EP1101391A1 - Infrarot strahlenquellen, zusammenstellung von fühlen und quellen, und das verfahren der herstellung - Google Patents

Infrarot strahlenquellen, zusammenstellung von fühlen und quellen, und das verfahren der herstellung

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Publication number
EP1101391A1
EP1101391A1 EP99938909A EP99938909A EP1101391A1 EP 1101391 A1 EP1101391 A1 EP 1101391A1 EP 99938909 A EP99938909 A EP 99938909A EP 99938909 A EP99938909 A EP 99938909A EP 1101391 A1 EP1101391 A1 EP 1101391A1
Authority
EP
European Patent Office
Prior art keywords
emission
reflector
detector
detection element
radiation
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP99938909A
Other languages
English (en)
French (fr)
Other versions
EP1101391A4 (de
Inventor
Edward A. Johnson
W. Andrew Bodkin
John S. Wollam
James T. Daly
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Ion Optics Inc
Original Assignee
Ion Optics Inc
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Ion Optics Inc filed Critical Ion Optics Inc
Priority to EP10184853A priority Critical patent/EP2326139A3/de
Publication of EP1101391A1 publication Critical patent/EP1101391A1/de
Publication of EP1101391A4 publication Critical patent/EP1101391A4/de
Withdrawn legal-status Critical Current

Links

Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/02—Details
    • G01J3/10—Arrangements of light sources specially adapted for spectrometry or colorimetry
    • G01J3/108—Arrangements of light sources specially adapted for spectrometry or colorimetry for measurement in the infrared range
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J5/00—Radiation pyrometry, e.g. infrared or optical thermometry
    • G01J5/0014—Radiation pyrometry, e.g. infrared or optical thermometry for sensing the radiation from gases, flames
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J5/00—Radiation pyrometry, e.g. infrared or optical thermometry
    • G01J5/02—Constructional details
    • G01J5/06—Arrangements for eliminating effects of disturbing radiation; Arrangements for compensating changes in sensitivity
    • G01J5/061—Arrangements for eliminating effects of disturbing radiation; Arrangements for compensating changes in sensitivity by controlling the temperature of the apparatus or parts thereof, e.g. using cooling means or thermostats
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J5/00—Radiation pyrometry, e.g. infrared or optical thermometry
    • G01J5/02—Constructional details
    • G01J5/08—Optical arrangements
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J5/00—Radiation pyrometry, e.g. infrared or optical thermometry
    • G01J5/02—Constructional details
    • G01J5/08—Optical arrangements
    • G01J5/0801—Means for wavelength selection or discrimination
    • G01J5/0802—Optical filters
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J5/00—Radiation pyrometry, e.g. infrared or optical thermometry
    • G01J5/02—Constructional details
    • G01J5/08—Optical arrangements
    • G01J5/0806—Focusing or collimating elements, e.g. lenses or concave mirrors
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J5/00—Radiation pyrometry, e.g. infrared or optical thermometry
    • G01J5/02—Constructional details
    • G01J5/08—Optical arrangements
    • G01J5/0815—Light concentrators, collectors or condensers
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J5/00—Radiation pyrometry, e.g. infrared or optical thermometry
    • G01J5/02—Constructional details
    • G01J5/08—Optical arrangements
    • G01J5/0893—Arrangements to attach devices to a pyrometer, i.e. attaching an optical interface; Spatial relative arrangement of optical elements, e.g. folded beam path
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J5/00—Radiation pyrometry, e.g. infrared or optical thermometry
    • G01J5/02—Constructional details
    • G01J5/08—Optical arrangements
    • G01J5/0896—Optical arrangements using a light source, e.g. for illuminating a surface
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J5/00—Radiation pyrometry, e.g. infrared or optical thermometry
    • G01J5/10—Radiation pyrometry, e.g. infrared or optical thermometry using electric radiation detectors
    • G01J5/20—Radiation pyrometry, e.g. infrared or optical thermometry using electric radiation detectors using resistors, thermistors or semiconductors sensitive to radiation, e.g. photoconductive devices
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J5/00—Radiation pyrometry, e.g. infrared or optical thermometry
    • G01J5/52—Radiation pyrometry, e.g. infrared or optical thermometry using comparison with reference sources, e.g. disappearing-filament pyrometer
    • G01J5/53—Reference sources, e.g. standard lamps; Black bodies
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
    • G01N21/3504—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light for analysing gases, e.g. multi-gas analysis
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/02—Details
    • G01J3/0205—Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows
    • G01J3/0216—Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows using light concentrators or collectors or condensers

Definitions

  • This invention relates to optical radiation sources and relates in particular to blackbody radiation sources whose wavelength-dependent surface emissivity is modified in order to radiate with higher power efficiency and/or to radiate within a desired wavelength controlled spectral band.
  • the invention also relates to sensors incorporating sources and to systems and methods embodying same.
  • Non-dispersive Infrared (NDIR) techniques utilizing the characteristic abso ⁇ tion bands of gases in the infrared have long been considered as one of the best methods for gas measurement. These techniques take advantage of the fact that various gases exhibit substantial absorption at specific wavelengths in the infrared radiation spectrum.
  • non-dispersive refers to the type of apparatus incorporating the NDIR technique, typically including a narrow band pass interference filter (as opposed to a "dispersive" element such as a prism or a diffraction grating to isolate and pass radiation in a particular wavelength band from a spectrally broad band infrared source.
  • a prior art NDIR gas analyzer typically includes an infrared source with a motor- driven mechanical chopper to modulate the source so that synchronous detection is used to discriminate spurious infrared radiation from surroundings; a pump to push gas through a sample chamber; a narrow band-pass interference filter; a sensitive infrared detector, inexpensive infrared optics and windows to focus the infrared energy from the source onto the detector.
  • the radiation source is one such component; and in order for NDIR methodology to work efficaciously, this source must provide broad spectral output and give high power in the band of interest.
  • a blackbody radiation source is generally used for this component because it can be heated to a temperature that provides high intensity radiation within any wavelength region. Because the blackbody output spectral distribution and intensity are uniquely determined by the temperature of the source, the spectral peak intensity can be adjusted by varying the temperature of the blackbody source.
