WO2011081600A1

WO2011081600A1 - Thin optical devices with means for filtering off -axis light

Info

Publication number: WO2011081600A1
Application number: PCT/SE2010/051490
Authority: WO
Inventors: Lars LANDSTRÖM; Torgny MÖLLER
Original assignee: Serstech Ab
Priority date: 2010-01-04
Filing date: 2010-12-28
Publication date: 2011-07-07

Abstract

The current invention relates to thin planar optical devices and how undesired light rays in certain directions can be filtered in a controlled way. By reducing light in unwanted directions, a better performance can be achieved for, e.g., grating-based constructs such as a spectrometer/spectrograph. The present invention allows for filtering of the unwanted light inside the optical construct, thus minimizing the need for external manipulation of the incoming light fed into the optical device. To achieve the filtering effect, light is allowed to be partially transmitted into at least one of the interfaces of interest. By choosing suitable material properties and geometries of the whole construct, the filtering effect can be tuned within a wide range.

Description

THIN OPTICAL DEVICES WITH MEANS FOR FILTERING OFF -AXIS LIGHT

Technical Field This invention relates to optics in general and, in particular, to relatively thin optical devices, for example relatively thin grating based optical devices, such as spectrometers and the like.

Background Thin optical, grating based devices, such as spectrometers or spectrographs with a small footprint have various applications, e.g., identification, sensing, color measurements, etc. Such thin grating based spectrometers are usually referred to as two dimensional (2D) planar spectrometers. For example, by using lithographic or molding fabrication methods, the cost of each unit can be lowered drastically for large volume applications. The word "thin" in this context may refer to a thickness of the optical device in the order of a few tens or hundreds of pm.

Usually, the actual sensing or detection is based on well-known spectroscopic techniques such as absorption, reflection, or transflection techniques, where a known incoming broadband field is altered by an analyte, and the resulting field is analyzed by the spectrometer. Other techniques, where monochromatic light is modified by the analyte (e.g., Raman scattering) have also proven to be useful for chemical sensing and/or identification of different compounds/mixtures. Unfortunately, the Raman scattered light is very weak relative to the incoming light. If bulk quantities are not available, the direct sensing, or detection, by this technique can be cumbersome. However, amplification of the Raman signal can be achieved by so-called Surface Enhanced Raman Spectroscopy (SERS), allowing detection of trace levels of certain molecules. Seeing the market demand for small, mobile, and cheap detector solutions rapidly increasing over the last decade, platforms as described above have proven to be strong candidates for further integration into units meeting this high demand. Numerous designs of different planar optical devices are available in the literature today. Commonly, these optical devices are manufactured monolithically and based on total internal reflection (TIR) where the light is guided and confined within the dielectric material by waveguides, or waveguide slabs, having a higher refractive index than the surrounding medium. However, especially for diffractive 2D devices with vertical gratings, this classical TIR construction may introduce diffraction errors if out-of-plane light rays are incident and diffracted by the grating. (Here, Out-of-plane rays' denotes light rays being divergent, non-parallel, with respect to the grating normal.) That is, better performance is achieved if the incoming light is collimated to be in-parallel to the grating normal, for example using a lens. However, such collimation may be difficult to accomplish in a miniaturized optical device.

Summary

Embodiments of the present invention are based on an understanding from the inventors that an alternative to classical collimation using lenses, that is better suited in a miniaturized optical device, is to employ a directional filtering of the light input to the optical device. Thus, embodiments of the present invention does not achieve collimation in its classical sense. Instead, the unwanted out- of-plane rays are being filtered out by being partially transmitted through at least one of the interfaces parallel to the grating normal. By choosing the optical properties of the materials of the device in combination with geometry considerations, the amplitude of these unwanted rays can be significantly reduced. Thereby, diffraction errors can be reduced, which may e.g. result in a better performance of a spectrometer or spectrograph, for instance in terms of resolution and noise. According to a first aspect, there is provided an optical filter construct adapted for use in a range of optical wave lengths and having a main optical propagation plane. The optical filter construct comprises a main optical transmission medium for providing an essentially lossless transmission of light in said range of optical wavelengths in said main optical propagation plane. At least one interface of the main optical transmission medium, which is essentially parallel to the main optical propagation plane, is at least partially covered with a filtering medium, which is either a metal or a medium with higher refractive index than the main optical transmission medium material for at least partly filtering out light incident that interface from further propagation in the main optical transmission medium. (Note that a few nm thin layer of, e.g., native oxide on the dielectric or metal will not alter the filtering effect significantly.)

