Electro-optic comb doppler thermometer

Abstract

An exemplary thermometer combines an electro-optic frequency comb with a Doppler thermometry cell. This combination allows primary temperature measurements that are fast and accurate enough for industrial applications.

Description

[0001] ELECTRO-OPTIC COMB DOPPLER THERMOMETER

[0002] Related Applications

[0003] This application claims the benefit of U.S. Provisional Patent Application Serial No. 63 / 763,389 (filed February 26, 2025) and U.S. Provisional Patent Application Serial No. 63 / 700,037 (filed September 27, 2024), both of which are hereby incorporated herein by reference in their entireties.

[0004] Federally-Sponsored Research and Development

[0005] This invention was made with United States Government support from the National Institute of Standards and Technology (NIST), an agency of the United States Department of Commerce. The Government has certain rights in this invention.

[0006] Copyright Notice

[0007] This patent disclosure may contain material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure as it appears in the U.S. Patent and Trademark Office patent file or records but otherwise reserves any and all copyright rights.

[0008] Field of Invention

[0009] The present invention relates generally to high-precision thermometry, and more particularly to electro-optic comb Doppler thermometry.

[0010] Summary of Invention

[0011] There are no known commercially available, primary, high-precision thermometers. In high performance applications, platinum resistance thermometers (PRTs) have dominated the market for decades. However, PRTs require expensive and time-consuming calibration. Furthermore, PRTs lose calibration due to, for example, mechanical shock, humidity variation, and device aging. Embodiments of the invention provide an intrinsically stable, accurate thermometer that never requires calibration. Exemplary embodiments will reduce downtime and improve accuracy in high-precision thermometry applications, such as pharmaceutical manufacturing and improve thermometer reliability when recalibration is difficult or impossible (for example, on a satellite).

[0012] Disclosed herein is a physical configuration that combines an electro-optic frequency comb with a Doppler thermometry cell as well as a process for measuring the temperature with said configuration. Doppler thermometers use the Doppler effect to spectroscopically measure the thermal velocity distribution of a gas, and thus its temperature. Because the gas temperature is determined without reference to another thermometer or artifact, Doppler thermometers are primary ( / .e. they do not require calibration and do not drift over time). The gas is in thermal equilibrium with, and contained in, a Doppler thermometry cell, so Doppler thermometers can act as contact thermometers. However, prior art Doppler thermometers used large thermometry cells (approximately 100 cm3) and required long averaging times to reach target accuracy. As a result, they are unable to provide useful temperature measurements outside of a laboratory environment.

[0013] According to an aspect of the invention, an electro-optic comb Doppler thermometer includes a first splitter configured to divide light from a light source into a local oscillator path and a comb path; an acousto-optic modulator optically coupled to and downstream from the first splitter along the local oscillator path; an electro-optic modulator optically coupled to and downstream from the first splitter along the comb path; a frequency synthesizer configured to produce a frequency-swept, sinusoidal signal and apply said signal to the electro-optic modulator, and wherein the electro-optic modulator is configured to modulate light from the comb path according to the frequency-swept, sinusoidal signal into an electro-optical ly generated optical frequency comb; a Doppler thermometry cell in a thermal environment to be measured, wherein the Doppler thermometry cell is optically coupled to and downstream from the electro-optic modulator, wherein the cell includes a gas vapor having an optical transition overlapping a frequency range of light from the laser, and wherein the cell is configured to pass the optical frequency comb through the gas vapor, producing a comb Doppler spectrum; a second splitter optically coupled to and downstream from the acousto-optic modulator and the Doppler thermometry cell, wherein the second splitter is configured to combine the comb Doppler spectrum with light from the local oscillator path; a photodiode, coupled to and downstream from the second splitter and configured to convert the comb Doppler spectrum into an electric signal via optical intensity oscillations produced by superposition of the comb Doppler spectrum and light from the local oscillator path; and a Doppler analyzer module configured to analyze the electrical signal to determine a temperature of the thermal environment by determining a width of an atomic / molecular vapor velocity distribution of the cell.