  • the peak intensity of a blackbody radiation source of temperature T is at a wavelength ⁇ max equivalent to 2.898 x 10- 3 /T where T is measured in degrees Kelvin (1°K) and ⁇ max is measured in meters.
  • T is measured in degrees Kelvin (1°K)
  • ⁇ max is measured in meters.
  • the Nernst Glower gives off an amount of infrared energy with an emissivity close to unity, its power efficiency (i.e., efficiency from electrical to useable optical energy) is notoriously low.
  • the large amount of unwanted heat is also a major drawback in the use of the Nernst Glower in any NDIR gas measurement systems.
  • the large heat capacity of the Nernst Glower makes it a necessarily slow device in terms of intensity modulation. In many cases, it can only be used as a steady state or DC radiation source; and thus a mechanical chopper is needed to generate synchronous modulated signals, further adding to the complexity of NDIR gas measurement systems.
  • the prior art has attempted to replace the blackbody radiator with hot filament as an approximation to a blackbody.
  • a filament does not have high emission over all wavelengths because there is spatial temperature variation within the filament; it is not an ideal blackbody source.
  • Optical radiation sources such as a hot filament lamp, are thus often referred to as "quasi-blackbody" sources.
  • hot filament radiators typically are used by a quartz bulb which is substantially opaque for wavelengths, longer than about 4.5 ⁇ m reducing its applicability to gas detection through longer wavelength abso ⁇ tion lines. Prior art hot filaments are thus not particularly good infrared radiation sources for use with NDIR gas sensors.
  • Bridgham describes an infrared radiation source that includes a thin film resistor heater with highly emissive material Cr 3 Si on a substrate.
  • the thin film heater is positioned between a pair of thin metal elements serving as sensing electrodes on a very thin ( ⁇ 0.005" typical) insulating substrate.
  • the entire thin film heater is packaged in a standard TO-5 heater equipped with a focusing reflector and pins supporting the heater structure.
  • One object of the invention is to provide sources which function as tuned waveband emitters that preferentially emit radiation into a wavelength band of interest as compared to blackbody or gray-body radiators.
  • Another object of the invention provides in integrated circuit sensor include selectively tuned radiation source.
  • Still another object of the invention provides methods and devices for sensing gas constituents without certain of the difficulties and problems of prior art NDIR devices.
  • on chip refers to integrated circuits and the like which are processed through microelectronic fabrication techniques to provide circuitry and/or processes (including A/D processing) on a semiconductor element or chip.
  • SOURCE generally means tuned waveband emitters constructed according to the invention that preferentially emit radiation into a wavelength band of interest.
  • Certain SOURCES include those tuned radiation sources described in the commonly assigned and copending U.S.S.N. 08/905,599, filed on August 4, 1997, now U.S. Patent No. 5,838,016, granted November 17, 1998, and U.S.S.N. 08/511,070, filed on August 3, 1995, now abandoned (the latter also being inco ⁇ orated herein by reference).
  • Still other SOURCES including tuned cavity emitters, chemical and microelectronic/semiconductor emitters, and other etched electromechanical, chemically -treated, ion-bombarded and lithographically created surfaces which permit spectral control and which provide narrow band incoherent emissions tailored to specific end-user needs and requirements.
  • the invention has many objectives in providing improved sources and sensors. It also utilizes and is complimentary to certain art encompassing other areas including NDIR gas measurement technology, silicon micro-machined thermopile, optical beam configurations and fabrication techniques. As such, the following patents and publications providing information and background to the specification and are herein inco ⁇ orated by reference.
  • the invention provides an integrated circuit sensor (ICS).
  • the ICS has a tuned narrow band emitter (e.g., a SOURCE of the invention) and a tuned narrow band detector fabricated on a single silicon die, Optics (e.g., reflective optics or a silicon slab waveguide) capture source emissions and refocus the captured energy onto the detector.
  • Control electronics are preferably included and are preferably inco ⁇ orated "on chip" to facilitate packaging issues.
  • a solar cell can also be integrated with the sensor to provide power.
  • the sensor can further be integrated with other features described herein or contained in related applications.
  • the invention provides a Hybrid Infrared Gas and Thermal
  • a SOURCE is integrated with an uncooled detector (e.g., a microbolometer detector), and a reflector (e.g., a parabolic reflector) into a package such as a single transistor can.
  • Source drive electronics and detector readout electronics and preferably included with the HIRGS and can also be provided "on chip”.
  • the detector is also preferably "tuned” to the waveband emissions of the SOURCE.
  • the invention provides an Integrated Gas Sensor (“IGS”).
  • a SOURCE is coupled to illumination optics which pass a beam (of tuned magnetic radiation) into a gas sample or stream.
  • a detector (preferably with receiving optics) is arranged to capture the beam and convert the beam into electrical signals.
  • An electronics system is used to diagnose the beam intensity and spectral characteristics to analyze the gas, e.g., to determine the % concentration of a particular gas.
  • an alternative IGS includes a SOURCE which inco ⁇ orates a gas cell sample (or gas flow cell).
  • Wavelength compatible optics preferably collect SOURCE emissions to efficiently illuminate the cell.
  • a mirror e.g., a flat mirror
  • the detector is positioned to collect the reflected beam adjacent to the SOURCE.
  • An electronics subsystem is used to diagnose the beam intensity and spectral intensities to determine characteristics of the gas, e.g., concentration of a particular gas.
  • methodology is provided for manufacturing a microbolometer- based SOURCE and integrated sensor.
  • the sensor includes one microbolometer arranged to emit tuned radiation, optics to collect and transmit the radiation to a sample under study for a specific pu ⁇ ose (e.g., to illuminate a gas cell or a solid surface), and another microbolometer arranged to collect at least part of the post sample (i.e., transmitted through the gas or scattered from the surface) radiation.
  • microbolometers function by efficiently collecting infrared radiation within an absorbing microbridge cavity; and the absorbed energy changes the resistance of the bridge electronics, indicating an amount of radiation.
  • the efficient thermal mass of the bridge permits very fast dissipation of radiant energy so that multiple microbolometer can efficiently take high speed IR picture frames (i.e, each microbolometer operating as a pixel in a picture in accord with the invention, the source microbolometer operates in reverse: electric energy drives the microbolometer to radiate heat and to emit radiation in the band of the cavity.
