An improved sensitivity fourier transform raman spectrometer for light sensitive samples

Abstract

A Fourier Transform (FT) spectrometer and method are provided comprising: an excitation light source configured to generate an excitation light signal; a detector; an interferometer optical system comprising a beam splitter, wherein the beam splitter is configured to receive the excitation light signal, direct a portion of the excitation light signal along a sample path toward a sample, and receive a spectroscopy signal generated at the sample; and a collimating optical interface disposed in the sample path. The collimating optical interface is configured to: receive the portion of the excitation light signal from the beam splitter and deliver the excitation light signal to the sample as a non-focused beam and receive the spectroscopy signal generated at the sample and collimate the spectroscopy signal prior to the spectroscopy signal reaching the beam splitter; or receive a non-collimated spectroscopy signal between the beam splitter and the detector and collimate the spectroscopy signal toward the detector.

Description

An Improved Sensitivity Fourier Transform Raman Spectrometer for Light Sensitive SamplesCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of United States provisional application no. 63 / 727,138, filed December 2, 2024, which is hereby incorporated by reference as though fully set forth herein.BACKGROUNDField

[0002] The instant disclosure relates to a Fourier Transform (FT) Spectrometer. In particular, the instant disclosure relates to an FT Spectrometer including an interferometer.Background

[0003] Fourier Transform (FT) Raman spectrometry is a method of acquiring Raman spectra using the principle of Michelson interferometry. The Michelson interferometer splits the Raman scattered light into two paths, with at least one moving mirror (e.g., one path with a fixed mirror and one path with a moving mirror or two paths each with a respective moving mirror) and detects the interference pattern when the two paths are recombined at a beam splitter. If the incoming light is not well collimated the off-axis components do not interfere and create a constant signal that leads to signal-related noise in the detector. For example, shot noise increases by the square root of the signal. Therefore, the optimal signal is obtained with the incoming light is collimated.BRIEF SUMMARY

[0004] In one embodiment, A Fourier Transform (FT) spectrometer is provided comprising: an excitation light source configured to generate an excitation light signal; a detector; an interferometer optical system comprising a beam splitter, wherein the beam splitter is configured to receive the excitation light signal, direct a portion of the excitation light signal along a sample path toward a sample, and receive a spectroscopy signal generated at the sample; and a collimating optical interface disposed in the sample path. The collimating optical interface is configured to: receive the portion of the excitation light signal from the beam splitter and deliver the excitation light signal tothe sample as a non-focused beam and receive the spectroscopy signal generated at the sample and collimate the spectroscopy signal prior to the spectroscopy signal reaching the beam splitter; or receive a non-collimated spectroscopy signal between the beam splitter and the detector and collimate the spectroscopy signal toward the detector

[0005] In another embodiment, a method of collecting a spectroscopy signal using a Fourier Transform (FT) spectrometer is provided. The method comprises generating an excitation light signal; directing the excitation light signal through a beam splitter of a spectrometer optical system toward a sample; receiving a spectroscopy signal generated at the sample based on the excitation light signal; and collimating the spectroscopy signal between the sample and a detector. The spectrometer optical system is configured to: receive at least a portion of the excitation light signal from the beam splitter and deliver the excitation light signal to the sample as a non-focused beam and receive the spectroscopy signal generated at the sample and collimate the spectroscopy signal prior to the spectroscopy signal reaching the beam splitter; or receive a non-collimated spectroscopy signal between the beam splitter and the detector and collimate the spectroscopy signal toward the detector.

[0006] The foregoing and other aspects, features, details, utilities, and advantages of the present invention will be apparent from reading the following description and claims, and from reviewing the accompanying drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Figure 1 illustrates the concept of a Michelson interferometer and an FT-Raman spectrum.

[0008] Figure 2 illustrates two example methods to input scattered light (a spectroscopy signal) into the interferometer.

[0009] Figure 3 is a block diagram showing an example of a Fourier Transform (FT) Raman spectrometer that includes a focusing lens configured to focus the excitation light signal onto a sample and collect a spectroscopy signal generated at the sample by the excitation light signal.

[0010] Figure 4 shows an example optical path of another example Fourier Transform (FT) spectrometer utilizing a collimating lens to focus an excitation light signal onto a sample andreceive and collimate a spectroscopy signal generated at the sample in response to the excitation light signal.

[0011] Figure 5 shows an example of an interferometer as a Michelson interferometer comprising a first, fixed mirror and a second, moving mirror.

[0012] Figure 6 illustrates a portion of an optical system of an embodiment of a Fourier transform (FT) spectrometer configured to input a collimated spectroscopy light signal into the interferometer utilizing a collimating lens disposed internally in the optical system of the spectrometer from the beam splitter.

[0013] Figure 7 shows the effects of the optical input method on the interferograms.

[0014] Figure 8 further shows the advantage of the alternative internal collimating element method to input collimated light into an interferogram.

