Abstract:
A planar nanospectrometer formed as a single chip that uses diffraction structures, which are combinations of numerous nano-features placed in a predetermined configuration and providing multiple functionalities such as guiding light, resonantly reflecting light at multiple wavelengths, directing light to detectors, and focusing light on the detectors. The diffraction structure can be described as a digital planar hologram that comprises an optimized combination of overlaid virtual sub-gratings, each of which is resonant to a single wavelength of light. Each device includes at least one sensor, at least one light source, and at least one digital planar hologram in an optical waveguide. The device of the present invention allows detection of small amounts of analytes in gases and liquids or on solid surfaces and can be particularly advantageous for field analysis of environmental safety in multiple locations because of its miniature size and low cost.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present patent application is related to the following pending patent applications: (1) U.S. patent application Ser. No. 405,160 filed by V. Yankov et al on Apr. 2, 2003 entitled “Planar holographic multiplexer/demultiplexer”; (2) U.S. patent application Ser. No. 137,152 filed by S. Babin et al on May 2, 2002 entitled: “Photonic multi-bandgap lightwave device and methods for manufacturing thereof”; and (3) U.S. patent application Ser. No. 167,773 filed by L. Polonskiy et al on Jun. 11, 2002 entitled: “Integrating elements for optical fiber communication.” 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention generally relates to optical spectrometry for detecting small quantities of analytes and for other related applications. In particular, the present invention provides a miniature integrated optical spectrometer based on nano-structures embedded into planar waveguides. 
       BACKGROUND INFORMATION 
       [0003]    Last century witnessed multiple improvements in optical spectrometer design and dramatic reduction in size. As a result, spectrometers have moved from optical laboratories to industrial, field, aerospace and other areas of application where compactness, ruggedness, reliability, and low cost are crucially important. 
         [0004]    Several companies supply compact spectrometers of traditional configuration for ultraviolet, visible, and near-infrared spectral bands. For example, two such companies are Hamamatsu Photonics Co., Ltd. and Ocean Optics (see links below); however, new achievements in nanotechnology make it possible to develop even smaller spectral devices.
       http://sales.hamamatsu.com/assets/pdf/parts C/c9407ma etc_kacc1136e0.pdf   http://www.oceanoptics.com/products/usb2000+.asp.       
 