For example, at least one of the interfaces perpendicular to the propagation direction of light (in the main optical propagation plane) may be to a medium (i.e. the filtering medium) with higher refractive index than the material of the main optical transmission medium, thus allowing partial transmission of light incident that interface out from the main optical transmission medium for performing the filtering. The optical filter construct may be adapted to be coupled to a light source.

In use of the optical filter construct, photons of the filtered out light may be used to initiate photovoltaic phenomena and/or thermal excitation of the filtering medium.

According to a second aspect, there is provided a spectrometer adapted for use in a range of optical wavelengths. The spectrometer comprises a main optical transmission medium for providing an essentially lossless transmission of light in said range of optical wavelengths in a main optical propagation plane. The main optical transmission medium has an input interface for receiving input multi-wavelength light in said range of optical wavelengths, and an output interface for outputting light of different wavelengths on separate focal spots on said output interface and in said main optical propagation plane. At least one interface of the main optical transmission medium, which is essentially parallel to the main optical propagation plane, is at least partially covered with a filtering medium, which is either a metal or a medium with higher refractive index than the main optical transmission medium material for at least partly filtering out light incident that interface from further propagation in the main optical transmission medium.

As for the optical filter construct of the first aspect, at least one of the interfaces perpendicular to the propagation direction of light (in the main optical propagation plane) may, for example, be to a medium (i.e. the filtering medium) with higher refractive index than the material of the main optical transmission medium, thus allowing partial transmission of light incident that interface out from the main optical transmission medium for performing the filtering.

The spectrometer may further comprise a dispersive element and a focusing element integrated with the main optical transmission medium. The dispersive element and focusing element may be a same element or separate elements. The dispersive element and focusing element may be adapted to separate the input multi-wavelength light to said light of different wavelengths to be output on said separate focal spots on the output interface.

The dispersion from the dispersive element may be predominantly in the main optical propagation plane, which may be the same plane as the normal of the dispersive element. The spectrometer may further comprise a detector system for detecting the light of different wavelengths output on said separate focal spots of the output interface. The detector system may be located at the output interface. At least one interface of the main optical transmission medium, which is not covered (or partially covered) by the filtering medium, may be configured to provide total internal reflection for certain angles.

A slit may be arranged on the input interface to define a width of an incoming light field. For example, the slit may be defined by an opening in a non- transparent material, such as a metal, by a waveguide-like construction being straight or tapered, or by an optical fiber aligned to the input interface that determines the input light field characteristics.

The dispersive element may comprise a holographic grating, a reflection grating, and/or a transmission grating.

The focusing element may comprise a lens or a mirror.

Focusing or de-focusing optics may be placed either between the input interface and the dispersive element and/or between the dispersive element and the output interface. The output interface may be substantially aligned on a straight line.

A guide structure may be fabricated with the main optical transmission medium to facilitate the attachment of the detector system.

The detector system may comprise an electronic system for powering, control and signal collection. The main optical transmission medium may be fabricated via lithographic methods, via molding techniques, such as injection molding and/or embossing molding, via casting methods, via etching, via material removal by a focused ion beam, and/or via chemical or physical deposition techniques.

The spectrometer according may be adapted for Raman spectroscopy, including surface enhanced Raman spectroscopy. The spectrometer may be adapted for absorption spectroscopy, fluorescence spectroscopy, photoluminescence spectroscopy, emission spectroscopy, reflection/transflection spectroscopy, and/or light scattering measurements.