[0015] Figure 9 illustrates two calculations of the MPE following ANZI Z136.1.

[0016] Figure 10 shows examples of two common Raman sampling geometries.

[0017] Figure 11 shows a 30 mm spacer and a conjugate lens pair collimating sampling element and experimental results achieved with the spacer and sampling element.

[0018] Figure 12 shows experimental results for the same sucrose sample measured without collimation at the sample.

[0019] Figure 13 shows four example sampling elements.

[0020] Figure 14 shows another example embodiment of a spectrometer that includes a fixed focusing lens configured to focus the excitation light signal onto a sample and collimate a returning spectroscopy signal generated at the sample in response to the excitation light signal when used independently.DETAILED DESCRIPTION

[0021] The following description of the invention is provided as an enabling teaching of the invention in its best, currently known embodiment. To this end, those skilled in the relevant art will recognize and appreciate that many changes can be made to the various aspects of the invention described herein, while still obtaining the beneficial results of the present invention. It will also beapparent that some of the desired benefits of the present invention can be obtained by selecting some of the features of the present invention without utilizing other features. Accordingly, those who work in the art will recognize that many modifications and adaptations to the present invention are possible and can even be desirable in certain circumstances and are a part of the present invention. Thus, the following description is provided as illustrative of the principles of the present invention and not in limitation thereof.

[0022] As used throughout, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a” component can include two or more such components unless the context indicates otherwise. Also, the words “proximal” and “distal” are used to describe items or portions of items that are situated closer to and away from, respectively, a user or operator such as a surgeon. Thus, for example, the tip or free end of a device may be referred to as the distal end, whereas the generally opposing end or handle may be referred to as the proximal end.

[0023] All directional references (e.g., upper, lower, upward, downward, left, right, leftward, rightward, top, bottom, above, below, vertical, horizontal, clockwise, and counterclockwise) are only used for identification purposes to aid the reader’s understanding of the present invention, and do not create limitations, particularly as to the position, orientation, or use of the invention. Joinder references (e g., attached, coupled, connected, and the like) are to be construed broadly and may include intermediate members between a connection of elements and relative movement between elements. As such, joinder references do not necessarily infer that two elements are directly connected and in fixed relation to each other.

[0024] Ranges can be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another aspect includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another aspect. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint.

[0025] As used herein, the terms “optional” or “optionally” mean that the subsequently described event or circumstance may or may not occur, and that the description includes instances where said event or circumstance occurs and instances where it does not.

[0026] The term “substantially” as used herein may be applied to modify any quantitative representation which could permissibly vary without resulting in a change in the basic function to which it is related.

[0027] Figure 1 illustrates the concept of a Michelson interferometer 10 and an FT-Raman spectrum. The interferometer can have many designs to maximize the efficiency, common to the different designs is a moving mirror (MM) 12, a fixed mirror (FM) 14, and a beam splitter (BS) 16. If the beam splitter BS 16 has appreciable thickness in one optical path and reflects from the front surface, as drawn the fixed mirror FM 14 optical path would have an optical path distance that depends on the wavelength of the light due to the wavelength dependence of the index of refraction of the beamsplitter material. This dependence can be removed with a with a compensator (C) 18 of the same material as the beam splitter BS16 in the moving mirror MM 12 optical path to cause the same optical path effect as the beam splitter BS 16 material in the fixed mirror FM 14 path. The interference pattern produced by the collimated optical rays and the offset signal from uncollimated light is focused onto the detector by a lens (FL) 20. The detector can be any sensitive detector, in the case of FT Raman spectrometry it is often an Avalanche Photodiode (APD) 22. The interferometer is defined by a theoretical, and can be a physical, aperture that is defined by the diameter of the fixed and moving mirrors (FM and MM) 12, 14. This aperture is called a Jacquinot stop (JS) 24 and can be defined by its diameter (D).

[0028] The signal produced by the Michelson interferometer is called an interferogram and is illustrated with an example in Figure 1. The abscissa of the plot is the distance that the moving mirror (MM) has moved, for Raman this would be in the units centimeters. The interferogram shown in Figure 1 include the area where the optical path difference between the mirrors is zero and all wavelengths interfere constructively. This is the point with the highest intensity in the interferogram. When the interferogram is processed through the Fourier transform algorithm the result is a spectrum of the frequencies contained in the algorithm and by the principle of the Fourier transform the abscissa now becomes the inverse of the interferogram or wavenumbers (cm1).

[0029] Figure 2 illustrates two methods to input scattered light (a spectroscopy signal) into the interferometer. The method titled Input Optics takes light from an excitation light source such as a laser source, commonly a diode laser (DL), illuminates a sample (S) and the Raman scattered light is passed back through the beam splitter and is introduced into the interferometer. The amount of scattered light that is detected is defined by the cone of light that passes through the Jacquinot stop (JS). The problem with the input optics described in Figure 2 is that light going through the Jacquinot stop JS is not collimated and most of it will not be modulated by the moving mirror MM into an interferogram. This will lead to a large DC signal which contributes noise to the interferogram and to the resulting spectrum.