         [0007]    For example, U.S. Pat. No. 4,923,271 to Henry et al (“Henry”) issued on May 8, 1990 describes an optical multiplexer/demultiplexer comprising cascaded elliptic Bragg reflectors (gratings). All gratings are formed by means of microlithography in a planar waveguide. Each grating is tuned to a definite light wavelength corresponding to one of the working channels. The gratings have one common focal point but different elliptical ties so that the location of the remaining focus can be chosen to provide adequate spacing between input and output. Preferably, the plurality of elliptical Bragg gratings is ordered such that the grating associated with the shortest wavelength is positioned closest to the input of the device. In principle, this type of optical chip can be used as a spectral device for limited amount of wavelengths; however, extending this type of optical chip to a large number of channels is not feasible, and this is the main disadvantage of the approach. The gratings are separated spatially for sequential processing of light. As the number of channels and correspondingly the number of wavelengths to be processed grows, the size of the device increases, the path of light to the remote gratings grows, and, consequently, intrinsic losses grow as well. Also, building large devices is difficult and expensive due to limited precision of the lithographic process and limited uniformity of the waveguide used for gratings. 
         [0008]    A new approach to spectral planar integrated devices is based on superposition of multiple sub-gratings on the same planar area. Each sub-grating resonates to a fixed wavelength, but a super-grating comprising many sub-gratings can be deployed as a spectral instrument. Several devices and systems based on this new approach are disclosed in several pending U.S. Patent Applications such as U.S. patent application Ser. No. 405,160 filed by V. Yankov et al on Apr. 2, 2003 entitled “Planar holographic multiplexer/demultiplexer”; U.S. patent application Ser. No. 137,152 filed by S. Babin et al on May 2, 2002 entitled “Photonic multi-bandgap lightwave device and methods for manufacturing thereof”; U.S. patent application Ser. No. 167,773 filed by L. Polonskiy et al. on Jun. 11, 2002 entitled “Integrating elements for optical fiber communication.” However, none of these publications discloses how the new approach can be introduced into the structure of a spectrometer. 
         [0009]    The overlaying of multiple sub-gratings for optical multiplexer/demultiplexer applications was further developed by Vladimir Yankov et al as disclosed in “Multiwavelength Bragg Gratings and Their Application to Optical MUX/DEMUX Devices,” Photonic Technology Letters, vol. 15, pp. 410-412, 2003. 
         [0010]    Based on the above principle, several optical systems were patented by Thomas Mossberg et al (see U.S. Pat. No. 7,120,334 issued on Oct. 10, 2006 entitled “Optical Resonator Formed in a Planar Optical Waveguide with Distributed Optical Structures.” However, the inter-laser cavity spectrometer proposed by T. Mossberg in U.S. Pat. No. 7,120,334 has a narrow band limited by laser spectral properties and a cavity-free spectral range, works only on the absorption principle, and analyzes only liquids. The remaining two patents do not teach a compact spectrometer. 
         [0011]    S. Grabarnik et al reported information on a miniature spectrometer with a volume of 0.135 cm 3  and dimensions of 3×3×11 mm mounted directly on the surface of a charge-coupled device (CCD) sensor (see  Optics Express,  Vol. 15, No. 6, pp. 3581-3588, 2007). The spectrometer is formed by two flat diffraction gratings that are designed to perform both the dispersion and imaging functions, eliminating the need for spherical optics. Two separate parts of the device were fabricated with single-mask 1/Jm lithography on a single glass wafer. The wafer was diced, and the device was assembled and directly mounted onto a CCD sensor. The resolution of 3 nm, spectral range of 450 to 750 nm, and the optical throughput of ˜9% were measured to be in a complete agreement with the model used for development of the device. 
         [0012]    In “Investigation of the use of CCDs as high-resolution position-sensitive detectors of ionizing radiation (Lawrence Berkeley Laboratory) (http://www.slac.stanford.edu/cgi-wrap/getdoc/icbp82-009.pdf), A. Bross reported successful use of charge-coupled devices (CCDs) as analog shift registers, optical imagers, and high-density memories. In fact, the device comprises a CCD Planar spectrometer operable in either one- or two-dimensional modes. 
         [0013]    A common disadvantage of the above-described known optical spectrometers is their relatively large dimensions, and the applicants are unaware of the existence of miniature optical planar spectrometers designed and operating on the principle of digital planar holography. 
       SUMMARY OF THE INVENTION 
       [0014]    An object of the present invention is to provide a nanospectrometer on the basis of digitally generated diffraction structures in planar optical waveguides. Another object of the invention is to provide a method of manufacturing the aforementioned nanospectrometer by means of microlithography. It is a further object to provide a nanospectrometer with super-gratings that comprise multiple sub-gratings consisting of standard binary features such as dashes or grooves etched or formed in a planar waveguide by means of microlithography. 
         [0015]    The nanospectrometers of the invention use diffraction structures, which are combinations of numerous nano features placed in a configuration and providing multiple functionalities such as guiding light, resonantly reflecting light at multiple wavelengths, directing light to detectors, and focusing light on detectors. A diffraction structure can be described as a super-grating because it is an optimized combination of overlaid sub-gratings, each of which is resonant to a single wavelength of light. Each device includes at least one sensor, at least one light source of spectrum, at least one super-grating in an optical waveguide, and at least one array of detectors. The device of the present invention allows detection of small amounts of analytes in gases and liquids or on solid surfaces and can be particularly advantageous for field analysis of environmental safety in multiple locations because of its miniature size and low cost. 