[0014] Optionally, the electro-optic comb Doppler thermometer also includes a variable attenuator in a reference environment and selectively optically couplable to the electro-optic modulator and the second splitter, wherein the variable attenuator is configured to tune the optical frequency comb to a reference optical frequency comb that contains equal total power as the comb Doppler spectrum; wherein the Doppler thermometry cell is selectively optically couplable to the electro-optic modulator and the second splitter, wherein the optical frequency comb is intermittently switched between the variable attenuator and the Doppler thermometry cell, and wherein the Doppler analyzer module is configured to normalize the comb Doppler spectrum to the reference optical frequency comb yielding high uniformity normalized comb Doppler spectra.

[0015] Optionally, the electro-optic comb Doppler thermometer includes a first switch selectively optically coupling the electro-optic module to the variable attenuator and the Doppler thermometry cell; and a second switch selectively optically coupling the variable attenuator and the Doppler thermometry cell to the second splitter.

[0016] Optionally, the first and second switches are configured to switch such that time in which the optical frequency comb passes through the sample path and the reference path are equal.

[0017] Optionally, the first and second switches are configured to switch at a time interval based on anticipated time constant of temperature changes in the thermal environment and a desired temperature measurement accuracy. Optionally, the first and second switches are configured to switch such that time in which the optical frequency comb passes through the sample path is greater than time it passes through the reference path.

[0018] According to another aspect of the invention, an electro-optic comb Doppler thermometer includes a first splitter configured to divide light from a light source into a local oscillator path and a comb path; an acousto-optic modulator optically coupled to and downstream from the first splitter along the local oscillator path; a second splitter optically coupled to and downstream from the acousto-optic modulator and configured to split light from the acousto-optic modulator into two paths; an electro-optic modulator optically coupled to and downstream from the first splitter along the comb path; a frequency synthesizer configured to produce a frequency-swept, sinusoidal signal and apply said signal to the electro-optic modulator, and wherein the electro-optic modulator is configured to modulate light from the comb path according to the frequency- swept, sinusoidal signal into an electro-optically generated optical frequency comb; a third splitter optically coupled to and downstream from the electro-optic modulator and configured to split the optical frequency comb into two paths; a Doppler thermometry cell in a thermal environment to be measured, wherein the Doppler thermometry cell is optically coupled to and downstream from the third splitter, wherein the cell includes an gas vapor having an optical transition overlapping a frequency range of light from the laser, and wherein the cell is configured to pass the optical frequency comb through the gas vapor, producing a comb Doppler spectrum; a variable attenuator in a reference environment and optically coupled to and downstream from the third splitter, wherein the variable attenuator is configured to tune the optical frequency comb to a reference optical frequency comb that contains equal total power as the comb Doppler spectrum; a fourth splitter optically coupled to and downstream from the second splitter and the variable attenuator, wherein the fourth splitter is configured to combine light from the acousto-optic modulator with the reference optical frequency comb; a first photodiode optically coupled to and downstream from the fourth splitter and configured to digitize the reference optical frequency comb; a fifth splitter optically coupled to and downstream from the second splitter and the Doppler thermometry cell, wherein the fifth splitter is configured to combine light from the acousto-optic modulator with the comb Doppler spectrum; a second photodiode optically coupled to and downstream from the fifth splitter and configured to digitize the comb Doppler spectrum; and a Doppler analyzer module configured to analyze the digitized reference optical frequency comb and the comb Doppler spectrum to determine a temperature of the thermal environment by determining a width of an atomic / molecular vapor velocity distribution of the cell.

[0019] The foregoing and other features of the invention are hereinafter described in greater detail with reference to the accompanying drawings.

[0020] Brief Description of the Drawings

[0021] FIG. 1 shows a schematic of an exemplary electro-optic comb thermometer.

[0022] FIG. 2 shows an example electro-optic comb spectrum. Vertical lines denote optical frequency components with optical power indicated by the line height.

[0023] FIG. 3 shows an example electro-optic comb spectrum after passing through a Doppler thermometry cell.

[0024] FIG. 4 shows a practical embodiment of the invention. The optical frequency comb can be switched between a reference path and the Doppler thermometry cell (cell #1 ) to produce normalized comb Doppler spectra with lower noise.

[0025] FIG. 5 shows a fieldable embodiment of the invention. Switches #1 and #2 allow the optical frequency comb to be electronically switched between cell #1 and the reference path.

