in all three regions a to c.
The two materials 134 and 137 of the dielectric region 130 may be named as, for example, first and second materials 134 and 137. The first and second materials 134 and 137 may be dielectrics having different refractive indexes. In addition, the first material 134 may be a dielectric material with a relatively low refractive index and the second material 137 may be a dielectric material with a relatively high refractive index, but the present invention is not limited thereto. In other words, the first material 134 may be a material of a relatively high refractive index, and the second material 137 may be a material of a relatively low refractive index. The dielectric of the low refractive index may be, for example, a fluorine ultraviolet ray resin, a spin on glass, hydrogen silsesquioxane (HSQ), magnesium fluoride (MgF2), calcium fluoride (CaF2), or silicon oxide (SiO2). The dielectric of high refractive index may be, for example, a metal-oxide such as titanium oxide (TiO2).
In other words, the dielectric region 130 of the present invention may be a binary dielectric region 130 that is a kind of an effective medium of a combination of two dielectric regions 130 having different refractive indexes, and a relative ratio of the first and second materials 134 and 137 may become gradually different in one direction.
Relative ratios between the first and second materials 134 and 137 of the present invention may be defined as a duty cycle or a fill factor. The duty cycle of fill factor in the present invention is a relative volume ratio that a component of the first material 134 between the first and second materials 134 and 137 occupies on the basis of the first material 134. The refractive index of the dielectric region 130 is made variable in one direction by gradually changing a mutual duty cycle or fill factor between the first and second materials 134 and 137 in the dielectric region 130.
A wavelength of a light passing the optical filter 100 is controlled by the optical thickness of the dielectric region. The optical thickness may be defined as a value of the physical thickness multiplied by a refractive index, and a determinant of a transmission band wavelength of the filter of the present invention is the optical thickness of the dielectric region. Therefore, a central wavelength may also be controlled by a refractive index change besides the physical thickness.
On the other hand, a width of a pair of the first and second materials 134 and 137 has a relation with a wavelength of a filter to be transmitted. For example, the width of the pair of the first and second materials 134 and 137 may be made sufficiently small, and in this case, a light does not discriminate the two materials as individual materials and recognizes the same as one effective medium defined by a specific effective dielectric constant. At this point, an optical constant of the effective medium is determined by a geometrical distribution of the two materials and a relative volume fraction. For the dielectric region 130 of which an imaginary part of the dielectric constant is close to 0, the optical constant of the effective medium has an arbitrary value between optical constants of the two components.
According to a preferred embodiment, the buffer layer 140 may be the first material 134 or the second material 137. For example, when the buffer layer 140 is the second material 137, it is advantageous in manufacturing process. For example, first, in a situation where the first material is patterned, the second material 137 is spread on the entire patterned region of the first material 134 to fill a gap of the first material 134. For example, when a resin is used as the second material 137, the gap of the first material 134 is filled with the resin by spin coating, and the resin may also be spread widely on the top surface of the second material 137. In this case, a planarization work may be separately performed.
Referring to
Referring to
Referring to
After forming the second material 137, in order to make the formation of the second reflection layer 110 easy or enhance the property of an optical filter, a process of planarizing the top surface of the second material 137 may be further included.
Referring to
For convenience of explanation, a difference with
For the optical filters of
On the other hand, a planar disposal of the first material is performed in a periodic lattice structure, and various lattice structures may be available besides a hexagonal lattice shown in
For convenience of explanation, a difference with
On the other hand,
For convenience of explanation, a difference with
Hereinafter, graphs showing results of simulating an operation of each filter will be described.
The results are calculated under assumption that a lattice period is 200 nm, and the thickness of the dielectric region layer is 60 nm in
The calculation results show a variation aspect of the transmission band in a state where the period is fixed to 200 nm and the width of a low refractive index nanostructure is increased from 50 nm to 150 nm at an interval of 20 nm. It may be known that as a fill factor of the high refractive index nanostructure increases, a central wavelength of the transmission band moves toward a long wavelength region.
Since the calculation is conducted by using a metal mirror layer having a semi-transmission property, a full width at half maximum (FWHM) of the transmission band is wide and a transmissivity shows also a certain limit or lower. But when a DBR is alternatively adopted, a very narrow FWHM and high transmissivity may be achieved identically to a typical linear variable filter technology in which the thickness varies in a length direction.
Next,
Referring to
In
Referring to
When mainly describing a difference with
On the other hand,
In addition, for the second material 137, it is possible to vertically laminate two or more layers, or to be horizontally formed of two or more materials.
The optical filter of the present invention may include an anti-reflection coating layer 153 and/or a broadband transmission band filter 152. In addition, although not illustrated in
The anti-reflection coating layer 153 is a component adoptable for reducing an amount of a light that is incident to the optical filter from the outside and then is reflected to disappear to the outside. The broadband transmission band filter 152 is an effective component capable of adjusting a necessary wavelength band of a light incident to the optical filter.
