Patent Grant
7667882
The present disclosure is generally related to optical spectrum measurements and, more particularly, is related to spectrometers
Compact and efficient spectrometers are essential for biological and environmental sensing applications in which optical signals of interest are usually very weak and portability is highly desired. Due to their wavelength selectivity, gratings have been used in conventional spectrometers. For example, multiple wavelength channels of a spatially coherent input source can be separated by rotating a grating. For a spatially incoherent source, adding a narrow slit and lens (i.e., to make a collimated beam) in front of the grating is required to avoid the ambiguity between the incident wavelength and the incident angle. However, since most of the input power is blocked by the slit and some space is needed to add the lens, conventional spectrometers are inefficient and bulky.
Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.
Embodiments of the present disclosure provide systems and methods for analyzing an optical spectrum. Briefly described, one embodiment of a system includes a volume hologram for dispersing a general diffuse beam of light provided as input; a detector for receiving and detecting light dispersed by the volume hologram; and a Fourier transforming lens for forming the Fourier transform of the light dispersed from the volume hologram onto the detector.
Embodiments of the present disclosure can also be viewed as providing methods for analyzing an optical spectrum. In this regard, one embodiment of such a method, among others, can be broadly summarized by the following steps: recording a volume hologram such that an illuminating beam of light is separated into a plurality of components; and projecting a Fourier transformation of the separated light onto a detection plane, where positioning of the projection is dependent upon wavelength characteristics of the components.
Other systems, methods, features, and advantages of the present disclosure will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description and be within the scope of the present disclosure.
Many aspects of the present disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.
As shown in
By employing a volume hologram 110, the spectrometer 100 of the present disclosure is sensitive to both wavelength and direction of a beam from the light source 140. For example, the characteristics of the volume hologram 110 places components of the light beam having the same wavelength and different directions (e.g., incident angles) into the same area of the detector 130. In particular, in an exemplary embodiment, the Fourier spectrum of the diffracted beam, which is in a shape of a crescent, can be obtained and the position of this crescent only depends on the incident wavelength. The spectrum of the input signal (e.g., light beam) can be retrieved by measuring the position and power of the crescent at the detector. Thus, the ambiguity between incident angle and incident wavelength in a Fourier transform SBVH spectrometer 100 is eliminated by taking the Fourier-transform of the diffracted beam.
With those features, this spectrometer 100 can be used to detect a weak spatially incoherent signal. Because only a Fourier transforming lens 120, a hologram 110, and a detector 130 are required in the system 100, the system's dimension can be reduced to several centimeters squared, and stability can be dramatically improved. In one sense, the collimator (entrance slit and a lens or concave mirror) and wavelength dispersive medium of conventional design is integrated into one single spherical beam volume hologram.
To explain the properties behind the advantageous use of a SBVH 110 with a Fourier transforming lens 120 as a spectrometer, the following discussion is presented. The general shape of a diffracted crescent at the output face of the volume holograph 110 (which, in one embodiment, is recorded at wavelength λ=532 nm in a sample of Aprilis photopolymer with typical thickness of 100 to 400 μm) when it is read by a plane wave with wavelength λ1 and incident angle θ1 (between the direction of plane wave and normal to the hologram in air) can be represented as
gλ
where f(x) represents the shape of the crescent, kx(λ1) is the component of the propagation vector k(λ1) in the in-plane x direction (the “plane” is formed by the propagation vector of the plane wave and the point source), Δ(λ1,θ1) is the location of the center of the crescent (and it depends on both λ1 and θ1), and φ1 is a constant phase.
When the volume holograph 110 is read by a diffuse beam at wavelength λ1, crescents corresponding to different spatial modes (or different incident plane wave angles θ) exist at different locations Δ. As a result, the diffracted beam has a diffuse pattern resulted from the combination of all these crescents. The diffracted beam in this case (diffuse monochromatic incident beam) can be approximately represented by
where N is the number of different incident angles θm (or the number of spatial modes) of the incident beam. Note that the direction of propagation of all crescents (i.e., kx(λ1)) only depends on the incident wavelength and not on the incident angle θ]. A more accurate form of Equation (2) can be obtained by replacing
with ∫dθ and Δ(λ1,θm) with Δ(λ,θ). Taking the Fourier transform of both sides of Equation (2) and calculating the intensity of both sides results in
where ωx represents the spatial frequency variable.
