Waveguide spectrometer to carry out the integrated interferogram scanning

Abstract

A waveguide spectrometer includes at least one substrate layer with at least one surface waveguide extending from an inlet face to guide the received light; at least one evanescent field sampler in the waveguide to out-couple light along the waveguide; at least one light sensing unit to detect the out-coupled light, each electrically connected to an electronic read out system; and means to achieve counter propagating optical signals inside the waveguide to obtain interference between the counter propagating optical signals generating an interference pattern along the waveguide. A compact and simple construction with improved spectral range/bandwidth of the spectrometer can be achieved with at least one modulator integrated into the sampling waveguide structure to enable conditioning of the guided optical signals and for changing the refractive index. The integrated modulator is realized by electrodes placed aside directly neighboured to the guiding core resp. waveguide generating an optical phases shift required for scanning the interferogram.

Claims

1. A Waveguide Spectrometer to carry out integrated interferogram scanning, comprising: a substrate layer with a waveguide, the waveguide extending from an inlet face and configured to guide received light, at least one evanescent field sampler in the waveguide, configured to out-couple light out from the waveguide, at least one light sensing unit configured to detect the out-coupled light, each electrically connected to an electronic read out system, and means to achieve counter propagating optical signals inside the waveguide configured to obtain interference between the counter propagating optical signals generating an interference pattern along the waveguide in an interferogram, wherein the waveguide spectrometer comprises an integrated modulator configured to enable conditioning of the guided optical signals and configured for changing the refractive index, wherein said integrated modulator is realised by electrodes placed aside directly neighboured to the waveguide on the substrate next to the waveguide generating an optical phases shift required for modulating the interferogram and wherein a distance between the waveguide and each of the electrodes is d>0 mm.

2. The Waveguide Spectrometer according to claim 1, wherein the Waveguide Spectrometer comprises a reflecting element, wherein the waveguide is extending from an inlet face through the substrate layer to a reflecting element in order to achieve a Lippmann configuration.

3. The Waveguide Spectrometer according to claim 1, wherein the means to achieve counter propagating optical signals comprises a beam splitting element configured to split the received optical signal into two separate signals guided through two inlet faces into the waveguide in order to achieve counter propagation inside the waveguide in a Gabor configuration.

4. The Waveguide Spectrometer according to claim 1, wherein the electrodes are placed on the top surface towards the front side of the substrate layer aside the waveguide and proceeding on the top surface along a length.

5. The Waveguide Spectrometer according to claim 1, wherein the electrodes are placed on opposing side faces of the substrate layer aside the waveguide and proceeding perpendicular to the front side of the substrate layer along a length.

6. The Waveguide Spectrometer according to claim 1, wherein the at least one evanescent field sampler in the waveguide are at least one of photonic crystal light cones, etched grooves, and a metallic nano sampler.

7. The Waveguide Spectrometer according to claim 1, wherein the waveguide is directly inscribed into the substrate layer.

8. The Waveguide Spectrometer according to claim 1, wherein the integrated modulator uses the electro-optic effect to achieve modulation of the refractive index property.

9. The Waveguide Spectrometer according to claim 1, wherein the waveguide structure material contains LiNbO.sub.3, and wherein the index modulator is implemented directly on the LiNbO.sub.3.

10. A method for manufacturing the Waveguide Spectrometer according to claim 1, the method comprising: inscribing the surface waveguide in the substrate layer with a laser beam in direction of the length of the substrate layer, placing the at least one evanescent field sampler into the waveguide, configured to out-couple light out from the waveguide, positioning the means to achieve counter propagating optical signals inside the waveguide configured to obtain interference between the counter propagating optical signals generating an interference pattern along the waveguide, positioning the at least one light sensing unit configured to detect the out-coupled light connecting each electrically to the electronic read out system.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) A preferred exemplary embodiment of the subject matter of the invention is described below in conjunction with the attached drawings.