On the other hand, the anti-reflection coating layer may be formed on a surface other than a surface on which the optical filter is formed on a separate transparent substrate on which the optical filter of the present invention is formed.
According to configuration of
The present optical device is provided with filter regions a, b and c, and photo-detectors PD1, PD2, and PD3 respectively corresponding thereto. Although
Hereinafter, another optical device according to an embodiment of the present invention will be described. The optical filter of the present invention may be formed on the top surface of the transmissive substrate to be manufactured as a separate optical filter module (see
An array photodetector coupled to the filter array operates as a spectroscopic device through a mathematical digital signal processing algorithm. When assuming an ideal filter having a delta function property, the resolution simply becomes equal to a value obtained by dividing an operation wavelength region of the spectroscopic device by the number of filters. Accordingly, there is a limitation in that the number of filters required for an high resolution operation increases proportionally thereto.
The optical filter according to the present invention is a non-ideal filter defined by a Lorentzian function, and has a property that an FWHM of a transmission band is determined according to a design of a lower reflection layer. A case of using a metal reflection layer is more typical than a case of using a DBR, and as reflectance of the metal reflection layer is lower or as the period number of unit combination of a low refractive index layer and a high refractive index layer that configure the DBR is smaller, the FWHM increases.
A signal recovery principle in a spectrometer based on the non-identical filter array may be explained using
Since the number M of filters is typically smaller than the number N of wavelength samplings, the linear algebra equation of Equation (2) comes down to an ill-posed problem. Since an explicit inverse matrix of D(λ) having the size of M×N (M<N) does not exist, a spectrum signal may be recovered using a pseudo inverse matrix, but is vulnerable to a small fluctuation or system noise and shows an unstable result.
As a measure for obtaining a more effective and numerically stable solution, a regularization scheme is being used. The most representative scheme may be a Tikhonov regularization scheme. This scheme recovers a spectrum of an analysis target object by determining a solution to minimize a sum of a residual norm and a side constraint norm as Equation (3). Here, α is a regularization factor for determining a weight for minimization of the residual norm in contrast to minimization of the side constraint norm, and there exists an optimal value of α so as to obtain a robust solution. When using singular value decomposition (SVD) and L-curve analysis, the method adapts to a system and determines an optical regularization factor for itself to enable spectrum recovery in real time.
sα=arg min|Ds−r∥22+α2∥L(s−s*)∥22 (3)
When using such a regulation scheme, it is advantageous in that a spectrum may be recovered with a relatively high resolution, while a non-identical filter array having a wide FWHM is used. A signal recovery algorithm is not limited to the exemplified regularization scheme, but various schemes may be available.
On the other hand, the L-curve analysis is a method in which a solution of a Tikhonov regularization equation is obtained when a value is gradually increased and substituted, the obtained solution is substituted again to the residual norm ∥Ds−r∥22 and solution norm ∥L(s−s*)∥22, and resultant values are represented on log-scaled coordinate axes. Then an L-curve shaped graph is obtained and a corner value of the L-curve is adopted as an optimal value α. A scheme for obtaining the corner value is to take log-scaled values of the residual norm and solution norm as variables and to determine α having the smallest radius of curvature. The value obtained in this way is substituted again to the Tikhonov regularization to obtain Sa and recover the object's spectrum.
Next, for a presence of a system noise, there is a case where a digital signal recovery process by regularization does not properly operate and an unstable solution is output. In order to reduce such a problem, a per-unit cell strength distribution curve of a photodetector, in which a filter array is integrated, is applied to a Savitzky-Golay smoothing algorithm that is effective in noise filtering, and then an influence to signal recovery may be evaluated. A Savitzky-Golay filter is one of smoothing schemes for making waveforms of a data sequence including noises to smooth waveforms from which the noises are excluded while original signal disposition is not largely damaged, and is a filter for obtaining k-th order polynomials for fitting surrounding points best at an individual point by a least square method, and determining a data value at that point. The Savitzky-Golay filter relatively well conserves a maximum, minimum, or peak/valley value by applying a moving average in a scheme that data near a data disposition is more weighted and distant data is less weighted. There occurs a situation where when a noise is mixed in a per-unit cell strength distribution curve that is measured from a photodetector array, even when a Tikhonov regularization scheme is applied, an original spectrum may not be properly recovered. But when the Savitzky-Golay filter is applied, it has been checked that signal recovery performance by digital-signal-processing is excessively improved.
On the other hand, the inventors of the present invention find a fact that for spectrum recovery, an error ratio may be reduced when a transmission spectrum of each filter, namely, filter functions form a proper overlap. A description thereabout will be provided.
For quantitative finding, the overlap factor is defined as a value obtained by dividing a transmissivity or reflectance value at a point, at which spectrums of two adjacent filter functions cross each other, by a maximum transmissivity or reflectance value of the filter functions, and a variation in signal recovery error value according thereto is evaluated. It may be known that when an overlap degree of the two adjacent filters is lowered to a certain value or lower, the signal recovery error value largely increases.