[0026] FIG. 6 shows another fieldable embodiment of the invention. Splitters #3, #4, and #5 divide and combine light from the comb arm and local oscillator arm to allow simultaneous digitization of the comb Doppler spectrum and the reference optical frequency comb.

[0027] FIG. 7 shows a schematic of an exemplary computer system for controlling an exemplary thermometer.

[0028] Detailed Description Disclosed herein is a physical configuration that combines an electro-optic frequency comb with a Doppler thermometry cell as well as a process for measuring the temperature with said configuration. Doppler thermometers use the Doppler effect to spectroscopically measure the thermal velocity distribution of a gas, and thus its temperature. Because the gas temperature is determined without reference to another thermometer or artifact, Doppler thermometers are primary ( / .e. they do not require calibration and do not drift over time). The gas is in thermal equilibrium with, and contained in, a Doppler thermometry cell, so Doppler thermometers can act as contact thermometers. However, prior art Doppler thermometers used large thermometry cells (approximately 100 cm3) and required long averaging times to reach target accuracy. As a result, they are unable to provide useful temperature measurements outside of a laboratory environment.

[0029] One embodiment of the invention is shown in FIG. 1. Optical signals are shown in solid lines and electrical signals are shown in dashed lines. In an electro-optic comb Doppler thermometer 100, light from a first laser 110 enters a first splitter 120 which divides the light into a local oscillator arm directed to a first acousto-optic modulator (AOM) 130 and a comb arm directed to a first electrooptic modulator (EOM) 140. The AOM is configured to shift the frequency of the laser light in the local oscillator arm by a constant amount. This makes it possible to distinguish positive order and negative order comb teeth in the beatnote spectrum after the photodiode. A first frequency synthesizer 180 produces a frequency-swept, sinusoidal signal and applies said signal to the EOM 140. EOM 140 modulates the comb arm light according to the frequency-swept signal yielding an electro-optically generated optical frequency comb. The optical frequency comb is communicated to a first Doppler thermometry cell 150, which may be placed in a thermal environment 151 to be measured. Cell 150 may be filled with an atomic (or molecular) gas vapor and light from laser 110 may be tuned near an optical transition of said vapor. After the optical frequency comb passes through cell 150, frequency components are absorbed according to the density and velocity distribution of the vapor, producing a comb Doppler spectrum illustrated in FIG. 3. The comb Doppler spectrum may be combined with light from the local oscillator arm at a second splitter 160. The combined light is communicated to a first photodiode (PD), which converts the comb Doppler spectrum into an electric signal via the optical intensity oscillations produced by the interference of the comb Doppler spectrum and the local oscillator arm light. Subsequent digitization and fitting of the comb Doppler spectrum in a first Doppler analyzer module 190 then yields the width of the atomic / molecular vapor velocity distribution and, therefore, the thermal environment 151 temperature.

[0030] Exemplary lasers may include, for example, any type of continuous-wave laser based on a gas, dye, solid-state, fiber, or semiconductor lasing medium. Narrow-linewidth, tunable semiconductor or fiber lasers are preferable. The operating wavelength of the laser may be determined by the atomic transition of interest and may vary between 200 nm to 20000 nm, for example.

[0031] Exemplary splitters 120, 160 may include, for example, a fixed-ratio fiber beamsplitter, variable ratio acousto-optic beamsplitter, or the like. Exemplary splitters 120, 160 may be of the same type or of different types from each other.

[0032] Exemplary AOMs may include any optical frequency shifter or the like. Exemplary frequency synthesizers may include, for example, a direct digital synthesizer (DDS) as illustrated, an arbitrary waveform generator, or the like.

[0033] Details of exemplary frequency comb generation are provided in US Patent Application Publication No. 2024 / 0014903 A1 , the content of which are hereby incorporated herein in its entirety. The optical frequency comb consists of many regularly spaced optical frequency components with nearly identical optical power in each frequency component, as shown in FIG. 2.