(2) FIG. 1a shows a perspective view of a first preferred embodiment of a waveguide for a waveguide spectrometer according to the present invention with photonic crystal light cones as evanescent field samplers and electrodes on the side of the waveguide resp. guiding core (without photo-detectors and conductors);

(3) FIG. 1b shows a perspective view of a second preferred embodiment of a waveguide for a waveguide spectrometer according to the present invention with etched grooves as evanescent field samplers;

(4) FIG. 1c shows a perspective view of a third preferred embodiment of a waveguide for a waveguide spectrometer according to the present invention wherein electrodes are placed on opposing side faces of the substrate layer;

(5) FIG. 2a shows a perspective view of a fourth preferred embodiment of a waveguide for a waveguide spectrometer in a counter-propagative Gabor configuration with metallic nano samplers as evanescent field samplers;

(6) FIG. 2b shows a perspective view of a fifth preferred embodiment of a waveguide for a waveguide spectrometer in a counter-propagative Gabor configuration with engraved diffusers as evanescent field samplers wherein electrodes are placed on opposing side faces of the substrate layer in the area where the interferogram is located;

(7) FIG. 3a shows a perspective view of a sixth preferred embodiment of a waveguide for a waveguide spectrometer in a counter-propagative Gabor configuration with metallic nano samplers as evanescent field samplers;

(8) FIG. 3b shows a perspective view of a seventh preferred embodiment of a waveguide spectrometer in a counter-propagative Gabor configuration with engraved diffusers as evanescent field samplers written in the substrate layer;

(9) FIG. 4 shows an interferogram corresponding to a polychromatic light generated by a superposition of a forward propagating and a backward propagating wave with and without applying voltage to the electrodes.

DESCRIPTION

(10) FIG. 1a shows a perspective image of a first preferred embodiment of a waveguide 11 of the waveguide spectrometer according to the present invention with photonic crystal light cones 20 as evanescent field samplers. Photonic crystal light cones are conical deformations in the waveguide cladding resp. substrate layer 10 which enables extraction of evanescent field. Photonic crystal light cones can be achieved using e.g. focused ion-beam lithography. The evanescent field samplers are configured to out-couple light from the waveguide 11.

(11) In this first embodiment, the waveguide 11 is extending from an inlet face 12 through a substrate layer 10, along the direction of the length l of the substrate layer 10, to a reflecting element 13 acting as a back-reflecting mirror in order to achieve counter propagation inside the waveguide 11 (so-called Lippmann configuration). In an area of the inscribed surface waveguide 11 written e.g. by a femto-second pulse laser into the substrate layer 10, the refractive index is changed and differs from the not laser radiated substrate material. Inscribing the waveguide 11 directly within the substrate layer 10 advantageously improves the manufacturing process of the substrate layer 10 with surface waveguides 11 and stacks thereof in a cost effective way. As a further advantage, such manufacturing process by inscribing the waveguide 11 allows direct access to evanescent fields on the smooth surface of the substrate layer 10 required for the light out-coupling means. The substrate layer 10 further shows a substrate width w1 and a substrate height t1.

(12) For reasons of simplicity and better illustrations, the plurality of photo detectors functioning as light sensing units and typically arranged on a front side I of the substrate layer 10 and corresponding conductors for electrical connection of said plurality photo detectors are not shown in FIG. 1a and the following Figures of the present application. In a preferred embodiment, the plurality of photo detectors is directly coupled on the evanescent fields samplers of the waveguide. The photo detector may be in the form of thin film nano-sensors or a standard array.

(13) Alternatively, an image transfer system is placed between the photo detectors and the evanescent field samplers. Moreover, a plurality of such waveguides 11 in a substrate layer 10 may build a spectrometer stack.

(14) Preferably, the substrate layer 10 is realized by, but not limited to, a LiNbO.sub.3 crystal. Alternatively, the substrate layer 10 may be realized by other electro-optical materials (having a significantly large Pockels coefficient) allowing manufacturing low loss waveguide, e.g. gallium arsenide (GaAs), lithium niobate (LiNbO.sub.3), gallium phosphide (GaP), lithium tantalite (LiTaO.sub.3) or quartz. Among them, LiNbO.sub.3 is attractive because of its large electro-optic coefficients, large transparency range (0.4 to 4 μm) and wide intrinsic bandwidth.

(15) The evanescent field samplers (e.g. in the form of photonic crystal light cones as shown in FIG. 1a or etched grooves as shown in FIG. 1b) are preferably distanced by a pitch that needs to satisfy Nyquist criterion for spectral sampling in combination with the properties of the material and the detector pixels' size (for case of on-chip deposited detectors) and the image transfer system's image resolvability restricted by diffraction limiting factors (for the case of far-field detectors combined with optics). Typical numbers are comprised in the range, but not limited to, p=1 μm to 20 μm.