[0030] The conventional method to introduce light into the Michelson interferometer is the Focusing Optics method shown in Figure 2. This method focuses the laser beam onto the sample at one focal length distance from the collection lens (CL), and it collimates the cone of light that is backscattered into the collimating lens CL. The collimated light will get modulated by the interferometer and will produce lower noise, higher signals in the interferogram and the spectrum.

[0031] Figure 3 is a block diagram showing an example of a Fourier Transform (FT) Raman spectrometer that includes a focusing lens configured to focus the excitation light signal onto a sample and collect a spectroscopy signal generated at the sample by the excitation light signal. Although this example shows the use of a focusing lens configured to focus the excitation light signal on the sample and collimate the returning spectroscopy signal into the optical system of the spectrometer, the spectrometer shown in Figure 3 is an example of a spectrometer may also be used with an internal collimating lens (e.g., at the Jacquinot stop) or a sample collection collimating device (e.g., a conjugate lens pair) as described herein.

[0032] In this example, the spectrometer comprises a spectroscopy laser configured to provide an excitation light signal to excite a sample and produce a spectroscopy signal (e.g., Raman scattering, fluorescence, and / or near infrared (NIR) absorption) from the sample. The spectroscopy laser (e.g., a Raman laser) is shown directing the excitation light signal towards the sample with a first dichroic beam splitter which sends a relatively large percentage of the beam to the sample.

[0033] The excitation light signal passed by the beam splitter is focused onto a sample using a lens (e.g., a collimating lens). A spectroscopy signal generated at the sample in response to the excitation light signal is collected via the lens, and where the lens comprises a collimating lens, thespectroscopy signal is collimated and directed back into the spectrometer towards the beam splitter. The first dichroic beam splitter also permits longer wavelengths, such as the spectroscopy signal (e.g., a Raman scattering signal, a fluorescence signal, and / or a NIR absorption signal), to pass through to an interferometer.

[0034] A second laser called the metrology laser produces a metrology light signal that is directed into the interferometer. The purpose of the metrology laser is two-fold. First, the metrology laser is used to schedule the sampling of the spectroscopy signal (e.g., Raman scattering signal) and to ensure that sampling is greater than two times the frequency of the Raman signal. This is the Nyquist condition. Second, the principle of FT spectroscopy is that a moving mirror within the interferometer will produce an interferogram that has its abscissa as distance between two mirrors. The accuracy of this distance dictates the accuracy of the spectrum produced by the FT algorithm. A quasi-monochromatic source like a laser will produce a sinusoidal pattern that has maxima at precisely the wavelength of the laser and therefore is a standard for the distance between the mirrors. The third source that an FT spectrometer might utilize is a broadband source, such as a “white light” or other broadband source, to produce a large signal when the mirrors are exactly the same distance apart. This is shown in Figure 3 as an LED source to produce the broadband signal.

[0035] In the example shown in Figure 3, the interferometer comprises a Michelson design with a beam splitter placed between a moving mirror and fixed mirror. The source signals (spectroscopy signal, metrology light signal, and / or broadband signal) are introduced to the beam splitter. The beam splitter sends one portion of the source signals to the fixed mirror and the other portion of the source signals to the moving mirror. This produces an interference pattern determined by the distance between the mirrors and the wavelength of the sources. The interference pattern (interferogram) is projected onto transducers (e.g., photodetectors) to produce an electrical signal to record the interferogram. In the example shown in Figure 1, for example, three detectors are shown for the three sources.

[0036] In this example, the spectrometer comprises three detectors that provide respective detected signals. An LED detector produces an LED detection signal comprising a sharp peak at the Zero Path Difference (ZPD). This can be used by an FT algorithm that integrates the interferogram from 0 to infinity and this defines the signal at 0. The theory of FT states that a signal from a broad source will produce a large interferometer signal at the ZPD.

[0037] A metrology detector produces a metrology detection signal that is used to determine an Optical Path Difference (OPD), which can be used to correct for inaccuracy in the mechanical drive of the moving mirror. The theory of FT states that a sharp (monochromatic) signal will produce a continuous sinusoidal interferogram.

[0038] A spectroscopy detector (e.g., a Raman detector) produces a spectroscopy detected signal (e.g., a Raman detected signal) that is a combination of multiple sharp bands and therefore produces an interferogram that is a combination of the multiple sinusoidal signals at different frequencies.

[0039] When the FT algorithm is applied to these signals (intensity vs path difference) they produce a spectrum (intensity vs frequency). The FT algorithm in various implementations may be performed by a controller or other processor device and may include a Fast Fourier Transform (FFT) or Discrete Fourier Transform (DFT).