[0034] Exemplary vapor cells may include any material suitable for use with an optical frequency comb for determining temperatures. Any atom or molecule (e.g., CO, CO2, H2O, acetylene, or the like) with strong absorption lines would be preferrable (with appropriate laser technology to measure that absorption line). The choice of atom or molecule determines the size and vapor pressure needed in the cell. Therefore, more preferable choices include alkali or alkaline earth atoms due to their strong absorption lines which allows for very small, low vapor pressure cells. Such cells may then have a fit-for-purpose size and avoid systematic effects that increase with vapor pressure. Producing accurate temperature measurements with the embodiment of FIG. 1 requires high power uniformity of the optical frequency components of the electro-optically generated frequency comb. The required uniformity may not be achievable in practice and normalization of the comb Doppler spectrum to a reference optical frequency comb becomes desirable. To produce a reference optical frequency comb in the embodiment of FIG. 1 , light from laser 110 is tuned away from optical transitions of the vapor in cell 150. When optical frequency comb passes through cell 150 its frequency components are not absorbed and it maintains the nearly identical optical power in each frequency component as illustrated in FIG. 2; becoming a reference optical frequency comb. By intermittently switching of light from laser 110 between the near-resonance and far-from-resonance condition the comb Doppler spectrum can be normalized to the reference optical frequency comb in Doppler analyzer module 190. This process gives a uniform normalized comb Doppler spectrum from which the temperature of the thermal environment 151 can be computed by Doppler analyzer module 190.

[0035] The change between the near-resonance and far-from-resonance condition may occur at a predefined time interval that is set based on the anticipated time constant of temperature changes in the thermal environment 151 and the desired temperature measurement accuracy after a single comb Doppler spectrum and reference optical frequency comb are digitized by Doppler analyzer module 190. Additionally, the time interval can be ‘balanced’ or ‘imbalanced’. In balanced operation, the time in which light from laser 110 is far- from-resonance with the vapor in cell 150 is equal to the time in which light from laser 110 is near-resonance with the vapor in cell 150. In imbalanced operation, the time in which light from laser 110 is far-from-resonance with the vapor in cell 150 is less than the time in which light from laser 110 is near-resonance with the vapor in cell 150. The balanced process will attain the lowest uncertainty in a given measurement time, but the imbalanced process is more sensitive to fast temperature changes.

[0036] The required uniformity may not be achievable in practice and an alternative embodiment of the invention that overcomes this issue is shown in FIG. 4. The electro-optic comb Doppler thermometer 400 is substantially the same as the above-referenced electro-optic comb Doppler thermometer 100, and consequently the same reference numerals but indexed by 300 are used to denote structures corresponding to similar structures in the electro-optic comb Doppler thermometers. In addition, the foregoing description of the electro-optic comb Doppler thermometer 100 is equally applicable to the electro-optic comb Doppler thermometer 400 except as noted below. Moreover, it will be appreciated upon reading and understanding the specification that aspects of the electro-optic comb Doppler thermometers may be substituted for one another or used in conjunction with one another where applicable.

[0037] In the electro-optic comb Doppler thermometer 400, the optical frequency comb can be directed either to cell 450 in the thermal environment 451 or along a reference path containing a variable attenuator 452 in a reference environment 453. Attenuator 542 may be adjusted such that, after passing through attenuator 452, the optical frequency comb becomes a reference optical frequency comb that contains the same total power (summed over all frequency components) as the comb Doppler spectrum. By intermittently switching the optical frequency comb between the reference path with attenuator 452 and the sample path with cell 450, the comb Doppler spectrum can be normalized to the reference optical frequency comb in Doppler analyzer module 490. This process yields high uniformity normalized comb Doppler spectra from which the temperature of the thermal environment 451 can be deduced with high accuracy by Doppler analyzer module 490.

[0038] Exemplary variable attenuators 452 include, for example, manually or electrically adjustable fiber-optic attenuators, acousto-optic attenuators, a fiber or free-space delay line, or the like.

[0039] The switching method shown in FIG. 4 may be manual ( / .e. the optical fibers carrying the optical frequency comb are unplugged from the connections to cell 450 and plugged into the connections to the reference path). This manual switching process may be insufficient for a fielded instrument.

[0040] Referring now to FIG. 5, electro-optic comb Doppler thermometer 500 is substantially the same as the above-referenced electro-optic comb Doppler thermometers 100, 400, and consequently the same reference numerals but indexed by 100 are used to denote structures corresponding to similar structures in the electro-optic comb Doppler thermometers. In addition, the foregoing descriptions of the electro-optic comb Doppler thermometers 100, 400 are equally applicable to the electro-optic comb Doppler thermometer 500 except as noted below. Moreover, it will be appreciated upon reading and understanding the specification that aspects of the electro-optic comb Doppler thermometers may be substituted for one another or used in conjunction with one another where applicable.