(16) Electrodes 30; 30′ (i.e. anode and cathode), are placed aside directly neighboured to the guiding core respectively waveguide 11, while the electrodes 30; 30′ are functioning, while an electrical voltage is applied as refractive index modulator. A gap de defines the distance between the electrodes 30; 30′. Furthermore, a distance d is defined between the waveguide 11 and each of the electrodes.

(17) In this first preferred embodiment, the electrodes 30; 30′ are placed on the top surface towards the front side I of the substrate layer 10. The refractive index of the guiding core respectively the waveguide 11 is known and has a fixed value (n1). As soon as a voltage is applied between the electrodes 30, 30′, the refractive index in the waveguide 11 is shifted (n2=n1+Δn), resulting in a shift of the interferogram as illustrated in FIG. 4. This process is reversible, i.e. after voltage is switched off, the reflective index returns to the fixed value n1. The fact that the change of the reflective index can be changed by applying voltage has the advantage that this reversible process can be per-formed fast.

(18) In addition, the integrated modulator is can be configured to compensate for perturbations of the refractive property of the waveguide that may arise from stress, thermal effects or other perturbations.

(19) ls is the length of the sampled interferograms by the evanescent field samplers, while le is the length of the electrodes. The length of the sampled interferograms ls also defines the achievable spectral resolution (e.g. in the NIR at 766 nm, a ls=5 mm gives a spectral resolution of 0.025 nm). The difference between ls an le is a trade-off between minimizing waveguide transmission losses and minimizing possible uncontrolled straying of the light originating from non-perfect light in-coupling into the waveguide 11. Preferebly, difference le−ls can range, without being limited to, from 0 to 10 times ls. The length le of the electrodes essentially corresponds here to the length of waveguide 11.

(20) The first sampler i.e. closest to the reflective element 13 is distanced by distance m to the reflective element 13. Preferably, the first sampler is positioned at a distance m<p from the reflective element, with m as small as technically feasible ideally approaching zero. For example, with focussed ion beam mirror machining, distances m in the range of 50 nm can be reached.

(21) From this point on, the same reference numbers will in the following denote the same components on the figures.

(22) FIG. 1b shows a perspective image of a second preferred embodiment of a waveguide 11 of the waveguide spectrometer according to the present invention which differs from the first preferred embodiment according to FIG. 1a by etched grooves 21 as evanescent field samplers. Etched grooves 21 are deformations in the waveguide cladding resp. substrate layer 10 which enable extraction of evanescent field. Etched grooves 21 can be achieved using e.g. focused ion-beam lithography.

(23) FIG. 1c shows a perspective image of a third preferred embodiment of a waveguide 11 of the waveguide spectrometer according to the present invention. In this third preferred embodiment, the electrodes 30; 30′ are placed on opposing side faces resp. side walls of the substrate layer 10 and proceeding perpendicular to the front side I of the substrate layer 10 along a length le. Furthermore, in this third preferred embodiment, the width w1 of the substrate layer 10 equals the gap de between the electrodes, i.e. de=w1.

(24) FIG. 2a shows a perspective image of a fourth preferred embodiment of a waveguide 11 of the waveguide spectrometer 1 in a Gabor configuration with bridge-shaped metallic nano sampler 22 as evanescent field samplers. These metallic structures as evanescent field samplers are e.g. plasmonic antennas in the shape of nano-bars whose scattering efficiencies are optimized for the operation bandwidth. The fabrication is through e-beam lithography, sputter coating, two-photon lithography, etc.

(25) In the counter-propagative Gabor configuration as shown in FIG. 2a, means to achieve counter propagating optical signals are comprised by a beam splitting element configured to split the received optical signal into two separate signals guided through two inlet faces 12; 12′ into the waveguide 11 in order to achieve counter propagation inside the waveguide 11.

(26) An electrode 30 is placed directly neighboured to the waveguide 11 while another electrode 30′ (not visible in FIG. 2a) is placed on opposite sides. In this fourth embodiment of the waveguide 11, the electrodes 30; 30′ (i.e. the anode and cathode) are placed at an arm 41 of the two arms 41; 41′. According to the fourth preferred embodiment as shown in FIG. 2a, the electrodes 30; 30′ are placed on one arm 41 of the guiding core and not directly on the interferogram. The length of left side arm 41′ and right side arm 41 can be equal or unequal. In the latter case, the optical zero path difference (ZPD) is shifted from the centre and this enables physical displacement of the interferogram due to change of refractive index in the waveguide material. The refractive index across the path of light through the arm 41 can be modulated by applying an electrical voltage to the electrodes 30; 30′ along the length le. In this configuration, the length of the sampled interferogram ls also defines the achievable spectral resolution. Principally, the length of the electrodes 30; 30′ le is a trade-off between optimizing the modulation to be achieved through the electro-optical effect, minimizing waveguide transmission losses and minimizing possible uncontrolled straying of the light originating from non-perfect light in-coupling into the waveguide.