  • the sister microbolometer detector is tuned to the same band and the two microbolometer effectively emit and recover tuned electromagnetic radiation.
  • a tuned cavity band emitter SOURCE is provided.
  • SOURCE of this aspect has an emissivity curve which differs - selectively and beneficially-from a standard blackbody curve. It thus functions as a selective incoherent band emitting infrared source.
  • This SOURCE can thus include a metal foil filament such as provided in U.S. Patent Application Serial No. 08/905,599; and can further include a metal transistor can be coupled to the SOURCE.
  • the SOURCE of this aspect can also be constructed with spectral control, including supporting electronics, to function as an incoherent narrow band emitter.
  • the surface of the emitting SOURCE of this aspect is dark. It also includes a sha ⁇ spectral transition between a high absorbing band and a highly reflecting out of band, thereby functioning as a "band emitter”.
  • prior art incoherent sources radiate in a manner representative of a blackbody curve.
  • most applications and products utilize only a small band within the spectrum relative to that blackbody curve (by way of example, most devices are used to generate energy within a defined band; and other radiation can result in noise).
  • These prior art sources thus inefficiently emit energy into the band. Accordingly, when the prior art source is brought to temperature, the blackbody emissions at the center of the measurement band represent only a tiny fraction of the total radiant output falls within the desired measurement band. The invention solves this dilemma.
  • the SOURCE emitter surface of one aspect is modified to become an effective band emitter.
  • the SOURCE By suppressing out-of-band emission sensors and systems utilizing the SOURCE trade out-of-band photons (noise) for in-band photon (signal).
  • the SOURCE becomes a near-perfect reflecting surface for out-of-band radiation and a highly efficient absorber for in-band radiation corresponding to an array of absorbers.
  • the size, shape, and spacing of these cavities are designed to absorb at a pre-selected resonance wavelength.
  • this aspect of the invention includes a SOURCE with manually applied cavities disposed thereover with microlithography.
  • the invention provides mid-and long-wavelength infrared sensors for gas detector applications utilizing a non-blackbody, narrow-band radiator.
  • the SOURCE of this aspect includes sub-micron scale surface structures which facilitate control of the radiator emission spectrum.
  • the source thus combines blackbody radiator functionality with a surface that selectively filters wavelength emissions.
  • an optical radiation SOURCE which optimizes the following three parameters for improved waveband emissions: operating temperature T(°K), emitting area A, and surface emissivity e ( ⁇ ).
  • the SOURCE utilizes structure which emphasizes the emitting surface characteristics (e.g., cavity structure) of the radiator and which optimizes its emitting area and its emissivity to maximize power output in the optimized waveband.
  • Another aspect of the invention provides an optical radiation SOURCE with an output that is "spectrally tailored" relative to the emission a perfect blackbody by modifying surface emissivity so as to maximize power output in a desired and controlled spectral wavelength.
  • the optical radiation SOURCE of the invention has a thin structure with a well-defined outwardly facing surface that emits optical radiation.
  • the thin structure includes a resistive element in the form of a thin film or foil supported by a substrate and configured with surface structure as discussed above.
  • SOURCE is powered by electrical current passed through its resistive element.
  • the texture of the outwardly facing filament is methodically and controllably modified by well-known techniques - such as electrolytic plasma etching, ion-milling, and preferably in combination with photo-lithographic masking technology - such that the emissivity of the radiator SOURCE possesses an emissivity near to unity with well-defined and controllable spectral wavelength band.
  • One form of the invention is a relatively low power reflector infrared (IR) source with an improved texture, smaller (compared to the prior art) filament, and with an integral reflector.
  • IR infrared
  • a common design issue for IR sources is the requirement for significantly more useful signal, particularly in the LWIR, with significantly less required drive power.
  • the present invention effects such changes by including reduced filament size, improved filament texture, and by inco ⁇ orating a reflector into the source package. This allows, for example, for medical instruments (including anesthesia monitors or critical care systems), industrial safety instruments, and automotive emission monitoring.
  • FIG. 1 A shows an IR source 10 of the invention having relatively high usable signal with relatively low drive power.
  • a relatively small filament 110 is used with a molded reflector 12 to provide a "search light” type beam-former, illuminating a narrow forward cone (as shown in Fig. IB) and eliminating the need for discrete optical (lens) elements.
  • the smaller filament of the invention also uses less power compared to the prior art ("standard") as shown in FIG. IC.
  • This source 10 reduces the required parts count and integration complexity for instrument environments, by integrating relatively high functionality into a single component by reducing or eliminating the need for separate optical elements and providing an output illumination cone which is compatible with standard thin film interference filters.
  • FIG. ID shows an IR source 10' which is similar to source 10 but includes an "M"-shaped filament 11', and a formed-sheet reflector 12'.
  • IR source does not require extensive tooling or automation.
  • the source utilizes a laser cut filament cut from thin self-supporting metal foils which are treated in large (e.g., 10 cm square) sheets. Again, preferably, the filament size is relatively small (for example, 250 parts per 10 cm square).
  • FIG. 2 shows another exemplary form of IR source of the invention, particularly including straight parabolic, or compound parabolic collimators configured in combination with a textured source.
  • the embodiment of FIG. 2 shows a single loop, single bar filament with a "behind the source” concentrators.
  • FIG. 3A shows a "two- sided” emitter with a "behind the source” concentrator.
  • FIGS. 3B-3D show exemplary filament emitters.
  • FIG. 4 shows a "flat pack" construction source which allows a variety of reflector configurations. By utilizing double sided emission, the filament effectively doubles its active area and provides twice the usable in-band signal flux at a given temperature.
  • filament configurations provide additional "shine-through” illumination for rear-mounted reflectors, for example, as shown in FIGS. 5 and 6.
  • the filament is "sideways", i.e., the reflector axis is in the plane of the filament.