[0041] A first switch 554 and a second switch 556 are interposed around the reference path (having a reference environment 553 with attenuator 552) and the sample path (having a thermal environment 551 with Doppler thermometry cell 550). First switch 554 may be optically coupled to and between the EOM 540 and the two alternate paths (sample and reference). Second switch 556 may be optically coupled to and between the two paths and the second splitter 560. Exemplary switches may include, for example, MEMs switches, acousto-optic switches, or the like. First and second switches 554, 556 allow fast, electronic switching of the optical frequency comb between the sample and reference paths.

[0042] There are two processes that may govern the switching of the optical frequency comb between the paths. First, in ‘balanced’ switching, the time in which the optical frequency comb passes through the sample path and the reference path are the same. First switch 554 and second switch 556 are thrown at a predefined time interval that is set based on the anticipated time constant of temperature changes in the thermal environment 551 and the desired temperature measurement accuracy after a single comb Doppler spectrum and reference optical frequency comb are digitized in Doppler analyzer module 590. Second, in ‘imbalanced’ operation, the time in which the optical frequency comb passes through the sample path (and therefore cell 550) is greater than the time it passes through the reference path. The balanced process will attain the lowest uncertainty in a given measurement time, but the imbalanced process is more sensitive to fast temperature changes.

[0043] Referring now to FIG. 6, electro-optic comb Doppler thermometer 600 is substantially the same as the above-referenced electro-optic comb Doppler thermometers 100, 400, 500, and consequently the same reference numerals but indexed by 100 are used to denote structures corresponding to similar structures in the electro-optic comb Doppler thermometers. In addition, the foregoing descriptions of the electro-optic comb Doppler thermometers 100, 400, 500 are equally applicable to the electro-optic comb Doppler thermometer 600 except as noted below. Moreover, it will be appreciated upon reading and understanding the specification that aspects of the electro-optic comb Doppler thermometers may be substituted for one another or used in conjunction with one another where applicable.

[0044] In the electro-optic comb Doppler thermometer 600, the optical frequency comb is simultaneously communicated to both the reference path and the sample path (and therefore Doppler thermometry cell 650) by a third splitter 662. The first splitter 660 now divides the light from the local oscillator arm and sends it to a fourth splitter 664 and a fifth splitter 666. The fourth splitter 664 combines the light from the local oscillator arm with the reference optical frequency comb onto a first photodiode (PD) 670. The fifth splitter 666 combines the light from the local oscillator arm with the comb Doppler spectrum onto a second PD 672. The PDs 670, 672 convert the reference optical frequency comb and the comb Doppler spectrum, respectively, into electric signals via the optical intensity oscillations produced by the superposition with the local oscillator arm light. The electro-optic comb Doppler thermometer 600 allows simultaneous digitization of the reference optical frequency comb and the comb Doppler spectrum. It therefore halves the temperature measurement time compared to the balanced process of FIG. 5 and provides greater suppression of intensity noise in the optical frequency comb. The downside of this embodiment is that it requires twice as much optical power as the embodiment of FIG. 5.

[0045] There are no commercially available, primary, high-precision thermometers. In high performance applications, platinum resistance thermometers (PRTs) have dominated the market for decades. However, PRTs require costly and time-consuming calibration. Furthermore, PRTs are known to lose calibration due to, for example, mechanical shock, humidity variation, and device aging. Exemplary embodiments provide an intrinsically stable, accurate thermometer that never requires calibration. Exemplary embodiments reduce downtime and improve accuracy in high-precision thermometry applications, such as pharmaceutical manufacturing and improve thermometer reliability when recalibration is difficult or impossible (for example, on a satellite).