The spectrometer may be adapted to be integrated in a portable device. In short, embodiments of the invention applied in, e.g., a thin spectrometer provides better performance in terms of wavelength resolution and noise levels compared to the commonly used fully TIR based planar waveguide spectrometers. The main optical device can be micro-fabricated by processes such as lithography methods using light-sensitive materials, or casting/molding methods, e.g., injection molding, embossing molding, or cast molding. However, the invention is not restricted to thin spectrometer devices only. Other applications where the controlled filtering of light in certain directions is desirable may also take advantage of the effect.

In one embodiment, the thin optical device includes a volume comprising an input plane, or input interface, for the optical light, optional mirrors, a transmission medium to the grating, optional mirrors, and again a transmission medium to the output plane, or output interface. Within this volume, the material of the transmission medium is chosen so as the light with wavelengths of interest can propagate with small loss. At least one of the interfaces parallel to the light propagation direction comprises or consists of a dielectric material with a higher refractive index than this transmission medium. Such an interface will allow a fraction of the incident light to be transmitted into the higher refractive index dielectric, thus reducing the off-axis rays for each reflection at said interface. The cumulative effect for multiple reflections will result in a controlled suppression of these off-axis rays which are detrimental to the overall performance of the spectrometer/spectrograph.

To deliver the light to be analyzed to and through the input plane of the optical device, an open beam solution or optical fiber solution may be used. An input slit may be placed at a certain distance and angle relative to the grating. This input may be incorporated within the above mentioned structure as a straight or tapered light guide where the slit width being determined by the geometry of said guide (or, more specifically, the geometry of the light modes inside said guide at the input plane) at the predetermined distance to the grating. An input slit may also be defined at the input plane by an optically thick, non- transmitting material, such as a metal. The width of the input field at a certain distance and angle relative to the grating may also be controlled by external optical components such as lenses, physical slits, and/or optical fibers. If using optical fiber, an index matching medium may be used between the fiber and input. In the open beam case, an anti-reflective coating may be used. The numerical aperture of the incoming field at the input plane will be optimized with respect to overall performance of the optical device.

The optical device may be constructed in such a way that the output interface (image plane) of the incoming light (after being dispersed at the grating) lies in one plane, i.e. aligned on a straight line.

In connection to the output plane, a signal collection system may be included. Such a system may comprise or consist of a light-sensitive semiconductor device, e.g., CCD , InGaAs, or Ge array systems, or an array of photodiodes, etc. An index matching medium or antireflective coating may be used to connect the output plane to the detector array.

Furthermore, in another embodiment, the directional filtering effect may also be used to harvest photons in a controlled way in combination with light sources in need of a lower divergence in certain directions. By proper geometry and material choices, a certain amount of light in determined directions can be used to induce, e.g., thermal, photovoltaic, or similar phenomena in the device which also acts as a slit.

Brief description of figures

FIG.1 shows a schematic of a concave grating spectrograph with input and output planes. FIGS.2 and 3 depict possible material combinations (with certain optical properties) which will result in the desired effect of the current invention.

FIG.4 shows the calculated reflection coefficients for an interface with refractive indices η = 1.6 and n₂ = 3.6. FIG.5 show different solutions to create the wanted input slit. Some solutions will also allow manipulation of the numerical aperture of incoming light.

FIG.6 shows example measurements obtained through a spectrometer with physical dimensions ~25 mm ^χ 25 mm ^χ 40 pm and with a Free Spectral Range of about 1 15 nm. The measured resolution was around 1 nm. Detailed Description