         [0016]    For successful development of nanospectrometers it is crucially important to properly design their dispersive elements, i.e., super-gratings. The super-grating comprises multiple sub-gratings, which consist of standard binary features like dashes, or grooves, etched or otherwise formed in a planar waveguide in order to generate local modulations of effective refractive index inside a planar optical waveguide. Positions of standard features are determined by a generating function, which is calculated based on desired parameters of the super-grating. Essentially, these gratings are sets of zeroes and ones engraved on the planar waveguide. A typical grating consists of several million features, which could be used in huge number of combinations, but only a few of them are appropriate for efficient operation; therefore, it is necessary to optimize the generating function. 
         [0017]    According to the present invention, each super-grating is generated as a mathematical superposition of elliptic, parabolic, or hyperbolic sub-gratings with a spatial period of approximately one-half wavelength by a method characterized by the following steps. The first to be created is a two-dimensional analog-generating function A(x,y) representing a superposition of modulation profiles of the refractive index. Each modulation function corresponds to the equivalent of a sub-grating. Determined in this step is a two-dimensional generating function A(x,y), which resembles an interference pattern of wavelengths emitted from multiple sources at different wavelengths. The generating function A(x,y) is a mathematical linear superposition of integration of elliptic sub-gratings, wherein each sub-grating is tuned to resonantly reflect at one of the N spectral channels. 
         [0018]    The next step is binarization of a two-dimensional analog-generating function A(x,y), which was produced in the previous step. Binarization is achieved by applying a threshold value and assigning 1 to all areas above the predetermined threshold and 0 to the remaining areas in order to obtain a digital two-dimensional generating function B(x,y). 
         [0019]    Next, the complex shape islands in B(x,y) with the value of 1 are simplified in order to be presented as a combination of standard microlithographic features (short straight lines and dashes). This is accompanied by conversion to function C(x,y). 
         [0020]    The last step is lithographic fabrication of the calculated standard microlithographic features by etching all binary features as function C(x,y) to a calculated depth on a planar waveguide. 
         [0021]    The present invention further provides the step of applying an apodization function to a function representing a plurality of binary features to be written by using single-layer (binary) microlithography. This is necessary in order to suppress side lobes of the transfer function. In particular, according to the present invention, an apodization function g(r) is determined and applied to binary features by removing some of the binary features to the extent that an average density of the binary features becomes proportional to g(r). 
         [0022]    The present invention further provides the step of correcting crosstalk by imposing a linear relationship on geometrical positions of input/output ports and central frequencies of channels. According to another modification of the present invention, crosstalk can be suppressed by precompensation of generating function A(x,y). 
         [0023]    With use of the above-described super-grating, the present invention makes it possible to develop nanospectrometers of different types that can be integrated on a chip for detection of solid, liquid, or gas analytes. Examples of these nanospectrometers are the following: a laser-induced breakdown (LIB) spectrometer, an absorption spectrometer, or a Raman spectrometer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  illustrates an exemplary super-grating embedded into a planar waveguide according to one modification of the present invention. 
           [0025]      FIG. 2  illustrates a fragment of an exemplary realization of a function C(x, y) for a super-grating with eight resonant wavelengths made in accordance with the invention. 
           [0026]      FIG. 3A  shows the simulated transfer function for a 4-channel PBQC according to the invention. 
           [0027]      FIG. 3B  shows the transfer function for the same device as in  FIG. 3A , measured experimentally. 
           [0028]      FIG. 4A  demonstrates the transfer function of a 4-channel super-grating with low channel isolation (high crosstalk) according to the invention. 
           [0029]      FIG. 4B  demonstrates the transfer function for the same device as in  FIG. 4A , measured experimentally. 
           [0030]      FIG. 5  shows the wave vectors of sub-gratings participating in the synthesis of a super-grating made in accordance with the invention. 
           [0031]      FIG. 6  illustrates the grating apodization function g(r) formed by the method of the invention. 
           [0032]      FIG. 7  shows the configuration of the laser-induced breakdown (LIB) nanospectrometer corresponding to the present invention. 
           [0033]      FIG. 8  shows the configuration of the absorption nanospectrometer of the invention with multiple super-luminescent diode light sources. 
           [0034]      FIG. 9  demonstrates the absorption nanospectrometer of the invention with a bare fiber probe. 
           [0035]      FIG. 10  presents the Raman nanospectrometer on a chip made in accordance with the invention. 
           [0036]      FIG. 11A  shows the layout of the Raman nanospectrometer with a fiber probe made in accordance with the invention. 
           [0037]      FIGS. 11B through 11C  illustrate the shapes of the fiber face used in the spectrometer of  FIG. 11A . 
           [0038]      FIG. 12  illustrates a folded nanospectrometer of the invention that has improved resolution. 
       