[0046] Exemplary embodiments have several advantages for achieving a primary fieldable thermometer. First, atomic vapors exhibit extremely strong optical transitions, which allows significant absorption (the depth of the intensity dip in FIG. 3) in a Doppler thermometry cell with length < 1 cm. Exemplary electro-optic comb Doppler thermometers can therefore act as a primary contact thermometer due to the small cell size. Second, the time to acquire the full Doppler spectrum is set by the repetition rate of the frequency-swept signal from first frequency synthesizer 180, 480, 580, 680 which may be > 10 MHz. An exemplary electrooptic comb Doppler thermometer will therefore average Doppler spectra extremely quickly and be able to produce high accuracy temperature readings in seconds.

[0047] Using an exemplary electro-optic comb allows Doppler thermometry at much lower cost than previously demonstrated approaches. Doppler thermometry typically requires a very accurate frequency scale on which to perform the line-shape fitting necessary to compute the temperature. Conventional approaches have used an optical reference to set the frequency scale: either a second stabilized, research-grade laser or an octave-spanning frequency comb, which are both costly. The frequency scale of an electro-optic comb is set by the RF or microwave clock controlling frequency synthesizer 180, which is low cost.

[0048] Conventional Doppler thermometers suffer from systematics due to optical pumping distortion of the absorption spectrum. To prevent this systematic distortion, prior stepped-scan approaches to Doppler thermometry have used exceptionally low optical power to perform the temperature measurement. The low optical power limits the signal-to-noise ratio of the measurement, which leads to long measurement times to achieve high accuracy. An electro-optic comb divides the optical power entering the thermometry cell between the teeth of the comb. Because the systematic distortion is related to the optical power per comb tooth, the comb can employ much higher total optical power without distorting the absorption spectrum in an exemplary system. As result, an exemplary electro-optic comb Doppler thermometer can achieve a given measurement precision much faster than a stepped-scan Doppler thermometer. The speed increase is roughly given by the number of teeth in the electro-optic comb.

[0049] Exemplary thermometers are especially useful in applications where the prevention of long-term drift is a critical feature such as harsh environments. Exemplary embodiments may be particularly useful in the pharmaceutical industry (such as for bio-reactors, e.g.), the nuclear power industry (such as in water cooling loops, e.g.), the space industry (such as for satellites, e.g.), and the nuclear waster industry (such as for nuclear waste monitoring, e.g.).

[0050] It should be understood that the calculations may be performed by any suitable computer system, such as that diagrammatically shown in FIG. 7. Data is entered into system 700 via any suitable type of user interface 716, and may be stored in memory 712, which may be any suitable type of computer readable and programmable memory and is preferably a non-transitory, computer readable storage medium. Calculations are performed by processor 714, which may be any suitable type of computer processor and may be displayed to the user on display 718, which may be any suitable type of computer display. Processor 714 may be associated with, or incorporated into, any suitable type of computing device, for example, a personal computer or a programmable logic controller. The display 718, the processor 714, the memory 712 and any associated computer readable recording media are in communication with one another by any suitable type of data bus, as is well known in the art.

[0051] Examples of computer-readable recording media include non-transitory storage media, a magnetic recording apparatus, an optical disk, a magnetooptical disk, and / or a semiconductor memory (for example, RAM, ROM, etc.). Examples of magnetic recording apparatus that may be used in addition to memory 712, or in place of memory 712, include a hard disk device (HDD), a flexible disk (FD), and a magnetic tape (MT). Examples of the optical disk include a DVD (Digital Versatile Disc), a DVD-RAM, a CD-ROM (Compact Disc- Read Only Memory), and a CD-R (Recordable)ZRW. It should be understood that non-transitory computer-readable media include all computer-readable media except for a transitory, propagating signal. The processes described herein may be embodied in, and fully automated via, software code modules executed by a computing system that includes one or more general purpose computers or processors. The code modules may be stored in any type of non-transitory computer-readable medium or other computer storage device. Some or all the methods may alternatively be embodied in specialized computer hardware. In addition, the components referred to herein may be implemented in hardware, software, firmware, or a combination thereof.

[0052] Many other variations than those described herein will be apparent from this disclosure. For example, depending on the embodiment, certain acts, events, or functions of any of the algorithms described herein can be performed in a different sequence, can be added, merged, or left out altogether (e.g., not all described acts or events are necessary for the practice of the algorithms). Moreover, in certain embodiments, acts or events can be performed concurrently, e.g., through multi-threaded processing, interrupt processing, or multiple processors or processor cores or on other parallel architectures, rather than sequentially. In addition, different tasks or processes can be performed by different machines and / or computing systems that can function together.