A schematic of a typical flat-field output spectrometer is shown in FIG.1. Different suggestions for grating geometry are well known to a person skilled in the art, and is therefore not described in any further detail herein. As an example, a concave grating 11 is frequently used, which both disperse and focus/collimate the divergent incoming light (emerging from the input plane 10) to the output plane 12. Commonly, the light travels inside the same medium 13 on its path from the input plane 10 via the grating 11 to the output plane 12. This light is incident on the diffractive construct and, after being dispersed, directed onto the image plane which is substantially a straight line. On this image line, which coincides with the output plane 12, the different wavelengths of the incoming light have been separated into different positions (illustrated by the different lines in FIG.1). The output plane may be connected to a light sensitive semiconductor device, such as an array of photodiodes, CCD array, InGaAs array, etc. The dash-dotted lines serve as an example of a preferred geometry that could be used as confinement of medium 13. A typical height of the medium 13 is in the order of a few tens or hundreds of pm, typically <500 pm. FIG. 1 only serve as a schematic example, a multitude of other geometries resulting in a similar operation can also be used. For example, collimating/focusing mirrors or other optical components such as focusing/de-focusing optical elements can be integrated to manipulate the light and other grating designs may be used. The above described design with a relatively small height of the device, are usually denoted as 2D optical constructs, and where the relatively thin layers are used as optical guiding material. Usually, the light is confined within the guiding material by means of total internal reflection (TIR), i.e., the refractive indices of all surrounding material is smaller than the guiding material. For most device applications these guiding materials support multiple optical modes. However, and related to the current invention, light rays in certain directions may actually decrease certain performance parameters of the 2D optical construct.

A certain divergence within the optical plane (or "main optical propagation plane") will be necessary for achieving best performance of the device depicted in FIG.1 ; e.g., the numerical aperture of the light at the input plane 10 needs to be optimized as to illuminate the largest amount of the effective grating 11. The width of the field (slit) at the input plane 10 is also greatly affecting the performance. Light rays divergent in the perpendicular plane will, however, introduce diffraction errors at the output plane 12 with respect to the correct position of rays of wavelength λ which are scattered by the grating normal to the grating plane. Hence, the possibility to reduce/filter these out-of- plane light rays will optimize the performance of the device in terms of resolution and noise levels.

Since waveguiding is based on TIR phenomenon, the waveguide material is surrounded by media with lower refractive index. Thus, these devices based on TIR waveguiding are manufactured in such a manner. Embodiments of the current invention increases certain performance by reducing the light rays being divergent in certain directions, namely out-of-plane relative to normal incidence on the grating 11. FIGS 2 and 3 schematically show examples of layered structures that will result in the wanted filtering effect. In one embodiment, see FIG.2a, the medium 13 in which the light propagates, a material 21 with either lower or higher refractive index is attached. The material 21 may also act as a protective cover for the structure 13. The material 20, however, has such properties that its refractive index is higher than 13, thus allowing partial transmission into this material for every interaction/reflection. The medium on the other two sides of 13 consists of lower refractive index, e.g., air, and at these interfaces TIR will occur for light incident at angles fulfilling the criteria for TIR.

In another embodiment, see FIG.2b, the optical medium 13 is again on a support consisting of a higher refractive index material 20, and the whole slab being covered by a highly reflective material 22, such as a metal. Metal, as opposed to the above mentioned material with lower refractive index, does not provide a total reflection of light and actually also acts to filter the light to some extent. Therefore, in some embodiments of the present invention, metal may be used instead of the above-mentioned material with higher refractive index to provide the filtering effect. In some embodiments, the filtering, or absorbing, material (dielectric or metal) may be patterned in a symmetrical 2D or 3D structure, allowing certain wavelengths of certain angles being coupled out in said material.

In another embodiment, see FIG. 3a, the optical device 13 is supported by a thick slab 13b consisting of the same material as 13. Here, the support and the optically active device may be manufactured monolithically. The upper material 21 consists of a lower or higher refractive index material and may also serve as a protection of the structures created in material 13. Importantly, if the height of 13 is much smaller than the whole thickness (13 + 13b), and fabricated in the same material, only rays with smaller angular dispersion than; tan^"1 (^{0 5 " Gratln}9 ^heidht/_{Distance input t0} grating) (assuming reflection at the upper interface) may hit the grating and thus being useful in a spectrograph/spectrometer set-up. The embodiment shown in FIG.3b is similar to the one in FIG.3a except here the 13 + 13b slab is being covered by a highly reflecting medium 22 such as a metal. The material 22 may also consist of a lower refractive index material compared to 13, such as, e.g., air, thus allowing low-loss confinement of the light by means of TIR. The filtering effect in this embodiment may be accomplished by that at least part of the light entering into the support 13b stays in the support 13b and never reenters the region 13.