[0040] Calculations and correction algorithms are performed via a controller. The controller may be an integrated component of the FT spectrometer or may be remotely connected to the FT spectrometer, such as wirelessly (e.g., Bluetooth, WiFi or other or wireless connection) or wired (e g., USB or Lightning connection). A wearable device, for example, may be adapted to be wirelessly connectable to an external processor or controller, such as to a smart phone, tablet, laptop computer, personal computer, or other computing device. In one example, the FT spectrometer is adapted to send the interferograms (metrology and spectroscopic) to an Android (or other portable computing) device through Bluetooth. In this example, a controller is provided on the interferometer to create the waveform to control the motor and there will be a second wirelessly connected "controller" to perform the Fourier transform and correction / calibration methods.

[0041] Figure 4 shows an example optical path of another example Fourier Transform (FT) spectrometer utilizing a collimating lens to focus an excitation light signal onto a sample and receive and collimate a spectroscopy signal generated at the sample in response to the excitation light signal. Again, although this example shows the use of a focusing lens configured to focus the excitation light signal on the sample and collimate the returning spectroscopy signal into the optical system of the spectrometer, the spectrometer shown in Figure 4 is an example of a spectrometer may also be used with an internal collimating lens (e.g., at the Jacquinot stop) or a sample collection collimating device (e.g., a conjugate lens pair) as described herein.

[0042] In this example, the FT spectrometer comprises a spectroscopy laser source but eliminates the broadband light source and the metrology light source shown in the FT spectrometer shown in Figure 4, such as described in PCT Application no. PCT / US24 / 11713 filed on January 16, 2024, which is incorporated by reference as if fully set forth herein. In the example FT spectrometer shown in Figure 4, the functionality of the broadband light source and the metrology light source shown in Figure 3 is replaced with the spectroscopy light source.

[0043] The FT spectrometer also comprises a mirror and a filter (e.g., a neutral density filter). The mirror and neutral density filter are adapted to isolate, attenuate, and forward a small amplitude light signal from the spectroscopy laser that leaks through the dichroic beam splitter into the interferometer. This signal is used as a metrology signal from the spectroscopy laser source. In this embodiment, for example, the FT spectrometer is adapted to function without the requirement of a secondary metrology laser. The Fourier transform (FT) spectrometer also produces a metrology signal that is at the spectroscopy laser source frequency, and this can be used to calibrate the spectroscopy signal.

[0044] The mirror may be used without a filter to reflect the leakage portion of the spectroscopy light signal toward the interferometer as a metrology light signal where the signal is not needed to be attenuated or where the mirror is adapted to attenuate the leakage portion of the spectroscopy light signal. In one embodiment, for example, the surface of the “mirror” could be modified to reduce the reflectivity. For example, the “mirror” may comprise a glass plate, a bead blasted plate, a poorly reflecting metal surface, or even a slight misalignment of the optic or the detector. Also, the neutral density filter could be placed at or near the detector. In yet another example, the beam shaping optic could defocus the light on the detector.

[0045] In the example shown in Figure 4, the Fourier transform (FT) spectrometer is adapted to use the leaked, attenuated portion of the excitation laser light signal as a metrology laser. The laser signal generated by the excitation laser source is directed to a first beam splitter. The first beam splitter reflects predominantly all of the spectroscopy laser signal toward the sample. The spectroscopy laser signal is focused onto the sample via one or more beam shaping optics, such as a lens. In the example shown in Figure 4, the lens of the beam shaping optics comprises a collimating lens that receives the cone shaped spectroscopy signal from the sample and collimates and directsthe collimated spectroscopy signal back into the spectrometer optical system toward the beam splitter (see signal between the collimating lens and the beam splitter).

[0046] A small fraction of the spectroscopy laser signal, however, will pass through the first beam splitter toward the mirror and neutral density filter as shown in a dotted line pattern. The mirror and neutral density filter are used to attenuate the portion of the spectroscopy light signal that leaks through the first dichroic beam splitter via the neutral density filter and to reflect the attenuated light signal back to the first beam splitter via the mirror. The first dichroic beam splitter, in turn, reflects the attenuated leakage portion of the spectroscopy light signal into the interferometer. In the interferometer, an interferometer beam splitter divides the attenuated light signal between a fixed mirror and a moving mirror of the interferometer. The split attenuated light signal is then passed to a third beam splitter.

[0047] A compensator is used in some implementations, such as where the beam splitter has a surface that is coated to create the splitting of the light. In such an example, one arm of the interferometer has a longer optical distance due to the refractive index of the beam splitter material. In other words, the path of one arm is longer by n (refractive index) x the thickness of the beam splitter. Since the refractive index is also wavelength dependent, the difference in the distance of both arms changes with wavelength. In this example, the compensator can comprise the same material as the beam splitter.