In Equation (3), the ambiguity term Δ(λ,θm) has been transferred into the phase term and the Fourier spectrum of a crescent (i.e., F(ωx)) is shifted by an amount of kx(λ) which only depends on the incident wavelength and the plane wave used for recording the hologram. The value of the sum of the complex phase terms in Equation (3) depends strongly on the degree of spatial coherence of the incident light source. For spatially coherent light illumination, this term is a complicated function of ωx because all phase constant terms φm are correlated. Thus, the Fourier spectrum |Gλ(ωx)|2 is in general a crescent multiplied by a speckle pattern. On the other hand, all phase constant terms φm are uncorrelated under spatially incoherent (or diffuse) light illumination simplifying Equation (3) to
|Gλ(ωx)|2=N|F(ωx−kx(λ))|2 (4)
By using ωx=x/λf for the Fourier transforming lens 120 with focal length f, the output intensity at the Fourier plane (where the detector 140 is placed) can be written as
Equations (4) and (5) show that all crescents diffracted by different spatial modes of a monochromatic diffuse input to this spectrometer appear at the same location in the output plane, and these crescents are incoherently added together. Furthermore, the location of the final crescent at the Fourier plane depends only on the incident wavelength λ and can be used for spectroscopy.
The Fourier transform volume holographic spectrometer 100 in accordance with the present disclosure has several advantages over conventional spectrometers. First, it does not require any slit and input lens allowing for simpler, lighter, and more compact design. Secondly, it is not sensitive to the input alignment as it works optimally under spatially incoherent light illumination which is useful for diffuse source spectroscopy, especially in biological and environmental sensing applications. For example, one use, among others, for the spectrometer of the present disclosure is in Raman spectroscopy for blood alcohol and glucose content measurement.
Furthermore, by using holograms with more complicated grating structures, the sensitivity of the spectrometer 100 can be further improved. Also, no moving part (i.e., rotation of the hologram 110) is required for limited operation bandwidth (a few hundred nanometers).
Note that the operation spectrum (as well as resolution) can be modified by changing the design parameters such as material thickness and the divergence angle of the recording spherical wave. Further increase in the range of wavelength can be achieved by simply rotating the volume hologram 110.
Further, to make a sophisticated diffraction mechanism, utilizing the volume hologram 110, to get a sophisticated spatial spectral pattern, no complexity is added to the spectrometer design of the present disclosure. All that is needed is more sophisticated equipment at the time of recording the hologram, although the hologram materials are the same in complexity. Therefore, any level of sophistication may be added to the spectrometer 100 of the present disclosure without adding any complexity to the system.
For example, as opposed to a sophisticated grating as used in some conventional systems, the volume hologram 110 is constructed of polymer media. Accordingly, while a conventional grating is often very expensive, a volume holograph 110 may be manufactured at a considerably lower cost. Further, while a conventional grating is also often very thin and very detailed (e.g., 3 to 4 microns in thickness), a volume hologram 110 is significantly more flexible in design with a size of 400 to 500 microns being generally acceptable.
Advantageously, the spectrometer 100 of the present disclosure utilizes the Bragg selectivity of the hologram 110 along with Fourier transformation to separate all acceptable wavelengths at the output.
Advantageously, the performance of the Fourier transform holographic spectrometer 100 can be further improved by using optimal volume holograms that have an optimal holographic pattern (instead of a simple spherical beam volume hologram).
Referring now to
Referring now to
Any process descriptions or blocks in flow charts should be understood as representing modules, segments, or portions of code which include one or more steps in the process, and alternate implementations are included within the scope of present disclosure in which functions may be executed out of order from that shown or discussed, including substantially concurrently or in reverse order, depending on the functionality involved, as would be understood by those reasonably skilled in the art of the present disclosure.
It should be emphasized that the above-described embodiments of the present disclosure are merely possible examples of implementations, merely set forth for a clear understanding of the principles of the disclosure. Many variations and modifications may be made to the above-described embodiment(s) without departing substantially from the spirit and principles of the disclosure. All such modifications and variations are intended to be included herein within the scope of this disclosure.
This application claims priority to copending U.S. provisional application entitled, “Fourier Transform Volume Holographic Spectrometer,” having Ser. No. 60/618,436, filed Oct. 12, 2004, which is entirely incorporated herein by reference.
This invention was made with Government support under Grant No. N01AA23013, awarded by the National Institutes of Health. The Government has certain rights in this invention.
| Number | Name | Date | Kind |
|---|---|---|---|
| 5638173 | Smith et al. | Jun 1997 | A |
| 5978112 | Psaltis et al. | Nov 1999 | A |
| 6486948 | Zeng | Nov 2002 | B1 |
| 7158228 | Psaltis et al. | Jan 2007 | B2 |
| Number | Date | Country | |
|---|---|---|---|
| 20060103904 A1 | May 2006 | US |
| Number | Date | Country | |
|---|---|---|---|
| 60618436 | Oct 2004 | US |