  • the inco ⁇ orated PCT Application No. PCT/US98/25771 discloses IR sources with metal foil filaments, and particularly shows an embodiment in a Tx metal transistor can. That form of source effects spectral control, and teaches use of spectral control to construct an incoherent narrow band emitter. In the prior art, the emitting surface has been modified to make it dark, but significant spectral control has not been accomplished. In accordance with the present invention, there are sha ⁇ spectral transitions between highly absorbing, in band, and highly reflecting, out of band, thereby effecting a band emitter. Traditional incoherent sources are constrained by the blackbody curve. The blackbody curve assures that, even if the source temperature is brought to peak with the blackbody distribution at the center of the measurement band, only a small fraction of the total radiant output falls within the desired measurement band.
  • the emitter surface (and the consequent surface spectrum) is shaped and textured so that the blackbody curve helps to define a band emitter.
  • this allows an instrument designer to trade out-of-band photons (noise) for in-band photons (signal.)
  • This is effected with a highly reflecting surface having an array of absorbing cavities thereon.
  • the size, shape, and spacing of these cavities are established to absorb at a pre-selected resonance wavelength.
  • the size of these cavities is preferably about l/2p for the desired wavelength. For wavelengths in the MWIR and/or LWIR gas bands, these sizes are well within the limits of current microlithography and microfabrication techniques.
  • the blackbody curve naturally rolls off more steeply on the short wavelength side, so it naturally defines the short wavelength band edge, even if the surface emissivity does not exhibit a sha ⁇ short wavelength cut-off.
  • the small surface texture features which make the infrared source of the invention work are preferably made by a random seed texturing process. These features make the surface a selective emitter with full blackbody-like emissivity at short wavelengths but little or no emissivity at longer wavelengths.
  • Spectral selection is determined using random textured infrared emitters for gas detector applications.
  • a non-blackbody, narrow-band radiator replaces the function of a traditional interference filter and dramatically improves the efficiency of the surface as an in- band emitter.
  • the ratio of usable bandpass flux-to-total flux maximized, and therefore maximizing instrument sensitivity.
  • Sub-micron scale surface structures control the radiator emission spectrum.
  • the function of the blackbody radiator surface is combined with the function of the bolometer element, and together with some of the function of a thin film interference filter into a single component. To achieve device miniaturization, it is important to achieve tight spectral control of the infrared emission. This enables maximum infrared abso ⁇ tion signal with minimum parasitic heating of the neighboring detector element.
  • the infrared emitters for gas detection are preferably made using random seed texturing of the radiator surfaces.
  • FIG. 7 shows spectral irradiance (relative units) as a function of wavelength for a textured metal foil filament of the invention, as compared with a conventional 575CBB filament.
  • the exemplary metal foil radiator displays spectral narrowing dl/1 - 0.5 (FWHM), by random texture. A mixture of small and large surface feature sizes accounts for the spectral width. Lithographic techniques are used to produce well controlled surface features for narrow emission waveband
  • An exemplary emission device is based on the mask pattern shown in FIGS. 7A(a) and 7A(b).
  • the patterns in the center of the mask labeled 8, 10 and 12B or N are crosses, either broad (B) or narrow (N) as shown in the detail. Numbering corresponds to the wavelength (i.e., 8, 10, or 12 microns) of the peak abso ⁇ tion.
  • Other patterns can be annular rings and tripoles.
  • the patterns are etched into polished silicon wafers, 1-3, ⁇ m deep.
  • thermal emission from photonic bandgap surface structures show distinct peaks with a wavelength proportionally to the geometry of the structure. This permits sensor-on-a-chip configurations, for example, where thermal radiation from a "designed" textured surface can be concentrated into a narrow band with low values of ⁇ / ⁇ .
  • the sensitivity of the detector approaches theoretical limits.
  • FIG. 7B shows emission spectra from aluminized patterned silicon surface heated to 500°C showing distinct peaks with wavelength varying as geometrical scale factor. In that figure, all sites on the wafer were heated to the same temperature.
  • the surface structure causes sha ⁇ emission peaks which appear over a spectral range of nearly an octave.
  • FIG. 7B shows a curve fit to the date from FIG 7B for a cross design as shown in FIG. 7B.
  • the upper (black) curve illustrates the emission spectrum from an ideal blackbody at 773 K.
  • the lower curve is the semi-empirical thermal model used for estimating instrument signal to noise. It consists of a graybody (with emissivity of 0.48) at a temperature of 773 K, plus excess emission at a peak wavelength of 6.89 microns and an emission width of ⁇ / ⁇ 0.16.
  • the upper curve in FIG. 7B shows a measured emission data, demonstrating a short wavelength cutoff. Random ion-beam produced texture provides devices with long wavelength cutoff.
  • the background blackbody emission from the silicon wafer has a relatively high emissivity of at least 0.48 whereas the emissivity of a good alv ⁇ minum film is below 0.1. Patterns with a larger fraction of open area (and hence cleaner coatings) show sha ⁇ er emission peaks.
  • FIGS. 7D(a) and 7D(b) show emission wavelength versus etched cavity size (measured as width of cross' leg) and versus cavity-to-cavity spacing. For a 4.65 ⁇ m emitter, the cross' leg is about 1.45 ⁇ m wide with center-to-center spacing of about 4 ⁇ m.
  • Substantially regular patterns on thin foils are formed by plating or depositing metal films.
  • a silicon substrate is used as a mechanical support for the foil during subsequent processing.
  • Subtractive processes based on deliberate, lithographic seed island formation are used with ion beam surface texturing processes. Lithographic masking and exposure schemes are used to achieve the required patterning.
  • Typical lead solder glass materials, used for fabrication of thick film circuits have a low sputter removal rate, and they are screen printed and chemically removed after processing.
  • Diamond-like carbon films, deposited by chemical vapor deposition have extremely low removal rates but are not easily patterned.
  • Typical layer thicknesses of the solder glass are a few thousandths of an inch (50-100 mm) and this is comparable to the resolution achievable with screen printing equipment.
  • Conventional microlithography for electronics can produce lines and spaces at least an order of magnitude smaller (two orders of magnitude for state-of-the-art VLSI processes) although the ability to faithfully transfer a pattern to the metal foil is somewhat limited by the thickness of the mask layer. Since the typical size of the pillars which comprise the surface texture is around a micron, micron scale antenna replication is used.
  • the following method places a regular array of micron-scale structures of a refractory metal oxide on the surface of high TCR self-supporting metal foils.