[0048] The metrology signal path of the attenuated light signal is shown in this embodiment with a dotted path in Figure 4. The last beam splitter in the system reflects this short laser wavelength and passes the longer wavelength spectroscopic signal. The laser metrology signal is then focused onto a metrology detector (e.g., a photodiode) via one or more beam shaping optics, such as a lens. In this particular embodiment, for example, the metrology detector comprises the photodiode pictured on the left of the interferometer. Similarly, the longer wavelength spectroscopic signal (shown by the grey solid path) is passed by the third and final beam splitter toward a long-pass filter and then is focused onto a spectroscopic detector via one or more beam shaping optics, such as a lens. In this embodiment, the spectroscopic detector comprises a second photodiode pictured on the top of the interferometer.

[0049] A controller is provided to perform one or more calculations or operations for the FT spectrometer. In one embodiment, for example, one or more calculations and correction areperformed via a controller. The controller is a component of the FT spectrometer in one embodiment. In another embodiment, a controller is remotely connected to the FT spectrometer, such as wirelessly (e.g., Bluetooth, WiFi or other or wireless connection) or wired (e.g., USB or Lightning connection). A wearable device, for example, may be adapted to be wirelessly connectable to an external processor or controller, such as to a smart phone, tablet, laptop computer, personal computer, or other computing device. In one embodiment, for example, the FT spectrometer is adapted to send the interferograms (metrology and spectroscopic) to an Android device through Bluetooth. In this embodiment, there is a controller on the interferometer to create the waveform to control the motor and there will be a second wirelessly "controller" to perform the Fourier transform and correction / calibration methods.

[0050] Figure 5 shows an example of an interferometer as a Michelson interferometer comprising a first, fixed mirror and a second, moving mirror. The interferometer receives an input light signal, such as described with reference to Figures 3 and 4 and generates an interferogram output signal that is directed to a detector.

[0051] In the top graph shown on the right side of Figure 5, a mechanical path difference can be shown in the irregular, non-sinusoidal pattern shown. Changes in movement of the second, moving mirror or changes in componentry, such as due to temperature fluctuations, can provide an irregular signal.

[0052] In the bottom graph shown on the right side of Figure 5, a uniform, sinusoidal pattern corresponding to a known light source (e g., metrology laser source, spectroscopy laser source, broadband light source) can be used to map a plurality of data points from the irregular, non- sinusoidal pattern shown in the top graph to correct for the variations in mirror movement and / or ambient conditions. In Figure 5, the top graph shows a mechanical path difference observed from an imperfect motor. The Optical Path Difference is the true location. In other words, the Optical Path Difference is the actual location of the motor determined by the sinewaves.

[0053] Figure 6 illustrates a portion of an optical system of an embodiment of a Fourier transform (FT) spectrometer configured to input a collimated spectroscopy light signal into the interferometer utilizing a collimating lens disposed internally in the optical system of the spectrometer from the beam splitter (e.g., at the Jacquinot stop). In this embodiment, the collimating lens (CL) is moved internal to the beam splitter in the optical system of thespectrometer. By placing the collimating lens on the input side of the beam splitter, the collimating lens is adapted to collimate the spectroscopy signal received / passed through the beam splitter but does not focus the excitation light signal onto the sample. Rather, the spectrometer may be adapted to provide an unfocused excitation light signal to the sample.

[0054] In the particular example shown in Figure 6, for example, the collimating lens is disposed at the Jacquinot stop (JS) of the spectrometer. The result will again be efficient interference in the interferometer and larger signal and less noise in the interferogram and the spectrum.

[0055] The size of the beam at the sample is determined by the laser beam size and could be adjusted with a beam expander. If the diode laser (DL) is coupled into the sampling optics by a fiber optic it could depend on the diameter of the fiber.

[0056] Figure 7 shows the effect of the optical input method on the interferograms. On the right-hand side, is the Input Optics uncollimated method shown in Figure 2 where we see the signal modulation at the optical path of zero difference is only 0.4 volts peak to peak (P-P). This is shown in the trace labeled SS DC Signal that is not AC coupled (SS DC Signal) to show the offset. The offset from the light that is not collimated is 3.4 volts on average. This large offset will produce noise in the interferogram and the spectrum. On the left-hand side the oscilloscope’s trace is shown for the embodiment shown in Figure 6 having a collimating lens disposed internally within the spectrometer optical system from the beam splitter at the Jacquinot stop and it shows a smaller offset of 2.1 V and a much larger modulation of 1.75 V P-P. This is over a 4-fold increase in signal and a 1.6 fold decrease in the offset signal.