[0053] Any logical blocks, modules, and algorithm elements described or used in connection with the embodiments disclosed herein can be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, and elements have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. The described functionality can be implemented in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the disclosure.

[0054] The various illustrative logical blocks and modules described or used in connection with the embodiments disclosed herein can be implemented or performed by a machine, such as a processing unit or processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A processor can be a microprocessor, but in the alternative, the processor can be a controller, microcontroller, or state machine, combinations of the same, or the like. A processor can include electrical circuitry configured to process computerexecutable instructions. In another embodiment, a processor includes an FPGA or other programmable device that performs logic operations without processing computer-executable instructions. A processor can also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. Although described herein primarily with respect to digital technology, a processor may also include primarily analog components. For example, some or all of the signal processing algorithms described herein may be implemented in analog circuitry or mixed analog and digital circuitry. A computing environment can include any type of computer system, including, but not limited to, a computer system based on a microprocessor, a mainframe computer, a digital signal processor, a portable computing device, a device controller, or a computational engine within an appliance, to name a few.

[0055] The elements of a method, process, or algorithm described in connection with the embodiments disclosed herein can be embodied directly in hardware, in a software module stored in one or more memory devices and executed by one or more processors, or in a combination of the two. A software module can reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of non-transitory computer-readable storage medium, media, or physical computer storage known in the art. An example storage medium can be coupled to the processor such that the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium can be integral to the processor. The storage medium can be volatile or nonvolatile. While one or more embodiments have been shown and described, modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustrations and not limitation. Embodiments herein can be used independently or can be combined.

[0056] All ranges disclosed herein are inclusive of the endpoints, and the endpoints are independently combinable with each other. The ranges are continuous and thus contain every value and subset thereof in the range. Unless otherwise stated or contextually inapplicable, all percentages, when expressing a quantity, are weight percentages. The suffix (s) as used herein is intended to include both the singular and the plural of the term that it modifies, thereby including at least one of that term (e.g., the colorant(s) includes at least one colorants). Option, optional, or optionally means that the subsequently described event or circumstance can or cannot occur, and that the description includes instances where the event occurs and instances where it does not. As used herein, combination is inclusive of blends, mixtures, alloys, reaction products, collection of elements, and the like.

[0057] As used herein, a combination thereof refers to a combination comprising at least one of the named constituents, components, compounds, or elements, optionally together with one or more of the same class of constituents, components, compounds, or elements.

[0058] All references are incorporated herein by reference.

[0059] The use of the terms “a,” “an,” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. It can further be noted that the terms first, second, primary, secondary, and the like herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. It will also be understood that, although the terms first, second, etc. are, in some instances, used herein to describe various elements, these elements should not be limited by these terms. For example, a first current could be termed a second current, and, similarly, a second current could be termed a first current, without departing from the scope of the various described embodiments. The first current and the second current are both currents, but they are not the same condition unless explicitly stated as such.

[0060] The modifier about used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context (e.g., it includes the degree of error associated with measurement of the particular quantity). The conjunction or is used to link objects of a list or alternatives and is not disjunctive; rather the elements can be used separately or can be combined together under appropriate circumstances.

[0061] Although the invention has been shown and described with respect to a certain embodiment or embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a "means") used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several illustrated embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.

Claims

Claims1 . An electro-optic comb Doppler thermometer comprising: a first splitter configured to divide light from a light source into a local oscillator path and a comb path; an acousto-optic modulator optically coupled to and downstream from the first splitter along the local oscillator path; an electro-optic modulator optically coupled to and downstream from the first splitter along the comb path; a frequency synthesizer configured to produce a frequency-swept, sinusoidal signal and apply said signal to the electro-optic modulator, and wherein the electro-optic modulator is configured to modulate light from the comb path according to the frequency-swept, sinusoidal signal into an electro- optically generated optical frequency comb; a Doppler thermometry cell in a thermal environment to be measured, wherein the Doppler thermometry cell is optically coupled to and downstream from the electro-optic modulator, wherein the cell includes a gas vapor having an optical transition overlapping a frequency range of light from the laser, and wherein the cell is configured to pass the optical frequency comb through the gas vapor, producing a comb Doppler spectrum; a second splitter optically coupled to and downstream from the acousto- optic modulator and the Doppler thermometry cell, wherein the second splitter is configured to combine the comb Doppler spectrum with light from the local oscillator path; a photodiode, coupled to and downstream from the second splitter and configured to convert the comb Doppler spectrum into an electric signal via optical intensity oscillations produced by superposition of the comb Doppler spectrum and light from the local oscillator path; and a Doppler analyzer module configured to analyze the electrical signal to determine a temperature of the thermal environment by determining a width of an atomic / molecular vapor velocity distribution of the cell.