  • the resultant structure has a surface which is a selective infrared absorber in itself or alternatively, is an improved starting material for texturing using ion milling techniques.
  • the steps listed are conventional photolithographic procedures.
  • Thin metal foils are deposited on silicon wafers for process compatibility and ease of downstream handling.
  • self-supporting silicon microbridges are used as the individual emitter elements.
  • the drilled/milled area of the disc is coated with a film of positive photoresist, for example, applied with a clean brush, and the foil is placed over the holes and pressed or rolled into good contact with the disc. Effectively, the holes in the disc are covered, the disc is then held down on a vacuum chuck for the next step.
  • a film of positive photoresist for example, applied with a clean brush
  • Positive photoresist is next applied to the entire disc, using a conventional spinner followed by a conventional soft-bake step.
  • the ramp rate up to baking temperature is maintained to be relatively low to allow solvent in the resist layer which bonds the foil to the disc to be released slowly and thus avoid lifting the foil from the disc.
  • a resist formulation designed for use with reflective substrates is useful for this step.
  • Masking, exposure, and development of the top layer of photoresist is performed in a conventional manner. Clear areas in the mask are positive images of the features to appear on the foil.
  • the hard-bake phase drives off solvents to make the resist film vacuum-worthy.
  • the assembly is then given a short exposure to an oxygen plasma in apparatus for ashing photoresist.
  • the objective is insuring adhesion of the deposited film.
  • Deposition of a refractory oxide film is next done by laser deposition, which allows maintenance of low substrate temperatures.
  • the refractory film covers the entire disc/foil assembly, contacting and, preferably adhering to the foil through the openings in the photoresist.
  • the coated disc/foil assembly is soaked in acetone (or photoresist stripper), in some cases with gentle ultrasonic vibration, to complete the lift-off of photoresist and overlying refractory film and free the foil from the supporting disc.
  • the foil is then cleaned and used "as is", or mounted in an ion mill for texturing as a self-seeded material.
  • the most desirable packaging alternative is to leave the metal foil intact on the silicon wafer.
  • the radiator elements are heated electrically for ready measurement of input power and conversion efficiency. In order to achieve this, the radiator elements are thermally and electrically isolated from their surroundings. A "window-frame" pattern of cut-outs in the silicon substrate is used so that the actual radiator elements are suspended on the remaining silicon, as shown in FIGS. 8 A and 8B(a)-8B(c).
  • FIGS. 8 A (ten steps, 1-10) and 8B(a)-8B(c) show the basic window-frame construction technique for forming individual radiator elements on a silicon die. Elements are left on the silicon wafer as a support during microtexturing and then silicon via's are etched from underneath to produce suspended radiator elements.
  • Wavelength selective bolometer surface Computer modeling is used to determine the optimum surface shape for desired emission characteristics, based in part on transmission through conductive screen and dielectric-mounted mask arrays. In such models, an incoming electromagnetic wave is considered to produce surface excitations (plasmons) in the metallic screen or grid, and this in turn produces the transmitted wave. The plasmons are considered to be excited directly in order to produce a wave emitted from a surface which has the desired frequency band filter behavior.
  • FIGS. 9 A and 9B show the conversion efficiency gains using lithographic feature design and fabrication.
  • Such IR sources illustrate a spectral narrowing of 0.5, which gives them almost a factor of four efficiency advantage over an ideal blackbody.
  • FIG. 9A the width achieved to date for filter transmission width, another factor of 4-5x in conversion efficiency is obtained (FIG. 9B).
  • the size and shape of the repetitive surface feature control the details of the emission spectrum: the peak frequency in the transmission band; the width of the transmission band; the location of the minima which define the band; the steepness of the intensity falloff.
  • a two-dimensional array with 90° rotation symmetry serves to ininimize polarization dependence of the effect, and variation of the details of the repetitive feature (replace a square unit mask by a plus-sign shape, e.g.) enhances desired spectral characteristics.
  • the resonant frequency of the transmitted wave scales linearly with the length of the unit cell in the grid, as expected by analogy with grating studies. Other details depend sensitively upon feature shape (as mentioned previously), the width of lines which create the feature, and feature-to- feature variations. Other features of a textured surface - height of surface features, wall profile shape have an effect, as well.
  • transmission line analogs comprise the models for prediction of emission from grids and periodic masks.
  • each element (unit cell) of the grid is modeled as a simple circuit consisting of a resistance, capacitance and inductance whose values are adjusted so that circuit current matches output field strength of the unit cell.
  • a grid then consists of a large number of identical circuits coupled together with capacitors (for capacitive grids, consisting of conducting elements out of contact with one another) or inductors (for inductive grids, consisting of continuous conductors as in a wire mesh.)
  • the collection of connected circuits implies an output current which are solved by techniques common in transmission line analysis. Analyses such as these are used in calculating transmission properties of grids and masks.
  • a first principle approach using Maxwell's equations, is used to establish the validity of this model, and to determine the appropriate R, C, and L values for a given unit element size and shape. The coupling values are similarly found.
  • the model is used to simplify analysis of the complete grid. When variation in the unit cell parameters is to be investigated, the analysis becomes more complicated, and the results are cast into a matrix inversion problem where standard computer techniques are employed.
  • Maxwell's Equations are used as a basis to predict the surface emission, and polarization independence, and then to calculate the emission spectrum of the unit surface cell. Assuming 90 degree rotation symmetry and an infinite grid size, this result is expanded to represent emission from a large surface. Then, small square grids (10x10 features, for example) with random feature-to-feature variation are modeled to determine a representation of the effects of fabrication artifacts and variations.
  • the invention further includes a turbulence-based infrared hydrocarbon leak detector which is radically simpler than conventional infrared abso ⁇ tion instruments.
  • a simple structured IR instrument is configured by building a tuned infrared source and conventional infrared detector into a single package.
  • digital signal processing techniques are used to overcome the DC and low frequency drift which characterizes most nondispersive infrared measurements.
  • the sensor does not provide an absolute concentration measurement for hydrocarbons but it is particularly sensitive to the high frequency "noise" caused by the turbulence accompanying changes in local gas concentration. This provides a robust leak detection capability for gases which are not normally present in the sampling environment. Combined with a simple reflector plate to define the gas sampling region, this sensor provides a rugged, reliable, field- and flight- worthy hydrocarbon leak detection capability.