[0057] Figure 8 further shows the advantage of the alternative internal collimating element method to input collimated light into an interferogram. In the calculations we assume a laser power of 100 mW. The significant value to laser damage is the Intensity (Power / Area). On the left-hand side, we calculate the Intensity of the laser radiation in a 5 mm diameter spot. This calculation shows that the intensity is 5.1 mW / mm2. This is a relatively low intensity and below the damage threshold of many samples. On the right-hand side, we calculate what would happen with the Focusing Optics input method shown in Figure 2. Depending on the lens and the emission spot size from the laser, the focused beam could be very small. We use a typical value of 50 microns. This produces a large intensity of 12,820 mW / mm2. This is over 2500 times large than the internalcollimating element input method with a 5 mm diameter beam. A laser intensity of 12.8 Watts / mm2is above the damage threshold for many materials. Clearly, for sensitive samples the internal collimating element input is superior.

[0058] An example for using an internal collimating element is tissue samples, in particular in- vivo tissue samples. In this case it is important to use a large beam to average over the heterogeneity of a tissue sample to procure a spectrum that is representative of the sample (specimen). Even more important is the photodamage to the sample. There is an exposure level at which lasers are not permitted to radiate human tissue. This level is termed the Maximum Permissible Exposure (MPE). The MPE is set by ANZI Z 136.1 and is based on 8-hour exposure levels.

[0059] Figure 9 illustrates two calculations of the MPE following ANZI Z 136.1. On the lefthand side is a calculation from a typical Focusing Optics (Figure 2) input method with a 50-micron focused laser spot size. This illustrates that if the continuous acquisition time for the spectrum has an MPE that would be 0.007 mW. This is an impractically low laser power for a Raman spectrum. However, on the right-hand side, we find an MPE for continuous exposure of 71 mW which is a reasonable laser power for Raman spectroscopy using an internal collimating element.

[0060] In conventional FT-Raman spectrometer designs, a large-area excitation beam is often desired to reduce photodamage, average over heterogeneous samples, or improve reproducibility. In one embodiment, an internal collimating element (Figure 6) achieves this by not focusing the Raman excitation beam on the sample (and in some embodiments employing a beam expander to spread the excitation beam) and collimating the returning spectroscopy signal generated at the sample with the excitation beam with a lens placed after / internal to the beamsplitter in the spectrometer optical system (e.g., in what is commonly designated as the Jacquinot Stop). While this configuration is optically valid, it imposes significant geometric constraints. Specifically, the sample must be positioned close to the beamsplitter, and the collected Raman signal depends strongly on the distance between the sample and the collimating lens.

[0061] This distance dependence follows a 1 / d2law, where dis the sample-to-lens distance. The distance dependence arises from the solid angle of Raman-scattered light collected and subsequently collimated.Limitations of Conventional Designs

[0062] To illustrate, consider two common Raman sampling geometries (Figure 10):• Right angle attachment - optimized for surface analysis• Linear attachment - optimized for cuvette or vial-based measurements

[0063] Because of the large difference in sample-to-optics distance, the 1 / d2dependence causes linear sampling configurations to produce much higher Raman signals than right-angle geometries. This creates inconsistencies across accessories and complicates practical system design.Collimation at Sample

[0064] In one embodiment, the Raman-scattered light is collimated directly at the sample, before it propagates through any attachments. This ensures that the collected Raman signal is largely independent of sample-to-interferometer distance.

[0065] One practical implementation is a conjugate lens pair used as a collimator. Figure 11 shows a 30 mm spacer and a conjugate lens pair collimating sampling element and experimental results achieved with the spacer and sampling element. In this example, experimental results were obtained with sucrose and demonstrate the effectiveness of this approach:• Direct collection using (no spacer): SNR = 87.3• With conjugate lens pair, sample moved 50 mm away via the spacer: SNR = 97.6

[0066] These values are essentially equivalent, showing that collimation at the sample eliminates the distance dependence predicted by the 1 / d2law.Validation against Non-collimated Collection

[0067] For comparison, the same sucrose sample was measured without collimation at the sample (Figure 12). As predicted, the signal -to-noise ratio decreased significantly with increasing distance, confirming the expected distance dependence.

[0068] While the demonstration employed a conjugate pair of 19 mm lenses, this collimation method is not unique. Equivalent performance could be achieved using one or more of the following collimating sampling elements:• Mirror-based collimator• Beam-Expanding Optics• Other optical relays designed to capture and collimate Raman scattering at the sample

[0069] In these implementations, the early collimation of scattered light decouples Raman performance from sampling geometry and enables consistent performance across a wide range of sampling attachments.

[0070] Figure 13 shows four example sampling elements. The first sampling element 40 comprises a linear focusing sampling element (see, e.g., Fig. 10) that comprises a focusing laser configured to focus the excitation light signal onto the sample and receive and collimate the spectroscopy signal generated at the sample in response to the excitation light signal.

[0071] The second sampling element 42 comprises a right-angle focusing sampling element such as shown in Fig. 10 that comprises a mirror and a focusing lens configured to focus the excitation light signal onto the sample and receive and collimate the spectroscopy signal generated at the sample in response to the excitation light signal.