2. The electro-optic comb Doppler thermometer of claim 1 , further comprising: a variable attenuator in a reference environment and selectively optically couplable to the electro-optic modulator and the second splitter, wherein the variable attenuator is configured to tune the optical frequency comb to a reference optical frequency comb that contains equal total power as the comb Doppler spectrum; wherein the Doppler thermometry cell is selectively optically couplable to the electro-optic modulator and the second splitter, wherein the optical frequency comb is intermittently switched between the variable attenuator and the Doppler thermometry cell, and wherein the Doppler analyzer module is configured to normalize the comb Doppler spectrum to the reference optical frequency comb yielding high uniformity normalized comb Doppler spectra.

3. The electro-optic comb Doppler thermometer of claim 2, further comprising: a first switch selectively optically coupling the electro-optic module to the variable attenuator and the Doppler thermometry cell; and a second switch selectively optically coupling the variable attenuator and the Doppler thermometry cell to the second splitter.

4. The electro-optic comb Doppler thermometer of claim 3, wherein the first and second switches are configured to switch such that time in which the optical frequency comb passes through the sample path and the reference path are equal.

5. The electro-optic comb Doppler thermometer of claim 4, wherein the first and second switches are configured to switch at a time interval based on anticipated time constant of temperature changes in the thermal environment and a desired temperature measurement accuracy.

6. The electro-optic comb Doppler thermometer of claim 3, wherein the first and second switches are configured to switch such that time in which the optical frequency comb passes through the sample path is greater than time it passes through the reference path.

7. An electro-optic comb Doppler thermometer comprising: a first splitter configured to divide light from a light source into a local oscillator path and a comb path; an acousto-optic modulator optically coupled to and downstream from the first splitter along the local oscillator path; a second splitter optically coupled to and downstream from the acousto- optic modulator and configured to split light from the acousto-optic modulator into two paths; an electro-optic modulator optically coupled to and downstream from the first splitter along the comb path; a frequency synthesizer configured to produce a frequency-swept, sinusoidal signal and apply said signal to the electro-optic modulator, and wherein the electro-optic modulator is configured to modulate light from the comb path according to the frequency-swept, sinusoidal signal into an electro- optically generated optical frequency comb; a third splitter optically coupled to and downstream from the electro-optic modulator and configured to split the optical frequency comb into two paths; a Doppler thermometry cell in a thermal environment to be measured, wherein the Doppler thermometry cell is optically coupled to and downstream from the third splitter, wherein the cell includes a gas vapor having an optical transition overlapping a frequency range of light from the laser, and wherein the cell is configured to pass the optical frequency comb through the gas vapor, producing a comb Doppler spectrum; a variable attenuator in a reference environment and optically coupled to and downstream from the third splitter, wherein the variable attenuator is configured to tune the optical frequency comb to a reference optical frequency comb that contains equal total power as the comb Doppler spectrum;a fourth splitter optically coupled to and downstream from the second splitter and the variable attenuator, wherein the fourth splitter is configured to combine light from the acousto-optic modulator with the reference optical frequency comb; a first photodiode optically coupled to and downstream from the fourth splitter and configured to digitize the reference optical frequency comb; a fifth splitter optically coupled to and downstream from the second splitter and the Doppler thermometry cell, wherein the fifth splitter is configured to combine light from the acousto-optic modulator with the comb Doppler spectrum; a second photodiode optically coupled to and downstream from the fifth splitter and configured to digitize the comb Doppler spectrum; and a Doppler analyzer module configured to analyze the digitized reference optical frequency comb and the comb Doppler spectrum to determine a temperature of the thermal environment by determining a width of an atomic / molecular vapor velocity distribution of the cell.

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