Apparatus and method for evanescent waveguide sensing
An apparatus and an associated method is provided for waveguide spectroscopy of a sample inside a container. The apparatus may include a substrate with a window that is transparent at the wavelengths of interest and is coupled to the container holding a sample; a waveguide core of a material that is transparent at the wavelengths of interest located on an inner surface of the transparent window adjacent the sample, the waveguide core having a refractive index greater than that of the sample, an optical element configured to couple light into and out of the waveguide, and a light source and one or more detectors located outside of the container.
1. An apparatus for waveguide spectroscopy of a sample inside a container, the apparatus comprising:
a substrate with a window that is transparent at the wavelengths of interest and is coupled to the container holding a sample;
a waveguide core of a material that is transparent at the wavelengths of interest located on an inner surface of the transparent window adjacent the sample, the waveguide core having a refractive index greater than that of the sample,
an optical element configured to couple light into and out of the waveguide,
a light source located outside of the container,
one or more detectors located outside of the container; and
at least one polarizer positioned between the light source and the waveguide core, positioned between the waveguide core and the detector, or in both positions, the at least one polarizer configured to adjust a transmitted light intensity.
2. The apparatus of claim 1 , wherein the waveguide comprises a cladding and the waveguide core has a refractive index greater than that of the cladding deposited between the core and the substrate.
3. The apparatus of claim 1 , wherein the waveguide comprises a cladding having a refractive index less than that of the substrate, and wherein the cladding has a thickness that is three or more wavelengths.
4. The apparatus of claim 1 , wherein the waveguide core has a refractive index greater than that of the substrate.
5. The apparatus of claim 1 , wherein the optical element comprises one of a prism and a grating.
6. The apparatus of claim 5 , wherein the prism is situated on the window outside of the container.
7. The apparatus of claim 5 , wherein the grating is located adjacent or within the waveguide on the inside of the container.
8. The apparatus of claim 1 , wherein the waveguide core has a thickness configured to transmit a single TE polarized waveguide mode.
9. The apparatus of claim 1 , wherein the waveguide core has a thickness configured to transmit a single TE polarized waveguide mode and a single TM polarized mode.
10. The apparatus of claim 1 , wherein the core is made of a material selected from the group consisting of Si3N4, Al2O3, Ta2O5, SixOyN1-x-y, Si, Ge, diamond, ZnS, and ZnSe.
11. The apparatus of claim 1 , wherein the waveguide has a cladding made of a material selected from the group consisting of SiO2, MgF2, CaF2, and SixOyN1-x-y.
12. The apparatus of claim 1 , wherein the substrate is made of a material selected from the group consisting of Si, glass, fused silica, Al2O3, ZnS, ZnSe, diamond, KBr, BaF2, and CaF2.
13. The apparatus of claim 1 , further comprises a device located outside of the container and configured to make spectroscopic measurements.
14. The apparatus of claim 13 , wherein the device comprises at least one of a spectrometer and a filter.
15. The apparatus of claim 1 , wherein the polarizer rotates at a specific frequency causing the polarization of the transmitted light to rotate.
16. The apparatus of claim 1 , wherein the polarizer comprises a multi-segment polarizer that rotates at a specific frequency in which neighboring polarizer segments are oriented orthogonal to each other, and half of the segments are oriented to pass light appropriate for TE waveguide modes and half are oriented to pass light for TM waveguide modes.
17. The apparatus of claim 16 , wherein the polarizer or polarizer segments are made from one or both of a sheet polarizer and a wire grid polarizer.
18. The apparatus of claim 1 , wherein the polarizer does not rotate and is fixed at an orientation that passes light at 45° to that which excites either TE or TM waveguide modes.
19. The apparatus of claim 1 , wherein the light source is unpolarized or circularly polarized.
20. The apparatus of claim 1 , wherein the light source is linearly polarized such that an orientation of the light source is at 45° to the orientation for exciting either TE or TM waveguide modes.
21. The apparatus of claim 1 , wherein the light intensity for transmitted TE and TM modes are separately measured and subtracted or divided to obtain a final spectrum.
22. The apparatus of claim 1 , wherein an intensity of the light for combined transmitted TE and TM modes is measured by a single detector with a lock-in amplifier or a phase-sensitive synchronous detection electronics.
23. The apparatus of claim 1 , wherein the polarizer is a polarizing beamsplitter cube, a Glan-Thomson prism, a Wollaston prism, a Glan-Taylor prism, or any combination thereof.