[0072] The third sampling element 44 comprises a linear non-focusing sampling element that comprises a collimating sampling element (e.g., a conjugate lens pair, a mirror based collimator, a beam expanding optic, or other optical relays) configured to pass the excitation light signal onto the sample without focusing the signal and receive and collimate the spectroscopy signal generated at the sample in response to the excitation light signal.

[0073] The fourth sampling element 46 comprises a right-angle non-focusing sampling element that comprises a mirror and a collimating sampling element (e.g., a conjugate lens pair, a mirror based collimator, a beam expanding optic, or other optical relays) configured to pass the excitation light signal onto the sample without focusing the signal and receive and collimate the spectroscopy signal generated at the sample in response to the excitation light signal.

[0074] In the first two example sampling elements 40, 42 focus the excitation light signal onto a sample and collimate a returning spectroscopy signal generated at the sample in response to the excitation light signal. The third and fourth example sampling elements 44, 46 pass the excitation light signal to the sample without focusing the signal onto the sample and collimate a returning spectroscopy signal generated at the sample in response to the excitation light signal.

[0075] In one embodiment, a spectrometer, such as shown in Figures 1 and 3-6, may include two or more of the four example sampling elements that are interchangeable to provide different sampling options depending on the application of use for the spectrometer. Thus, in one embodiment, a spectrometer may comprise an excitation light source configured to provide anexcitation light signal, an optical system configured to direct the excitation light signal toward a sample, receive a spectroscopy signal from the sample and direct the spectroscopy to a detector such as an interferometer. The spectrometer comprises at least two interchangeable sampling elements configured to receive and collimate the spectroscopy signal and pass the collimated spectroscopy signal towards the detector.

[0076] Figure 14 shows another example embodiment of a spectrometer 100 that includes a fixed focusing lens 102 configured to focus the excitation light signal onto a sample and collimate a returning spectroscopy signal generated at the sample in response to the excitation light signal when used independently. The spectrometer 100 also comprises a removable sampling element attachment 110 that comprises a second lens 112 matched with the fixed lens to form a conjugate lens pair 114 when the removable sampling element is attached to the spectrometer. In this case, when assembled, the fixed lens and the attached removable sampling element together form a collimating, non-focusing sampling element configured to pass the excitation light signal to the sample without focusing the signal onto the sample and collimate a returning spectroscopy signal generated at the sample in response to the excitation light signal.

Claims

CLAIMSWhat is claimed is:

1. A Fourier Transform (FT) spectrometer comprising: an excitation light source configured to generate an excitation light signal; a detector; an interferometer optical system comprising a beam splitter, wherein the beam splitter is configured to receive the excitation light signal, direct a portion of the excitation light signal along a sample path toward a sample, and receive a spectroscopy signal generated at the sample; and a collimating optical interface disposed in the sample path, wherein the collimating optical interface is configured to: receive the portion of the excitation light signal from the beam splitter and deliver the excitation light signal to the sample as a non-focused beam and receive the spectroscopy signal generated at the sample and collimate the spectroscopy signal prior to the spectroscopy signal reaching the beam splitter; or receive a non-collimated spectroscopy signal between the beam splitter and the detector and collimate the spectroscopy signal toward the detector.

2. The FT spectrometer of claim 1, wherein the collimating optical interface comprises a collimating lens disposed internally within the interferometer optical system at a Jacquinot stop of the spectrometer.

3. The FT spectrometer of claim 1, wherein the collimating optical interface comprises a sampling element disposed adjacent to the sample and configured to relay the excitation light signal to the sample and collimate the spectroscopy signal at the sample4. The FT spectrometer of claim 2, wherein the sampling element is configured to relay the excitation light to the sample without focusing the light onto the sample.

5. The FT spectrometer of claim 3, wherein the sampling element comprises at least one of the group comprising: a conjugate lens pair, a mirror-based collimator, a beam expanding optic, and at least one optical relay.

6. The FT spectrometer of claim 1, wherein the collimating optical interface comprises a fixed focusing lens coupled to the spectrometer and a removable optical attachment comprising a second lens, wherein the fixed focusing lens and the second lens form a conjugate lens pair when assembled.

7. The FT spectrometer of claim 1, wherein the interferometer optical system comprises a fixed mirror and a moving mirror or a pair of moving mirrors.

8. The FT spectrometer of claim 1, wherein the non-focused beam has a beam area at the sample larger than a diffraction-limited focal point to reduce optical intensity at the sample.

9. The FT spectrometer of claim 1, wherein the collimating optical interface comprises a single collimating lens disposed internally within the interferometer optical system at a Jacquinot stop of the spectrometer defined by an aperture of the fixed mirror and the moving mirror.

10. The FT spectrometer of claim 2, wherein the single collimating lens is configured to pass the excitation light signal to the sample as a parallel beam having a diameter of at least 1 mm or at least 5 mm.

11. The FT spectrometer of claim 1, wherein the collimating optical interface comprises a sampling element disposed distal to the beam splitter, the sampling element comprising a conjugate lens pair configured to relay the excitation light signal to the sample and collimate the spectroscopy signal immediately at the sample.