    
    
     DETAILED DESCRIPTION 
       [0039]    In the context of the present invention, the term “super-grating” means a digital planar hologram that performs multiple functions and operates for a plurality of channels incorporated into a nanospectrometer. 
         [0040]    In the context of the present invention, the term “sub-grating” means a virtual component of the aforementioned digital planar hologram that provides operation of a single light-signal-transmitting channel. The same elements of different sub-gratings belong to the same super-grating. 
         [0041]    The physics of a spectral super-grating, deployed in the invented spectrometers, is complicated, and for this reason several theoretical models should be used to explain the properties of transfer function. In a first approximation, the super-grating works like a superposition of elliptical sub-gratings, each of which connects an input port with one of multiple output ports. The sub-gratings are structures that are composed of multiple nano-features that modulate the refractive index of a planar waveguide where propagating light is confined. The nano-features are positioned in a manner to provide resonant reflection of light of a predefined wavelength. The super-grating works like a superposition of sub-gratings, reflecting multiple wavelengths to assigned output ports. 
         [0042]    The super-grating can be also considered as a photonic bandgap quasi-crystal with a quasi-periodic structure and multiple periods corresponding to multiple bandgaps. In such devices, light propagates in any direction except specifically designed one, thus resulting in light reflection from one ellipse focus into another. These photonic bandgap quasi-crystals can be made by means of binary lithography, nano-imprinting, or other methods on planar waveguides and contain nano-features that modulate the refractive index, and are made for example, into the form of dashes. 
         [0043]    The super-grating is synthesized from multiple sub-gratings in a synergistic manner, which includes a mathematical superposition of modulation functions followed by binarization. This process is substantially different from direct superposition of sub-gratings because superposition originates as a mathematical step, which effectively averages a plurality of modulation functions having varying phases. 
         [0044]    As discussed above, the nano-features form a predetermined planar quasi-periodic pattern of the refractive index. Positions of features are chosen to optimize transfer functions of all wavelengths. 
         [0045]    According to the present invention, each super-grating is originally computed as a mathematical superposition of elliptic, parabolic, or hyperbolic sub-gratings with a spatial period of approximately one-half wavelength, for which this sub-grating will be resonant (reflective). An analog-generating function A(x,y) that describes modulation of the refractive index in a planar waveguide and resembling a superposition of a plurality of interference fringes of diverging and converging light beams is implemented according to the following expression: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       A 
                        
                       
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                         ) 
                       
                     
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         [0046]    where index i refers to a wavelength number as well as corresponding output port: 
         [0000]    
       
         
           
             
               
                 l 
                 i 
               
               = 
               
                 
                    
                   
                     r 
                     
                       ρ 
                       in 
                     
                   
                    