For parameters for simulations of
ERROR=100*(norm(recovered signal-Original Signal)/norm(Original signal)
Referring to
On the other hand, a description will be provided about a proper overlap range in realization of a spectrum of the present spectroscope. As checked in
Therefore, a preferred lower limit value of the overlap will be discussed. A range of the error value may be preferably smaller than about 30%, and more preferably, smaller than 10%. Accordingly, when it is converted based on the overlap, a preferable overlap is 0.2 or greater and a more preferable overlap is 0.4 or greater. Next, a preferred upper limit value will be discussed. The overlap is preferred to be as high as possible, when a spectrum of adjacent filters exceeds a noise signal level, is not overlapped and is distinguishable. At this point, the number of filters may excessively increase, a structural factor difference between adjacent filters is minute, and thus there may be a process limit. Accordingly, it is preferred that the structural factor difference between adjacent filters is not set to 1 nm or smaller, and there may be an overlap upper limit according thereto.
On the other hand, when filter functions of filters form proper overlaps, an error ratio may be reduced. This aspect may be variously applied without being limited to shapes or kinds of the filters. For example, it may be applied to
Accordingly, the spectroscopes operate as a spectrometer that enables conversion to a strength distribution according to a light wavelength, and a spectroscope based on a filter array may be realized.
Referring to
Referring to
The hyper spectral image sensor is an element configured to sense several (relatively narrow) wavelength parts or a wavelength band of an entire hyper spectrum emitted from or absorbed by an object.
As illustrated in
An optical filter described above are not limited to the configuration and the method in the embodiment described above, and the embodiment may have a configuration in which all or a part of each embodiment is selectively combined such that various modifications can be made.
The present invention may be carried out in other specific ways than those set forth herein without departing from the spirit and essential characteristics of the present invention. Therefore, the above embodiments should be construed in all aspects as illustrative and not restrictive. The scope of the invention should be determined by the appended claims and their legal equivalents, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.
Claims
1. An optical filter comprising:
- first and second reflection layers separated from each other;
- a dielectric region interposed between the first and second reflection layers and in which two materials of which refractive indexes are different are alternately disposed; and
- a buffer layer disposed between the dielectric region and at least one of the first and second reflection layers, wherein there are at least two filter regions in which relative volume ratios of the two materials alternately disposed are different.
2. The optical filter of claim 1, wherein the two materials are alternately disposed, and there are at least two filter regions in which relative width ratios of the two materials are different.
3. The optical filter of claim 2, wherein a pair of the two adjacent materials are allowed to respectively have same widths as a pair of two adjacent other materials in one direction.
4. The optical filter of claim 1, wherein the two materials are composed of a first material comprising in plurality and disposed at a pre-determined interval, and a second material surrounding the first material.
5. The optical filter of claim 2, wherein a width of a pair of the two materials is smaller than a wavelength of a light passing through the filter.
6. The optical filter of claim 1, wherein the filter regions, in which relative volume ratios of the two materials are different from each other, are in two or more directions in a plane.
7. The optical filter of claim 1, wherein the first and second reflection layers are a metal layer or a dispersion Bragg reflector (DBR).
8. The optical filter of claim 1, wherein the dielectric region comprises three or more materials.
9. The optical filter of claim 1, wherein the buffer layer is formed of the first or second material.
10. The optical filter of claim 1, wherein an upper part of the second reflection layer comprises a wideband transmission filter and/or anti-reflection coating.
11. The optical filter of claim 1, wherein the filter regions respectively comprise different buffer layers.
12. An optical filter comprising:
- first and second reflection layers separated from each other;
- a dielectric region interposed between the first and second reflection layers and at least two materials, of which refractive indexes are different from each other, are alternately disposed; and
- at least two filter regions in which relative volume ratios of the two materials alternately disposed are different from each other,
- wherein an intermediate reflection layer parallel to the reflection layers is added to a central region of the dielectric region.
13. The optical filter of claim 12, further comprising:
- buffer layers of which optical thicknesses are identical between the intermediate reflection layer and the first and second reflection layers.
14. An optical device comprising:
- filter regions in which relative volume ratios of the two materials of claim 1 are different from each other and through which different wavelengths are passed; and
- photodetectors respectively corresponding to the filter regions.
15. The optical device of claim 14, wherein the optical device is a spectroscope, a CMOS image sensor, or a hyper-spectra imaging device.
16. An optical device comprising:
- a transmissive substrate; and
- the optical filter of claim 1 provided on an upper part of the transmission substrate and integrated in a separate module type.
Type: Application
Filed: Jul 31, 2017
Publication Date: Jan 17, 2019
Applicant: Samsung Electronics Co., Ltd. (Suwon-si, Gyeonggi-do)
Inventors: Kyeong Seok LEE (Seoul), Gyu Weon HWANG (Seoul), Won Mok KIM (Seoul), In Ho KIM (Seoul), Wook Seong LEE (Seoul), Doo Seok JEONG (Seoul)
Application Number: 16/069,337