  • FIG. 10 shows a novel, low-cost infrared hydrocarbon leak sensor using an integrated source and detector in an open path atmospheric gas measurement.
  • the sensor discards low frequency "signal” in favor of the high frequency "noise” associated with changes in local gas concentration.
  • the entire sensor engine mounts on a TO-8 transistor header.
  • Hot bolometer sensor (with and without separate filter) An aspect of the invention can be used to form an infrared gas monitoring component to build an integrated on-board exhaust NOx meter. Silicon micromachining technology is used to build a sensor which is radically simpler than conventional infrared abso ⁇ tion instruments. Infrared abso ⁇ tion instruments traditionally contain a source of infrared radiation, a means of spectral selection for the gas under study, an abso ⁇ tion cell with associated gas sample handling and/or conditioning, any necessary optics, a sensitive infrared detector, and associated signal processing electronics.
  • the invention simplifies and reduces the cost of an IR instrument by integrating the function of the infrared source and infrared detector into a single self-supporting thin-film bolometer element.
  • This element is packaged with inexpensive molded plastic optics and a conventional spectral filter to make a transistor- size "sensor engine.”
  • this sensor engine Combined with a simple reflector plate to define the gas sampling region, this sensor engine provides a complete gas sensor instrument which is extremely inexpensive and which will approach the sensitivity of conventional infrared abso ⁇ tion instruments.
  • FIG. 11A shows a novel, low-cost infrared gas sensor engine 100 using a heated bolometer element as both source and detector in an open path atmospheric gas measurement.
  • the bolometer element reaches radiative equilibrium with its surroundings at a slightly lower temperature if gas abso ⁇ tion frustrates light re-imaging on the element.
  • the compound parabolic concentrator defines a relatively narrow illumination cone and the passive reflector is designed to provide a pupil-image of the spectral filter onto itself.
  • the entire sensor engine can be mounted on a TO-8 transistor header.
  • FIG. 1 IB shows a detailed view of the sensor engine 100. Achieving tight spectral control of the infrared emission is important in making the heated bolometer element work well.
  • the device is particularly effective if the amount of radiation absorbed by gas molecules under study is made measurably large in terms of the overall thermal budget of the bolometer surface.
  • a tuned cavity band emitter is constructed with spectral resolution (dl/1) around 0.1, roughly the performance achieved to date with micromesh reflective filters. This raises the conversion efficiency to nearly 15% for the NOx problem.
  • This level of surface topology (and therefore spectral) control is achieved through micro-electro-mechanical systems (MEMS) technologies
  • FIGS. 11 A and 1 IB include a single bolometer which both emits and detects radiation.
  • Those configurations permit construction of devices, such as gas detectors, which are compact and employ a minimum number of component parts. In some instances, however, it is useful to construct such devices with separate radiations sources and sensors. Exemplary devices using separate radiation sources and detectors are shown in FIGS. 1 lC-1 II.
  • radiation source 102 and radiation detector 104 are mounted on a silicon die 106, facing a reflector 108 disposed across a test region 110. The reflector 108 is positioned to effect an optical path between the source 102 and detector 104.
  • the test region 110 is constructed so that an applied test gas flow passes between the source 102/detector 104 pair and the reflector 108, intercepting the optical path.
  • the silicon die 106 includes integrated processing circuitry 112 coupled between the detector 104 and a data output port 116.
  • An electrical control 118 is coupled to source 102 for driving source 102 to emit radiation, preferably infrared radiation with a spectral range including an abso ⁇ tion line of the gas to be detected.
  • FIG. 1 ID shows a configuration similar to that in FIG. 1 IC, but where the source 102 and detector 104 are housed in a closed chamber C, and the reflector (mirror) 108 is external to the chamber C, with the optical path between the source 102/detector 104 and reflector 108 passing through an optically transmissive window W.
  • FIG. 1 IE shows an embodiments similar to that in FIG. 11 C, but where the source 102 and detector 104 are mounted within an enclosure E disposed on a single can 120. The enclosure E establishes a test gas flow path therethrough, which intercepts the optical path between source 102/detector 104 and the reflector 108.
  • FIG. 1 IF shows an embodiment wherein a source 102 is disposed opposite to a detector 104, wherein a collimating lens 122 establishes a collimated radiation beam through a test gas flow to a lens 124. Lense 124 focuses the resultant beam onto a detector 104.
  • FIG. 11G shows a source 102 which is activated by an optical beam B directed from a high energy laser L (by way of lens 132, reflector 134 and lens 136).
  • the source is activated by an optical beam B directed from a high energy laser L (by way of lens 132, reflector 134 and lens 136).
  • 102 is back-illuminated, so that emitted radiation propagates from an emission surface toward a reflector 108 A, which in turn directs that radiation across a test gas flow to a reflector 108B.
  • Reflector 108B focuses the radiation incident thereon to a detector 104 which is coupled to a data output port 116.
  • FIG. 11H shows source 102 and detector 104 on opposite sides of a support, each facing a respective one of reflectors 108 A and 108B.
  • Source 102 is electrically driven to emit radiation which propagates to reflector 108A.
  • Reflector 108 A directs the radiation incident thereon across the test gas flow to reflector 108B.
  • Reflector 108B focuses the radiation incident thereon to the detector 104.
  • FIG. 1 II shows source 102 and detector 104 mounted on the same substrate S, and opposite to a reflector 108.
  • a lens 142 directs radiation from source 102 to reflector 108.
  • Reflector 108 directs radiation incident thereon to a lens 144, which in turn focuses that radiation onto detector 104.
  • An optically transmissive test gas cell 148 (containing a gas to be identified) is disposed between the lenses 142 and 144 and the reflector 108.
  • Detector 104 provides via data out port 116, a signal to a reference data comparator 150, which compares a received signal (from detector 104) to reference information, for example to permit identification of one or more components in the gas in test gas cell
  • An individual emitter die is packaged, together with individual infrared detector pixel elements and thin film interference filter windows in TO-8 transistor cans using standard process equipment.