                 
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         [0047]    where: 
         [0048]               is a vector connecting the input port to an arbitrary point (x,y) on the planar surface; 
         [0049]               is a vector that connects this point with coordinates (x,y) to the output port i for a chosen wavelength λ i ; 
         [0050]    a i  is a weight coefficient associated with wavelength i; and 
         [0051]    φ i  is an arbitrary phase associated with wavelength; and 
         [0052]    f(x,y) is a function that compensates for variation of refractive index. (All of the parameters above are associated with wavelength λ i ) 
         [0053]    The A(x,y) function resembles holographic fringes with an omitted factor 1/r to avoid performance deterioration. A super-grating with variation of the effective refractive index n(x,y) described by the analog generating function 
         [0000]      A(x,y) n(x,y)˜A(x,y)   (2) 
         [0054]    would have the best performance, but, unfortunately, it cannot be fabricated by mass production technologies (microlithography, nano-imprinting, or the like). In planar waveguide technology, the analog-generating function A(x,y) can be implemented as its surface relief. This will modulate the effective refractive index as prescribed by formula (1), but fabricating that multilevel relief with modern lithography is very difficult if possible at all. Therefore, to make this approach practical, the relief must be reduced to a binary shape, meaning that there cannot be more than one nano-feature at each location. According to the present invention, this problem is solved by approximating the analog-generating function A(x,y) with proper positioning of standard nano-features, for example, dashes, or short straight grooves. 
         [0055]    According to this invention, in order to obtain a digital (binary) two-dimensional generating function B(x,y), binarization of function A(x,y) is further implemented by applying a threshold value by assigning 1 to all areas above the predetermined threshold and 0 to the remaining areas. For further simplification of manufacturing conditions with the use of microlithography and nano-imprinting techniques, the shape of function B(x,y) is simplified by replacing ditches with curved boundaries by a combination of standard microlithographic nano-features (short straight grooves or dashes). This operation can be described as quantization of binary function B(x,y) to produce a discrete function C(x,y), which is nothing but a collection of standard nano-features (dashes) that can be formed according to the aforementioned mass-production methods. 
         [0056]    The super-grating described by the discrete generating function C(x,y) preserves all spectral properties of the original analog-generating function A(x,y), but binarization and quantization could introduce additional artifacts. Therefore, careful approach and thorough optimization of conversion algorithms are required. 
         [0057]    The last parameter of the super-grating to be determined is the depth of dashes to be formed in a planar waveguide by microlithography or nano-imprinting. 
         [0058]      FIG. 1  illustrates an exemplary nanospectrometer  20  having an integrated super-grating  30  with a planar waveguide  31  that comprises several flat layers of transparent optical materials, each associated with different refraction indices. The materials are chosen so that one of them, referred to herein as the core  24 , has a refractive index, which is higher than the refractive indices of the cladding  22 . This provides a low-loss guiding of lightwaves through the core  24 . 
         [0059]    In the exemplary modification shown in  FIG. 1 , an input light signal  26  comprising multiple wavelengths to be spectrally analyzed enters the planar waveguide  31  through an input port  28  from an optical fiber or from a ridge waveguide (not shown in  FIG. 1 ) and propagates within a sector determined by the angular aperture of the input port. The super-grating  30  that incorporates multiple nano-features organized according to discrete generating function C(x,y) is embedded into one or more layer interfaces of the waveguide. The super-grating  30  works as a thick (volume) digital hologram directing light of various wavelengths to the assigned output ports  32  and  34 . 
         [0060]      FIG. 2  illustrates a fragment of an exemplary realization of discrete generating function C(x, y) for an eight-channel super-grating  36  (dark lines that represent grooves etched on the layer interface(s) of the waveguide). This super-grating  36  resonantly reflects eight various wavelengths to the assigned output ports  32  and  34  ( FIG. 1 ). 
         [0061]    An example of practical implementation of this invention is demonstrated in  FIGS. 3A and 3B .  FIG. 3A  shows a simulated transfer function for a four-channel super-grating, while  FIG. 3B  presents the experimentally measured transfer function of the same super-grating fabricated on a planar waveguide with a core thickness of 0.4 micron and a core refractive index of n core =1.75, the core being isolated by a cladding with the refractive index of n clad =1.44. In  FIG. 3B , the sub-grating (channel) transfer functions are denoted as A, B, C, and D. In  FIGS. 3A and 3B , wavelengths (nm) are plotted on the abscissa axis, and the intensity (arbitrary units) is plotted on the ordinate axis. In that case the effective refractive index for the TE mode is about 1.53, for the TM mode about 1.47, and for cladding modes about 1.44. As can be seen, the experimental data substantially coincides with theoretical assumptions described above. 
         [0062]    According to the present invention, optimization of the super-grating design consists of finding an analog-generating function A(x,y) that provides best possible performance for the super-grating after the aforementioned binarization and quantization procedures. The most dangerous and performance-degrading effect is crosstalk between the super-grating channels, which may be caused by insufficient channel isolation (crosstalk is reflection of light with different frequencies in the same direction). Binarization of the analog-generating function A(x,y) is a strongly nonlinear transform. In accordance with the rules of nonlinear transform, if A(x,y) includes just three Fourier components with wave vectors           and           the Fourier spectrum of the generated binary relief would include the beating-generated wave vectors           expressed by a linear combination of the three original wave vectors: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         k 
                         ρ 
                       
                       ijh 
                       b 
                     
                     = 
                     
                       
                         m 
                          
                         
                           
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                             ρ 
                           
                           i 
                         
                       
                       + 
                       
                         n 
                          
                         
                           
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                   , 
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0063]    where m, n, and i are arbitrary positive or negative integers. These parasitic Fourier harmonics may be responsible for high crosstalk (insufficient channel isolation). In fact, this effect was observed both in simulations and experiments, as illustrated in  FIG. 4A  (simulation) and  FIG. 4B  (experiment). In  FIGS. 4A and 4B , the wavelengths (nm) are plotted on the abscissa axis, and the intensity (arbitrary units) is plotted on the ordinate axis. The transfer function of a four-channel super-grating demonstrates high crosstalk and low channel isolation about 8 dB only, while it is typically required that isolation be not less than 25 dB. 
         [0064]    If for simplicity of consideration we approximate each channel by a single           and take into account that the wave vectors of close channels have almost the same values, then combinations of formula (3) with m and n having values equal or close to 1 and −1, respectively, e.g., those expressed by 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         k 
                         ρ 
                       
                       i 
                     
                     + 
                     
                       ( 
                       
                         
                           
