Patent Application
20150168213
Detailed day-by-day environmental sensing requires a large number of expensive, reliable, sensitive, high-resolution compact spectrometers. SOTA spectrometers based on regular optics (diamond turned gratings) are bulky and expensive. Even advanced miniature Ocean Optics spectrometers cost more than $3000 at a resolution ˜1 nm in NIR.
In conventional spectrometers spatial filters (narrow slits) are used to limit the angular range of the incident beam to get better resolution. This reduces the light throughput.
Slitless high light throughput spectrometer geometry capable of measuring the high-resolution spectra of the spatially coherent or incoherent light in any part of visible or NIR spectra based on reflection substrate-guided wave holograms (SGWH) is disclosed herein; the method of recording the high resolution spectra is also disclosed herein.
By this invention a spectrometer is disclosed for preparing spectra of coherent and non-coherent light that includes
whereby the first reflection SGWH lens diffracts an incoming light beam, the diffracted lights propagate along the substrate via internal reflection, reaching the second reflector SGWH lens, which diffracts the light to the 2D imaging sensor, thereby forming spectral lines. In this invention the numerical aperture of the first reflection SGWH lens (diameter and focal distance) determines the wavelength bandwidth of the spectrometer and wavelength separation, and the focal distance of the 2nd reflection SGWH lens determines the resolution of the spectral lines.
The subject invention also includes a method for preparing spectra of coherent and non-coherent light in a spectrophotometer comprising the steps of shining a light beam on a first reflection SGWH lens, diffracting the light beam and propagating it along a transparent substrate via internal reflection to a second reflection SGWH lens, diffracting the light beam with the second reflection SGWH lens to a 2D imaging sensor, and forming spectral lines. A further embodiment includes adjusting the diameter of the first reflection SGWH lens to adjust the wavelength bandwidth of the spectrophotometer, adjusting the focal distance of the first reflection SGWH lens to adjust the wavelength separation, and adjusting the 2nd reflection SGWH lens to adjust the resolution of the spectral lines.
b) shows the same lines of
a) is an Example of the spectrum image of the captured A-band spectrum using disclosed a SGWH-based spectrometer.
b) is an Example of the Spectrum plot of
Input light coming from a distance enters the spectrometer through the opening that, together with the holographic optics, determines the bandwidth of the spectrometer. This opening is rather large (in the range of millimeters). It has nothing to do with the spectrometer resolution, and determines the spectrometer bandwidth.
First reflection cylinder SGWH H1 lens accepts the input light 25 and diffracts/couples the required part of the light spectrum (diffracted light 30) in the transparent substrate. This light propagates along the substrate via total internal reflection (TIR) effect, and reaches the second cylinder SGWH H2 lens. This lens diffracts/couples this light out and focuses to the 2D imaging sensor 15 as a set of sharp spectral lines. Software processes these images and converts them to the plots that can be seen on the computer display screen 20.
Depending on the application area and specifics of the required extracted information, the opening size that determines the wavelength bandwidth can vary. Also the length and thickness of the substrate can vary. Accordingly the diffracted beam angle and the number of total internal reflection (TIR) bounces of the light beam inside the substrate can vary.