The embodiments illustrated in FIG.2 result in that light propagating at some angle from the grating normal will interact with a higher refractive index material several times (unless the light path is very short and the off-angle very small). Concerning rays being only 1-2° off the grating normal, these will not induce any significant error at the output plane, and extremely short distances between input - grating - output are unlikely if certain resolution and free spectral range are to be achieved for light at optical wavelengths.

FIG.4a show the calculated reflection coefficients for an interface between materials with refractive indices n = 1.6 and n₂ = 3.6. These indices correspond to the epoxy SU-8 and Si, respectively, at near infrared (NIR) wavelengths. The materials SU-8 and Si are suitable examples for use as the main optical transmission medium (i.e. 13) and as said material with higher refractive index, respecively. Other suitable materials for the main optical transmission medium may e.g. be olefin polymers (e.g., Zeonex®, Zeonor®), or other polymer or plastic material, or glass. The coefficients for reflection are angular and polarization dependent and given by the Fresnel equations:

where the subscripts s and p relates to the polarization of incident light. For 'unpolarized' (mixed polarization) light, the reflection coefficient is R = {R_s + R_p)/2. As can be seen in FIG.4a, any reflection will induce an intensity change of the reflected light ray (reflection coefficient <1 ). However, in a practical useful construct for spectrograph/spectrometer applications, the off- axis rays will undergo multiple reflections at this interface, see FIG.4b. The total number of reflections at the interface mentioned above; N(0;) = L/l is dependent on the physical geometry of the optical structure 13 and the out-of- plane angle a = 90 - e_t. For example, a height of the structure 13 (e.g., as shown in FIG.2) of 40 pm gives; I = 80 μπι/ ΐ3η( 0 - θ ).

The cumulative effect of the multiple interactions at the interface of interest can then be approximated by R^N^⁰ⁱ\ This cumulative reflection coefficient as function of incident angle (0_έ) is depicted in FIG.4c. The length L in this example is 42 mm. For the parameters given here, it can be seen that an efficient suppression is obtained already at grazing incident light, e.g., for a > 3° almost no light will reach the output. In addition, a change in n₂ refractive index of, say, ±0.5 does not affect the results in FIG.4 significantly. The values used here and plots shown in FIG.4 only serve as example data to illustrate the main idea of the current invention. Importantly, the optical parameters used are by no means fictional, instead taken from common materials used in semiconductor processing today. By altering the materials (i.e., refractive indices) exemplified in FIGS. 2 and 3, and adjusting of geometries allows for tuning of the filtering effect.

As mentioned earlier, it is important to control the width of the light at the input plane 10, i.e., by a slit, and also the numerical aperture in order to achieve the best performance of a grating spectrometer/spectrograph. FIG.5 shows some examples of preferred solutions how to control the slit width at the input plane 10 (the bow-parentheses in FIG.5 illustrate the width of the slit) and also some examples how to alter the numerical aperture of the input field.

In one embodiment, the optical device is manufactured in such a way that the input, with a slit defined by a straight or tapered asymmetric waveguide (see FIGS.5a-c), is fabricated monolithically with the volume 13 as an extension out from the input plane 10, by lithographic methods, or molding, casting, embossing, imprinting methods. In such a case, the material 51 will be the same as material 13. For this monolithically constructed optical device, with the associated input guide as shown as examples in FIG.5a-c, similar surrounding materials as depicted in FIG.2 and FIG.3 will be preferred. The preferred embodiments in connection with FIGS.5a-c may also allow for a manipulation of the numerical aperture of the incoming light field. For example, the FIG.5b and FIG.5c tapered input guides will alter the incoming numerical aperture by the same factor as the ratio of the width change. The light may be coupled into these straight or tapered guides by using an optical fiber.

In another embodiment, the input light parameters, such as width and numerical aperture at the input plane 10 are defined by external components such as lenses, mirrors and physical slits, see FIG.5d. The dashed lines illustrate the incoming light field. This light field may also originate from an optical fiber.