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         [0065]    will become close to one of the original wave vectors           and will reflect light of a different wavelength (another channel) to the output port assigned for           Additional analysis shows that the reflections are focused. Therefore, such approximation will increase the crosstalk to an unacceptably high level. 
         [0066]    In accordance with the present invention, the above problem can be solved by properly positioning the output ports. Let us assume now that at some point the directions of wave vectors vary with absolute value of sub-grating wave vectors linearly so that the tips of the vectors lie on a straight line, as shown in  FIG. 5 , where E, F, G, and H are the wave vectors of channel sub-gratings. In this case, any linear combination of wave vectors lies on the same straight line that is frequency of reflected light is a function of reflection direction and, consequently, crosstalk is avoided. [is unclear as written.] If the foci positioning is linear and coordinates of output ports R i  satisfy the following equation: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       R 
                       ρ 
                     
                     i 
                   
                   = 
                   
                     
                       
                         R 
                         ρ 
                       
                       0 
                     
                     + 
                     
                       δ 
                        
                       
                           
                       
                        
                       
                         
                           R 
                           ρ 
                         
                         · 
                         
                           ω 
                           i 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
         [0067]    where ω i  is the central frequency of the channel, then, in approximation of a small numerical aperture and small sub-grating ellipticity, the wave vectors of the channel sub-gratings will lie on straight lines, as shown in  FIG. 5 . The positions of the input port as well as channel spacing are arbitral. It is understood that the input port receives light having specific spectral characteristics or a spectrum obtained from a light source, which is conventionally shown by reference numeral  37  in  FIG. 1 . This procedure of correcting binarization nonlinearity by properly positioning the channel was confirmed by both simulations and experiments, as shown in aforementioned  FIGS. 3A  (simulation) and  3 B (experiment). It can be seen that the aforementioned correction provides channel isolation of 28 dB, which is almost 20 dB better than without the aforementioned correction. 
         [0068]    Another source of artifacts is substitution of an infinite-size periodic structure with a finite one having sharp edges. This leads to appearance of additional Fourier harmonics and, thus, additional out-of-band reflections. Such a problem is well known in the theory of fiber Bragg gratings. The remedy, which consists of smoothening (apodization) of the back and front ends of the grating, is known as well. Usually, the grating apodization leads to gradual variation of the refractive index modulation depth in accordance with a certain (apodizing) function g(r), where r is the distance to the input point (where light enters the grating). Inside the apodized super-grating the modulation function smoothly grows in a central zone of the super-grating from zero (no n(x,y) modulation) to unit (maximum n(x,y) modulation) and then slowly drops to zero at its end. Full-scale modulation occurs only in the central part of the super-grating, which is surrounded with areas of variable modulation depth to provide a smooth transition from a nonmodulated to a fully modulated refractive index. Because the present invention uses binary nano-features, apodization can be implemented by removing some nano-features in the transitional areas so that the average density of the binary nano-features becomes proportional to g(r). 
         [0069]    In the next step, a compensation function is applied in order to compensate for variations in the average refractive index. In particular, a digital planar hologram creates a variation of the average effective refractive index so that the light wavelength within the digital planar hologram differs from that within the blank part of a planar waveguide. To avoid undesirable distortions due to this nonuniformity, it is necessary to compensate [for] the refractive index variation caused by patterning the planar waveguide, including variation caused by apodization. According to one modification of the present invention, a compensation function can be defined by the following equation: 
         [0000]        f ( x, y )=1 +Δn/n =1 +ag ( r ),   (6) 
         [0070]    where Δn is the averaged variation of the effective refractive index in the vicinity of a given point, a is the scaling parameter, and r is the distance to an input port. 
         [0071]    The super-grating apodization is illustrated in  FIG. 6 , where  23  and  25  are transitional areas and  24  is the central super-grating zone with the area of maximum modulation of the planar waveguide refractive index. 