  • Drive and readout schemes using a microprocessor controlled, temperature- stabilized driver are used to determine resistance from drive current and drive voltage readings. That information shows that incidental resistances (temperature coefficients in leads and packages and shunt resistors, for instance) do not overwhelm the small resistance changes used as a measurement parameter.
  • a straightforward analog control circuit, the Wheatstone bridge is used to perform that function. It is very simple, very accurate, quite insensitive to power supply variations and relatively insensitive to temperature.
  • the circuit is "resistor" programmable but depends for stability on matching the ratio of resistors.
  • an adjacent "blind" pixel - an identical bolometer element filtered at some different waveband - is used as the resistor in the other leg of the bridge, allowing compensation for instrument and component temperatures and providing only a difference signal related to infrared abso ⁇ tion in the gas.
  • An optics test bed is used to evaluate different configurations and perform measurements of the device under benchtop conditions.
  • the device is operated as instrumented tube furnaces and to calibrate the infrared readings against a conventional gas analyzer.
  • the Wheatstone bridge provides a simple computer interface and since it is implemented with relatively robust analog parts is not susceptible to radiation damage at high altitudes or in space.
  • FIG. 13 shows a test configuration. Turbulence detection and signal processing
  • Emitter elements are used with filtered pyroelectric detectors.
  • a D converters are used to collect and analyze the data and optimize the signal processing sequence.
  • thermopile detectors Although the high frequency components are difficult to see in the power spectral density plots, the thermopile detectors used have a poor high frequency response. Lead selenide and pyroelectric detectors have a much better high frequency response than the thermopile detector.
  • FIG. 14A shows a typical nondispersive infrared gas sensor test bed including infrared radiator and detector, suitable filters, and a small environmental chamber for performing thermal cycle and gas flow tests.
  • FIG. 14B shows a typical signal processing stream for data reduction.
  • FIG. 15 shows measured data from a pulsed-source and thermopile detector combination, illustrating the high (20 - 40 Hz) ripples associated with changes in gas flow. Long considered a problem for IR gas detectors, this characteristic is used to exploit this "noise" as the primary "signal" for a leak detector.
  • a tiny, self-referencing band emitter is used for absolute radiometric calibration of infrared spectral channels.
  • This simple, rugged device can be inco ⁇ orated directly into a radiometric instrument, or into a field test kit.
  • Microelectromechanical (MEMS) fabrication techniques are used to produce a photonic band gap infrared emitter surface with a narrow emission band tuned to the infrared spectral channel under test.
  • a discrete single-channel (less than 1mm active area) radiometric calibration element is used.
  • This configuration is stable, repeatable, and has high power conversion efficiency and out-of- channel rejection.
  • the configuration can provide a transistor-sized multi-pixel, multichannel hyperspectral calibration tool.
  • a simple built-in radiometric and wavelength calibration device is used for hyperspectral imaging systems.
  • a silicon bridge emitter element has a photonic band gap (PBG) surface tuned to emit over a narrow infrared waveband. With very low thermal mass and high temperature coefficient of resistivity (TCR), this element acts as its own thermistor for self-referencing, electronic feedback control.
  • PBG photonic band gap
  • TCR temperature coefficient of resistivity
  • FIG. 16 shows a simple, built-in calibrator for hyperspectral imaging systems, using silicon micro bridges with a photonic band gap surface structure to achieve narrow band emission and absolute feedback temperature control in the infrared.
  • Passive infrared spectra contain critical information about chemical make-up while broad-band infrared images provide information on the location, presence, and temperature of objects under observation.
  • Hyperspectral imaging the ability to gather and process infrared spectral information on each pixel of an image, can ultimately provide two dimensional composition maps of the scene under study and this data is critical for ecosystem inventory and status monitoring.
  • These studies routinely require data in the near- and mid- infrared spectral bands, over large areas, at a reasonably high resolution suited to ecological studies.
  • Data goals include complementary surface moisture and surface (vegetation) temperature data taken in registration with the shorter wavelength data (allowing precise and straightforward correlation with LWIR data) for plant coverage, deforestation, and plant health monitoring studies.
  • Hyperspectral imaging can provide surface temperature and humidity mapping, at a resolution compatible with the shorter wavelength systems, over inaccessible regions where this data is not available from ground monitoring stations or other sources.
  • Hyperspectral image measurements are difficult to make in the field since the instruments must operate unattended (or with minimal attention) in hostile environments, over extreme field temperature and humidity ranges. Much of the important irradiance information is in the 1-3 mm spectral band where low signal and drift in infrared sources and infrared detectors make stability and calibration difficult. In fact, the standard NIST FAL, T-10 reference bulbs generally used for laboratory calibration have very low signal in this waveband. But the atmospheric research community has made steady progress in the accuracy and reliability of these measurements and there is now a broad consensus on absolute standards for bolometric radiometry established through bi-annual international inter-comparisons between instruments used by various groups and a gold triple point standard. In general terms, the state of the art for absolute accuracy in these measurements has progressed to around 0.3% (3s).
  • a miniature, electrically-modulated multispectral line source weighing a few grams and occupying a fraction of a cubic centimeter provides the solution to this problem.
  • the heart of this source is an array of photonic band gap narrow band emitter pixels. Each pixel radiates only within its designated spectral band and achieves temperature stability by heating and cooling on a time scale much faster than thermal diffusion.
  • the IR source of this aspect of the invention because it is based upon a few-micron-thick textured emitter element, does rely on its thermal mass for stability, but instead depends upon a stable electronic drive supply.
  • Miniature IR sources of the invention rely upon electrically heated filaments of such low thermal mass that their temperature at all times correlates precisely with the current flowing through them. They can be heated (and cooled) in milliseconds, delivering a current-following temperature profile exactly matching the drive pulse. Because they rely on electrical pulse shaping and current control rather than thermal mass for stability, these miniature sources have several important advantages.
  • an infrared resolution tester which is a thermal version of the familiar resolution test patterns for visible band optical systems.
  • the integrated resolution tester based on this device is light-weight, low power, and responds rapidly to rum on turn off commands.
  • This device is mounted on a temperature controlled stage and demonstrates performance with collimating optics.