In another embodiment, the slit width is defined directly on the input plane by deposition of optically non-transparent material 52 on the input plane, see FIG.5e. This non-transparent material for the wavelength-region of interest may be a metal. The dashed lines illustrate the incoming light field, which also may originate from an optical fiber.

In another embodiment, the side-walls of the input guides (see FIG. 5a-c) are manufactured so that they are non-parallel to each other and at least one of the side-walls being not at a straight angle to the support material (e.g., 20 or 13b). Such a construction allows for controlled attenuation of the light not only in the off-axis direction but also in the main direction (parallel to propagation direction).

For example, by using lithographic methods and a negative epoxy based photoresist (SU-8), ~40pm high structures have been created on a Si substrate. Opposite to the silicon was a thin layer of PMMA attached to the structure. Above the PMMA a glass plate was attached for protective purposes. (The other two interfaces were SU-8 to air.) In this particular sandwich structure total reflection will occur, within a certain angular interval, for the interfaces SU-8/PMMA and SU-8/air. At the SU-8/Si interface, each reflection will have some energy transfer into the Si support, thus diminishing the out-of-plane light rays as described earlier.

Attached to the output plane may be a detector unit able to measure light signals. The advantages of earlier mentioned manufacturing techniques are that low price per unit can be reached, and miniaturization of the devices is also quite straightforward. For example, a device ~25x25 mm can easily exhibit ~1 nm resolution with a free spectral range of ~120 nm. FIG.6a shows a measurement of a monochromatic light source (diode laser @ 785 nm width line-width < 0.2 nm) through a spectrometer design with an input slit (defined by an asymmetric tapered waveguide, see, e.g., FIG.5b). The total path length of the light within the optical device was ~45 mm and the graph was obtained by converting the light at the output plane to electrical signal by a CCD array. The FWHM of the laser line was measured to ~0.9 nm. In addition, Ar elemental lines, in the wavelength region 790-855 nm, were measured through a similar spectrometer, shown in Fig 6b, and again a practical resolution around 1 nm can be seen. (The inserted, written wavelengths are tabulated values.)

By altering the design of the components, it is possible to optimize the device for different applications, e.g., the device may be optimized to detect Raman scattered light, originating from samples being illuminated by laser light. A spectrometer incorporating the invention can by the ability to compact design be suitable, and essential for, in particular, for hand held Raman based instruments, where it is crucial to keep the physical dimensions down. For the Raman optimized design the Raman signal may originate from plasmonic structures amplifying the Raman signal (e.g., so called SERS surfaces). Other spectroscopic techniques of interest may be; absorption spectroscopy, emission spectroscopy (such as thermal emission, fluorescence and photoluminescence), reflection and transflection spectroscopy, scattering measurements, etc.

In another embodiment, the filtering effect may be applied in front of a light source, where the source may be a light emitting diode, laser source, or similar, where the slit-effect results in a lower divergence in one plane of the output light beam. In such a set-up, and proper material choices, the photons being filtered out into a higher refractive index dielectric can be used to initiate, e.g., photovoltaic or thermal effects in the same. Although the present invention has been described with example schematics and preferred embodiments, it is not intended to be limited to the specific form set herein. Rather, the scope of the present invention is limited to the accompanying claims. In the above descriptions, certain specific details of the disclosed embodiments were set forth for purposes of explanation and not limitation, so as to provide a clear thorough understanding of the present invention. However, it should be understood readily for those skilled in the art, that the present invention may be achieved in other embodiments which do not conform exactly to the details set forth herein, without departing from the scope of the invention. Further, in this context, and for the purposes of brevity and clarity, the description of well-known processes, devices and methodology have been omitted so as to avoid unnecessary detail and possible confusion.