         [0072]    The super-grating is the main component of any nanospectrometer made in accordance with the present invention; however, as explained below, in order to improve functionality, the spectrometer should include some additional components. It should be understood that depending on the proposed nanospectrometer configuration, all or almost all components will be integrated on the same planar waveguide as the super-grating. 
         [0073]    The first configuration of the nanospectrometer is a laser-induced breakdown (LIB) spectrometer, shown in  FIG. 7 . The LIB nanospectrometer is integrated on a base  100 , which can be a piece of silicon wafer or any other substance appropriate for attaching all components. The super-grating  108  is embedded into a planar waveguide  104 . A laser  101  is integrated on the same base  100  and is coupled with a ridge waveguide  102 , which, in turn, is coupled with the planar waveguide  104  so that the laser beam propagates directly to a narrowband concave grating  103 . This grating is embedded into the same planar waveguide  104  and is implemented as a sub-grating component of the super-grating with the function to reflect and focus the laser beam for coupling it into an optical fiber  105 . The laser beam is delivered by the fiber to an object having a solid or liquid surface  107 , which needs to be studied and on which the laser beam must be focused through a focusing lens  106 . In a small focus, the laser intensity gets high enough to ionize the superficial layer on the surface of the object, and the created plasma emits optical radiation, the spectrum of which is a unique determinant for the ionized substance. This optical radiation is acquired with the focusing lens  106  and is coupled back to the fiber  105 , which delivers it to the super-grating  108  for analysis. The super-grating separates light into channels and focuses them on the arrays  109  and  110  of detectors for converting them into electrical signals that can be displayed and processed. 
         [0074]    In the second modification, the device is made as an absorption nanospectrometer, as shown in  FIG. 8 . All spectrometer components are integrated on a single chip. The device comprises a base  200 , which can be a piece of silicon wafer or any other substance appropriate for attaching all components, several super-luminescent laser-emitting diodes (SLED)  201 ,  202 ,  203 ,  204 , and  205 , which radiate in various spectral bands in order to cover the spectral range appropriate for absorption analysis. All SLEDs are coupled at a point of coupling with ridge waveguides  206 ,  207 ,  208 ,  209 , and  210  into a bare ridge guide  211 . The ridge guide is referred to as “bare” because it does not have the upper cladding that provides better interaction with the environment and higher sensitivity. The bare waveguide spirals around the chip to accumulate a longer length for better sensitivity and is coupled into a slab waveguide  212  where the super-grating  212   a  is embedded. Light, analyzed by the super-grating, is focused on arrays  213  and  214  of detectors for conversion into electrical signals, which can be displayed and processed. This nanospectrometer can analyze liquids and gases. 
         [0075]    The third modification, which is shown in  FIG. 9 , provides an absorption nanospectrometer with a fiber sensor. This nanospectrometer is similar to the previous one, but the sensor is implemented as a bare fiber (a fiber without a cladding) rather than as a ridge waveguide on a chip. This provides more convenient access to narrow channels or small gaps. All spectrometer components, in addition to the fiber sensor, are integrated on a single chip. The nanospectrometer comprises a base  300 , which can be a piece of silicon wafer or other substance appropriate for attaching all components, several super-luminescent laser-emitting diodes (SLED)  301 ,  302 ,  303 ,  304 , and  305 , which radiate in various spectral bands in order to cover the spectral range appropriate for absorption analysis. All SLEDs are coupled with ridge waveguides  306 ,  307 ,  308 ,  309 , and  310  into an optical fiber, which consists of a core  311  and a cladding  312 , which participates in guiding the light. At some distance from the chip, the cladding is removed, and a bare fiber  313  (core only) is used for guiding the light. Removal of the cladding makes the fiber probe more sensitive for detecting environmental gas or determining liquid composition. Before returning to the chip, the bare fiber of a required length (longer length provides better sensitivity) is again coated with a cladding  314 . The fiber core  315  is coupled into a slab waveguide  316 , where the super-grating  317  is embedded. Light, analyzed by the super-grating  317 , is focused on arrays  318  and  319  of detectors for conversion into electrical signals, which can be displayed and processed. This nanospectrometer can analyze liquids and gases by submerging the bare fiber probe into them. 