(27) FIG. 2b shows a perspective image of a fifth preferred embodiment of a waveguide 11 of the waveguide spectrometer in a Gabor configuration with engraved diffusers in the form of photonic crystal light cones 20 as evanescent field samplers.

(28) According to this fifth preferred embodiment, the electrodes 30; 30′, i.e. the anode and cathode (30′ not visible in FIG. 2b), are placed on opposite sides directly on the waveguide arm 40.

(29) FIG. 3a shows a perspective image of a sixth preferred embodiment of a waveguide 11 of the waveguide spectrometer according to the present application in a Gabor configuration with a plurality of metallic nano samplers as evanescent field samplers.

(30) The waveguide 11 may be written in the substrate layer 10 e.g. by a femto-second pulse laser.

(31) FIG. 3b shows a perspective image of a seventh preferred embodiment of a waveguide 11 of the waveguide spectrometer according to the present application in a Gabor configuration with engraved diffusers in the form of photonic crystal light cones 20 as evanescent field samplers written in the substrate layer 10.

(32) According to this seventh preferred embodiment, the electrodes 30; 30′ (i.e. anode and cathode), can be placed on the surface of the front side I of the substrate layer 10.

(33) FIG. 4 shows an interferogram corresponding to a polychromatic light generated by a superposition of a forward propagating and a backward propagating wave with (II) and without (I) applying voltage to the electrodes leading to a change of the refractive index exemplified in an LiNbO.sub.3 crystal using a waveguide according to one the preferred first to third embodiments as shown in FIG. 1a to 1c.

(34) In (II) where voltage is applied to the electrodes, a change of the refractive index Δn is achieved varying the effective optical path difference Δopd of the interferogram. As indicated in FIG. 4, the reflective surface realised by the reflective element 13 is located on the left at an optical path difference (opd)=0 cm. The wave propagates through the inlet face 12 from the right hand side to the reflective element 13, is reflected and forms a standing wave in the waveguide 11.

(35) In Lippmann configuration where a forwarding wave is back-reflected on one side of the waveguide and the reflective surface is located (at zero OPD), the interferogram is squeezed (in general terms modulated/deformed, i.e. squeezed, displaced or expanded) in front of the fixed samplers (i.e. photonic crystal light cones 20, etched grooves 21, metallic nano samplers 22) allowing interferogram scanning.

(36) In summary, the innovative solution is to scan the interferogram below a fixed samplers' configuration by taking advantage of electro-optical effect/thermal effect of specific waveguide material within the waveguide 11 itself. This is done by varying the index of refraction of waveguide module (e.g. Δn=0.4 according to FIG. 4) which is hosting the interferogram by application of an electric field. The electric field is created by applying voltage to the electrodes embedded in the waveguide spectrometer device. The change in the refractive index of the waveguide 11 changes the effective optical path length which results in expansion/squeeze (i.e. modulation) of the interferogram and its displacement along the optical path by Δopd (see FIG. 4). In principle, this phenomenon can be interpreted as the parade of an interferogram generated by a forward propagating and a backward propagating wave in front of the interferogram samplers which are in an immovable configuration.

LIST OF REFERENCE NUMERALS

(37) 1 Waveguide spectrometer 10 Substrate layer 11 Surface waveguides 12, 12′ Inlet/in-coupling waveguide face 13 Reflecting element with reflecting surface 20 Photonic crystal light cone 21 Etched groove 22 Metallic nano sampler 30; 30′ Electrodes 40 Main waveguide arm (of the interferometer) 41; 41′ Arm (of the interferometer) d Distance between each of the electrodes and the waveguide de Gap between electrodes I Front side (of the substrate layer) ls Length of sampled interferogram (by evanescent field samplers) le Length (electrodes) l Substrate length m Distance between reflective surface and first sampler p Pitch (between evanescent field samplers) t1 Substrate height w1 Substrate width