The invention pertains to spectroscopy and particularly to spectroscopy of scattered light.
The invention is a stand-off coded aperture spectrometer with mask gating.
To lead into a background of the present system, it may be noted that when light is scattered from an atom or molecule, most photons are elastically scattered (i.e., Rayleigh scattering). The scattered photons may have the same frequency as the incident photons. However, a small fraction of light (e.g., about 1 in 107 photons) may be scattered at frequencies different from the frequency of the incident photons. This may be a result of inelastic scattering. Such scattered light may provide information about the molecules' vibrational quantum states. Although Raman scattering may occur with a charge in vibrational, rotational or electronic energy of a molecule; a primary concern is the vibrational Raman effect.
There may be several kinds of Raman scattering. If a molecule absorbs energy (i.e., the resulting photon has lower energy), then one has Stokes scattering. If the molecule loses energy (i.e., the resulting photon has higher energy), then one has anti-Stokes scattering. The Stokes spectrum may be more intense than the anti-Stokes spectrum since a Boltzmann distribution may indicate that more molecules occupy lower energy levels than the higher levels in most cases. An absolute value should not depend on Stokes or anti-Stokes scattering. In many instances of scattering, the vibrational energy levels may represent a unique signature or footprint such that the material or substance may be identified. The intensities of the Raman bonds may be dependent just on a number of molecules occupying different vibrational states, when the scattering process occurs.
Stand-off Raman spectroscopy for detection of remote chemical or biological agents may involve low level spectral signal measurements done in a potentially high ambient background environment (i.e., daytime). A requirement for such a spectrometer may include a large etendue (i.e., light-grasping ability) along with a pulsed-light source gated-receiver pair. This may permit an improved signal-to-background ratio by a collection of as much signal possible along with a reduction in background signal by implementing a pulsed-laser source with detector gating such that the signal collection only occurs when the actual signal is present. Gating of a high etendue spectrometer might be a requirement for making such a measurement.
Gating of a high entendue spectrometer such as one based on a coded aperture and a CCD array detector may present issues. Gating of CCD arrays may often be accomplished by using an image intensifier where the gating actually occurs. This may limit the selection of potential detector arrays along with a low quantum efficiency (i.e., <10%) for wavelengths of 900 nm or greater which would be required for Raman illumination lasers of about 800 nm. If a coded mask of the spectrometer is constructed as a spatial light modulator (SLM), this would allow one to easily gate the receiver while also performing an encoding function.
Some systems may be a single detector Hadamard spectrometer requiring a reconfigurable coding mask. This approach would require multiple configurations of the mask to make a single measurement, thus not permitting gating of the receiver as an option for a single spectra measurement. For such system to make an “N” number of resolution element spectra measurements, it may require an “N” number of different mask encodements. The coding mask is typically positioned before the dispersing element of the spectrometer where the radiation is spatially separated in wavelength.
A spatial light modulator which may be used as a coded mask/gating element is a Texas Instruments digital micromirror device (DMD). The DMD may have 1024×768 individually addressable mirror-pixels or micromirrors. The micromirrors may be used as shutters to either pass incident light on to the detector or redirect the light to a light dump. The micromirrors may accomplish this by toggling between two angular positions. The toggling or switching may have a speed of about 15 micro seconds. In this application, all of the micromirrors may be normally positioned to the off position. However, when a Raman illumination source is pulsed, then a subset of the micromirrors which define the required encoding may be toggled to the on position during the Raman illumination pulse.
Mask gate 17 may be regarded as a coded aperture. The mask may be a two-dimensional array of reflective and non-reflective spots, which may represent ones and zeros, respectively. Various patterns of ones and zeros may represent different codes and effective throughputs.
If the mask gate 17 is off, then all of the mirrors 18 in the array of the mask should be in the off position and therefore all of light 14 would be reflected to dump 21, with no light going to the grating 22 via the lens 19. On the other hand, if the mask gate 17 is on, then some of the mirrors 18 may be in the on position thereby reflecting some of the light 14 through lens 19 to grating 22. In effecting an encoded pattern in mask 17 as a coded aperture, some of the mirrors 18 may be in the off position thereby reflecting some of the light to the light dump 21. The convolved light 14 from the mask gate 17 when on may go to grating 22 via the lens. Grating 22 may disperse light 14 according to wavelength.
Signals to mask 17 from a processor/interface 26 may turn on the mask gate 17 according to a preferred pattern of “on” and “off” mirrors in the array. Also the timing of when mask 17 should be on or off may be provided by processor/interface 26 via connection 27. This timing via the connection may be influenced by a signal to or from light source 11 via a connection 29.
An example mechanism to be use as the mask gate 17 may be a digital micromirror device (DMD™) by Texas Instruments Inc. This device may be a 768×1024 micromirror array. The mask or matrix of the DMD may have a 2 mm side dimension. Compared to a 50 micron slit, much more energy may be conveyed by mask 17 even if one-half of the mirrors are in an off position. It may work with wavelengths of light between 600 um and 2.5 microns. A code that may be implemented in the array for the present system 10 can have a masking pattern of a cyclic Hadamard S-matrix. The speed of this device may be about 15 microseconds. Speeds of other similar devices may be much faster. The time that mask gate 17 is on may be about the same amount of time as the laser pulse duration from light source 11 and as that of the Raman scattered light 14.
Light 14 in an encoded pattern from mask gate 17 via collimating lens 19 may impinge the grating 22 which diffracts the light 14 spectrally, and eventually impinges a CCD detector array 25. Light 14 may reach array 25 via a filter 23 and a focusing lens 24. An optical path of system 10 may be indicated generally by optical axes 20. Filter 23 may pass light having wavelengths about the same or shorter than the wavelengths of the Raman scattered light. This filter may reduce the effects of fluorescence. Signals of the light 14 on the CCD array 25 may go to the processor/interface 26 via a connection 28 to be deconvolved.
The following reveals a basic Matlab™ script which may be used in modeling a coded aperture spectrometer like system 10.
In the present specification, some of the matter may be of a hypothetical or prophetic nature although stated in another manner or tense.
Although the invention has been described with respect to at least one illustrative example, many variations and modifications will become apparent to those skilled in the art upon reading the present specification. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.