         [0076]    The fourth preferred modification, shown in  FIG. 10 , is a Raman nanospectrometer on a chip. All spectrometer components are integrated on a base  400 , which can be a piece of silicon wafer or other substance appropriate for attaching all components. The spectrometer comprises a laser  401  coupled to a ridge waveguide without an upper cladding  402 , a spiraling for higher sensitivity around a planar slab waveguide  404  and coupled thereto, a super-grating  405  embedded into the slab waveguide, and arrays  406 ,  407  of a detector. Deposited on the top of the ridge waveguide  403  is a Raman-enhancing layer  404  comprising nanoparticles of silver or other metal used for increasing the Raman-effect cross-section by many orders of magnitude, typically by 10 9 -10 12  times and sometimes even more. The laser beam propagates through the ridge waveguide, and each time it reflects from the top, the Raman spectrum caused by the environment is generated. After multiple reflections, the spectrum acquires higher intensity and can be analyzed by the super-grating  405 , which is designed to freely transmit the laser wavelength and to focus the Raman spectrum on the arrays of detectors  406  and  407  for converting light signals into electrical signals that can be displayed and processed. 
         [0077]    The fifth modification, shown in  FIG. 11A , is a Raman nanospectrometer with a fiber probe. All spectrometer components, in addition to the fiber sensor, are integrated on a single chip, which has a base  500  that may be a piece of silicon wafer or other substance appropriate for attaching all components. The spectrometer comprises a laser  501  coupled to a ridge waveguide  502  for guiding laser radiation to a planar slab waveguide  503 . The laser beam propagates directly to a narrowband concave grating  504 . The grating of this modification is embedded into the same planar waveguide and is implemented as any sub-grating component of the super-grating with the function to reflect and focus the laser beam for coupling it into an optical fiber  506 . The laser beam is delivered to the fiber end, which is coated with a Raman-effect enhancing layer  507  that comprises nanoparticles of silver or other metal for increasing the Raman-effect cross-section by many orders of magnitude, typically by an increase of 10 9 -10 12  times and sometimes even more. For higher sensitivity, the end of the probe is partially stripped of cladding and is made “D-shaped.” Shapes of fiber faces that are designated by reference numerals  505 ,  508 , and  507 , respectively, are shown in  FIGS. 11B to 11C . Such shapes are needed to provide direct contact over a significant area between the Raman-enhancing layer and the fiber probe core  505 . In addition, the fiber end is cleaved at an oblique angle to prevent direct reflection of the laser beam back to the chip. After multiple reflections and acquiring the Raman shift, which is the signature of the environment around the probe, the laser beam returns to the chip, where the narrowband mirror reflects the laser wavelength and transmits the Raman-shifted part of the spectrum to a super-grating  504  for analysis. 
         [0078]    The super-grating embedded into the slab waveguide  503  focuses the light spectrum on arrays  509  and  510  of the detectors for conversion into electrical signals, which can be displayed and processed. 
         [0079]    The sixth preferred modification, shown in  FIG. 12 , is a folded nanospectrometer of high resolution. The folded layout allows for more compact design and for compensation of optical nonuniformities in a planar waveguide. All spectrometer components are integrated on a single chip that has a base  600 , which can be a piece of silicon wafer or other substance, appropriate for attaching the components. The spectrometer comprises an input port  601 , from which the light beam to be spectrally analyzed propagates directly to a broadband concave grating  602  that is embedded into the same planar waveguide and operates as a folding mirror. The light beam sequentially goes through the multiple super-grating knees  604 ,  607 , and  610  steered with folding mirrors  605 ,  606 ,  608 , and  609 , all of which are broadband gratings embedded into the same planar waveguide as are the other nanospectrometer components. Each of the super-grating knees reflects spectrally dispersed light to array  611  of the detectors for converting light into electrical signals to be processed, analyzed, and displayed. 
         [0080]    Thus it has been shown that the present invention provides a nanospectrometer on the basis of digitally generated diffraction structures in planar optical waveguides. The invention also provides a method of manufacturing the aforementioned nanospectrometer by means of microlithography. The super-gratings of the proposed nanospectrometer comprise multiple sub-gratings consisting of standard binary features such as dashes or grooves etched in the planar waveguide by means of microlithography. 
         [0081]    Although the invention has been shown and described with reference to specific embodiments, these embodiments should not be construed as limiting the areas of application of the invention, and any changes and modifications are possible provided these changes and modifications do not depart from the scope of the attached patent claims. For example, optionally, all spectrometers according to the present invention can be used without integrated detector arrays in a spectroscopic mode.