Claims

1. An optical filter construct adapted for use in a range of optical wavelengths and having a main optical propagation plane, comprising a main optical transmission medium for providing an essentially lossless transmission of light in said range of optical wavelengths in said main optical propagation plane; wherein at least one interface of the main optical transmission medium, which is essentially parallel to the main optical propagation plane, is at least partially covered with a filtering medium, which is either a metal or a medium with higher refractive index than the main optical transmission medium material for at least partly filtering out light incident that interface from further propagation in the main optical transmission medium.

2. The optical filter construct according to claim 1 , wherein the optical filter construct is adapted to be coupled to a light source.

3. The optical filter construct according to claim 1 or 2, wherein, in use of the optical filter construct, photons of the filtered out light are used to initiate photovoltaic phenomena and/or thermal excitation of the filtering medium.

4. A spectrometer adapted for use in a range of optical wavelengths, comprising a main optical transmission medium for providing an essentially lossless transmission of light in said range of optical wavelengths in a main optical propagation plane and having:

- an input interface for receiving input multi-wavelength light in said range of optical wavelengths; and - an output interface for outputting light of different wavelengths on separate focal spots on said output interface and in said main optical propagation plane; wherein at least one interface of the main optical transmission medium, which is essentially parallel to the main optical propagation plane, is at least partially covered with a filtering medium, which is either a metal or a medium with higher refractive index than the main optical transmission medium material for at least partly filtering out light incident that interface from further propagation in the main optical transmission medium.

5. The spectrometer according to claim 4, further comprising a dispersive element and a focusing element integrated with the main optical transmission medium, wherein the dispersive element and focusing element may be a same element or separate elements, and are adapted to separate the input multi- wavelength light to said light of different wavelengths to be output on said separate focal spots on the output interface.

6. The spectrometer according to claim 4 or 5, wherein the dispersion from the dispersive element is predominantly in the main optical propagation plane, which is the same plane as the normal of the dispersive element.

7. The spectrometer according to any of the claims 4-6, further comprising a detector system for detecting the light of different wavelengths output on said separate focal spots of the output interface, wherein the detector system is located at the output interface.

8. The spectrometer according to any of the claims 4-7, wherein at least one interface of the main optical transmission medium, which is not covered by the filtering medium, is configured to provide total internal reflection for certain angles.

9. The spectrometer according to any of the claims 4-8, wherein a slit is arranged on the input interface to define a width of an incoming light field.

10. The spectrometer according to claim 9, wherein the slit is defined by an opening in a non-transparent material, such as a metal, by a waveguide-like construction being straight or tapered, or by an optical fiber aligned to the input interface that determines the input light field characteristics.

11. The spectrometer according to claim 5, wherein the dispersive element comprises a holographic grating, a reflection grating, or a transmission grating.

12. The spectrometer according to claims 5 or 11 , wherein the focusing element comprises a lens or a mirror.

13. The spectrometer according to claim 5, 11 , or 12, wherein the dispersive and focusing elements are combined into a single element.

14. The spectrometer according to any of the claims 5, 11 , 12, or 13, wherein focusing or de-focusing optics is placed either between the input interface and the dispersive element and/or between the dispersive element and the output interface.

15. The spectrometer according to any of the claims 4-14, wherein the output interface is substantially aligned on a straight line.

16. The spectrometer according to claim 7, wherein a guide structure is fabricated with the main optical transmission medium to facilitate the attachment of the detector system.

17. The spectrometer according to claim 7 or 16, wherein the detector system comprises an electronic system for powering, control and signal collection.

18. The spectrometer according to any of the claims 4-17 wherein the main optical transmission medium is fabricated via lithographic methods, via molding techniques, such as injection molding and/or embossing molding, via casting methods, via etching, via material removal by a focused ion beam, and/or via chemical or physical deposition techniques.

19. The spectrometer according to any of the claims 4-18, adapted for Raman spectroscopy, including surface enhanced Raman spectroscopy.

20. The spectrometer according to any of the claims 4-18, adapted for absorption spectroscopy, fluorescence spectroscopy, photoluminescence spectroscopy, emission spectroscopy, reflection/transflection spectroscopy, and/or light scattering measurements.

21. The spectrometer, according to any of the claims 4-20, adapted to be integrated in a portable device.