Different diffracted beams correspond to different wavelengths and propagate along the substrate at different angles (disperse) according to Eq. 2-1 [L. Gu, et al., “Bandwidth-enhanced Volume Grating for Dense Wavelength-division Multiplexer Using a Phase-compensation Scheme”, App Phys Lett, vol. 86, p. 181103, 2005.], which defines the relationship between incident angle, diffracted angle, hologram slant and wavelength
(Λ/sin φ(sin α+sin β)=λ/n, (2-1)
where Λ is the hologram period, φ is the hologram angle (slant), λ is the light wavelength in a vacuum, n is the refractive index of the hologram material, and α and β are the respective incident and diffracted angles of the input and diffracted beams. Diffraction angle β is equal to the total internal reflection angle. Its change with the input signal wavelength can be found by differentiating Eq. (2-1). Accordingly, the linear dispersion can be expressed by the differentiation of Eq. (2-1) as well, L. Gu, et al., supra:
dL/dλ=2d(d(β/dλ)/(cos β)2 , (2-2)
where d is the thickness of the substrate, L is the distance between input and output beams as shown in
It is clear that the lateral dispersion capacity is dependent on the designed diffraction angle of the hologram β (that in turn depends on the hologram period) and its slant angle φ. The larger the diffraction angle (3 is, the greater the dispersion capacity. This analysis is correct for the case when SGWH H1 and H2 are gratings or lenses. For lenses, the variation of slant across the hologram should be taken into account. For better wavelength separation (means higher resolution), in regular spectrometers the slit 10 (
When the SGWH lens H1 is played back with a monochrome collimated beam, the dark line in the passing zero order is visible, as shown in
The straight dark line of
This dark line is formed by varying the Bragg angle due to the hologram Bragg selectivity. Only a narrow angle range of beams corresponds to the Bragg condition and is diffracted (coupled) within the substrate.
The position of this line and its angle range and wavelength bandwidth depends on the SGWH lens numerical aperture, recorded and incident angles, wavelength, and hologram thickness.
Therefore, the thick hologram angular selectivity property replaces the slit function. When this property of thick holographic lens is combined with high angular and wavelength selectivity of thick SGWH, high resolution, a compact spectrometer can be built. This folded geometry has the advantage of high OOB (out of band) rejection ratio due to the minimized stray light that hits the 2D array sensor 15.
To accommodate the appropriate resolution and light throughput of the spectrometer, the SGWH H1 and H2 lenses should have appropriate numerical apertures. The numerical aperture of the lens H1 (diameter and focal distance) determines the wavelength bandwidth of the spectrometer and wavelength separation. The focal distance of the H2 lens determines the resolution provided that the 2D array sensor 15 can handle this resolution. There are appropriate sensors developed by Hamamatsu Photonics K. K., Sony, Toshiba that well fit this purpose.
General information about recording and playback of different types of SGWH can be found in F. Dimov et al, Patent Publication US2010/0157400 “Holographic Substrate-Guided Wave-Based See-Through Display”, which is incorporated herein in its entirety by reference.
The spectrometer that should work in the specific wavelength band SGWH recording setup should account for the wavelength change, because the available laser wavelengths and hologram recording materials are limited. The Bragg condition (playback angles θP for playback wavelengths λP different from recording wavelengths λR) can be calculated as
λP/λR=sin(θP)/sin (θR). (3-1)
Some of the most frequently used, convenient, and available materials for thick SGWH and laser wavelengths are included in the Table 1 below.
If the playback angles are known, the recording angles can be calculated with Eq. (3-1). For example, if a spectrometer is built that works in the atmospheric Oxygen A-band with the central wavelength 765 nm accounting for the normal incidence of the beams from atmosphere, a DuPont photopolymer can be chosen as a recording material for the SGWH. The reflection SGWH is chosen due to the higher than transmission SGWH dispersion. The recording wavelength can be chosen: either 457, or 532, or 647 nm considering the material sensitivity and convenience of recording setup, as understood from
A proof-of-concept spectrometer has been constructed for the atmospheric Oxygen A-band based on the above-described SGWH technology.
Appropriate plots of these scanned images along one horizontal line created with MATLAB software are shown in
Based on the captured spectra images, the estimated resolution of this spectrometer with reflection SGWH lenses has ˜0.1 nm resolution. An example of the captured A-band spectrum using the disclosed SGWH-based spectrometer is shown in
From the foregoing it will be observed that numerous modifications and variations can be effectuated without departing from the true spirit and scope of the novel concepts of the present invention. It is to be understood that no limitation with respect to the specific embodiments illustrated is intended or should be inferred. The disclosure is intended to cover by the appended claims all such modifications as fall within the scope of the claims.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 61904696 | Nov 2013 | US |
| Child | 14542156 | US |