12. The FT spectrometer of claim 11, wherein the conjugate lens pair is configured to maintain a Signal -to-Noise Ratio (SNR) of the spectroscopy signal that varies by less than 15% when a distance between the sample and the sampling element is increased by 50 mm.

13. The FT spectrometer of claim 1, wherein the collimating optical interface comprises: a fixed focusing lens coupled to a housing of the spectrometer; and a removable optical attachment comprising a second lens; wherein, when the removable optical attachment is coupled to the housing, the fixed focusing lens and the second lens align to form a conjugate lens pair that prevents the fixed focusing lens from focusing the excitation light signal onto the sample.

14. The FT spectrometer of claim 1, further comprising a beam expander disposed in an optical path of the excitation light source prior to the beam splitter, wherein the beam expanderand the collimating optical interface cooperate to deliver the excitation light signal to the sample with a power density below a Maximum Permissible Exposure (MPE) limit for human tissue.

15. The FT spectrometer of claim 1, further comprising a set of interchangeable sampling elements attachable to the spectrometer, the set comprising: a first sampling element comprising the collimating optical interface configured to deliver the non-focusing beam; and a second sampling element comprising a focusing lens configured to focus the excitation light signal onto the sample.

16. The FT spectrometer of claim 1, wherein the spectroscopy signal comprises a Raman scattering signal, and wherein the collimating optical interface is configured to reduce a DC signal offset component of the interferogram generated by the interferometer compared to a focused collection geometry.

17. A method of collecting a spectroscopy signal using a Fourier Transform (FT) spectrometer, the method comprising: generating an excitation light signal; directing the excitation light signal through a beam splitter of a spectrometer optical system toward a sample; receiving a spectroscopy signal generated at the sample based on the excitation light signal; collimating the spectroscopy signal between the sample and a detector; wherein the spectrometer optical system is configured to: receive at least a portion of the excitation light signal from the beam splitter and deliver the excitation light signal to the sample as a non-focused beam and receive the spectroscopy signal generated at the sample and collimate the spectroscopy signal prior to the spectroscopy signal reaching the beam splitter; or receive a non-collimated spectroscopy signal between the beam splitter and the detector and collimate the spectroscopy signal toward the detector.

18. The method of claim 17, wherein the spectrometer optical system comprises a collimating lens disposed internally within the interferometer optical system at a Jacquinot stop of the spectrometer.

19. The method of claim 17, wherein the spectrometer optical system comprises a sampling element disposed adjacent to the sample and configured to relay the excitation light signal to the sample and collimate the spectroscopy signal at the sample20. The method of claim 19, wherein the sampling element is configured to relay the excitation light to the sample without focusing the light onto the sample.21 . The method of claim 20, wherein the sampling element comprises at least one of the group comprising: a conjugate lens pair, a mirror-based collimator, a beam expanding optic, and at least one optical relay.

22. The method of claim 17, wherein the collimating optical interface comprises a fixed focusing lens coupled to the spectrometer and a removable optical attachment comprising a second lens, wherein the fixed focusing lens and the second lens form a conjugate lens pair when assembled.

23. The method of claim 17, wherein the interferometer optical system comprises a fixed mirror and a moving mirror or a pair of moving mirrors.

24. The FT spectrometer of claim 17, wherein the non-focused beam has a beam area at the sample larger than a diffraction-limited focal point to reduce optical intensity at the sample.

25. The method of claim 17, wherein conditioning the excitation light signal and collimating the spectroscopy signal are performed by a single collimating lens disposed at a Jacquinot stop internal to the FT spectrometer.

26. The method of claim 17, wherein conditioning the excitation light signal and collimating the spectroscopy signal are performed by a conjugate lens pair disposed at the sample, such that the spectroscopy signal is collimated prior to propagating from the sample to the beam splitter.

27. The method of claim 17, further comprising: detecting the interferogram; and processing the interferogram to generate a spectrum;wherein collimating the spectroscopy signal increases an AC modulation amplitude of the interferogram and reduces a DC offset of the interferogram relative to a method utilizing focusing input optics.

28. The method of claim 17, wherein the sample comprises biological tissue, and wherein delivering the excitation light signal as an unfocused beam comprises illuminating the biological tissue with a laser spot diameter of at least 1 mm to maintain an intensity below a photodamage threshold of the biological tissue.

29. The method of claim 17, further comprising attaching a removable optical module to the FT spectrometer to convert a fixed focusing lens of the FT spectrometer into a conjugate lens pair system that performs the steps of conditioning the excitation light signal and collimating the spectroscopy signal.

30. The method of claim 17, wherein collimating the spectroscopy signal decouples a signal intensity of the spectroscopy signal from a distance between the sample and the beam splitter, such that the spectroscopy signal intensity follows a non-inverse-square law dependence on said distance

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