  • This device is a flightworthy resolution tester for field and flight monitoring of infrared imaging systems.
  • MTF modulation transfer function
  • contrast contrast as a function of spatial frequency.
  • MTF modulation transfer function
  • FIG. 17A shows a resolution test pattern
  • FIG. 17B shows a resolution tester.
  • thermoelectric hot/cold stage By using ion beam lithography techniques, alternating bands of very high and very low emissivity are established on a lightweight, high conductivity metal foil surface. Using a tiny thermoelectric hot/cold stage to uniformly vary the temperature of this test pattern permits simultaneous radiometric and resolution tests. Modem high-performance IR systems have narrow fields of view, consequently long focal lengths and very high resolution. This complicates the problem of designing a built-in resolution tester, since calibration objects must appear to be very far away. In a conventional test arrangement, a collimator is used to project the image of the blackbody source with a slit pattern onto the aperture of the system under test.
  • the collimator acts as one lens and the fore-optics as another; the magnification of this two lens system is the focal length ratio of the two lenses.
  • the test object of the invention has extremely fine lines and spaces, as the key to calibrating high resolution systems with a device of modest size.
  • the invention includes a three- element, germanium f 1 lens system adapted from the "IR microcam,” to build a hand-held external test device.
  • FIGS. 18A and 18B shows the IR ⁇ cam lens as a 3-element germanium f/1 system. It has a 25mm aperture. It provides night vision capability for hand-launched unmanned aircraft (HL-UAV). This optical train is the basis of a hand-held resolution tester.
  • FPA Health Check Source HL-UAV
  • gain and offset in the AIT seeker depend on the local temperature (and pixel-to-pixel temperature variations) on the focal plane array.
  • One way to compensate for gain and offset variations is to use a flat-field IR reference source to provide uniform illumination on all pixels before (or during) flight. Such a source would also be extremely useful for an FPA health check on a Dem-Val flight.
  • the metal texturing technology developed by the M&S program makes it possible to build extremely compact, lightweight infrared reference sources.
  • Another aspect of the invention is an MWIR reference source for the BMDO STRV-2 mission.
  • the STRV-2 source is very small (6mm diameter x 3mm thick) since it was designed to go on the filter wheel very close to the FPA.
  • FIG. 19A shows a prototype reference source for STRV-2, mounted in an existing filter wheel slot for rninimum design and schedule impact. This source provides flat-fielding to correct pixel-to-pixel offset variations.
  • FIG. 19B shows exemplary sources adjacent to a metric.
  • This simple modular device provides a flat field warm-body reference with no parasitic heating of the optical train and it will allow correction of pixel-to-pixel offset variations on the focal plane.
  • infrared spectrometers and spectrographs of the prior art use cooled infrared detectors as well as separately-aligned optical elements. Cooling the detectors requires added bulk and heavy equipment, such as closed cycle refrigerators, cryogenic liquid, and thermoelectric coolers with large power supplies. Maintaining optical alignment is also a challenge if the instrument is to be subjected to environmental stresses.
  • Infrared spectroscopy for chemical analysis has typically used laboratory grade spectrometers, spectrographs, or Fourier Transform Infrared (FTIR) instruments. These instruments can be as small as a cigar box and as large as a table top. They generally consist of entrance and exit slits, reflective mirrors and a reflective diffraction grating made of glass which may or may not be rotated to scan the wavelengths of interest.
  • the FTIR for example, generally uses a Michelson interferometer with a moving mirror in one interferometer leg and a fixed wavelength reference such as a laser to scan the wavelengths.
  • external equipment including power supplies, scan motors and associated control electronics, and of course a cooled infrared detector.
  • an infrared chemical analysis instrument developed by Foster-Miller uses an FTIR about the size of a breadbox with a liquid nitrogen-cooled mercury cadmium telluride (HgCdTe) detector.
  • HgCdTe liquid nitrogen-cooled mercury cadmium
  • MMS 1 Monolithic Miniature-Spectrometer
  • the MMS 1 uses a APPENDIX - - PCT APPLICATION NO.
  • PCT/US99/07781 conventional silicon photodiode array and a cylinder of glass with an integral imaging grating.
  • PCT/US99/07781 conventional silicon photodiode array and a cylinder of glass with an integral imaging grating.
  • UV ultraviolet
  • the detector is also positioned away from the waveguide, presenting certain optical alignment difficulties, particularly under environmental stresses.
  • fabrication of the waveguide and grating is done by a process called LIGA (which stands for the German words for Lithography, Galvanoformung and Abformung) which uses a three-layer photoresist and deep-etch X-ray lithography.
  • LIGA which stands for the German words for Lithography, Galvanoformung and Abformung
  • Another object of the invention is to provide apparatus that solves or reduces the above-described problems in the prior art.
  • Another object of the invention is to provide a hand-held, monolithic, rugged IR spectrometer.
  • a further object of the invention is to provide methods of manufacturing uncooled IR spectrographs.
  • Yet another object of the invention is to provide a process of isolating chemical species over a large infrared band with a monolithicaily-constmcted IR spectrometer.
  • the spectrograph of the invention overcomes many of the problems in the prior art by using uncooled detectors, e.g., microbolometers, which eliminate the need for associated cooling equipment. Other detectors such as PbSe or PbS are also suitable.
  • the spectrograph of the invention is monolithically constructed with a single piece of silicon, which eliminates the need for alignment and which makes the device inherently rugged and light weight.
  • standard silicon microelectronics technology also makes the invention low-cost, as compared to the prior art. By way of example, these costs can be orders of magnitude lower than the conventional spectrographs, i.e., hundreds of dollars instead of tens of thousands of dollars.
  • the invention includes a solid optical grade waveguide (similar to a slab of glass) coupled to a line array of detectors.
  • a source e.g., earth emissions transmitted through gases
  • a source e.g., earth emissions transmitted through gases
  • the waveguide is silicon to transmit IR light; and the detector array is one of microbolometers, PbSe, PbS, or other IR sensitive detectors.
  • the waveguide is transmissive to visible light and the detectors are CCD elements.
  • the waveguide is ZnSe and the detectors are "dual band" so that visible and IR light energy are simultaneously captured at the detectors.
  • each pixel of the dual band detector can include a microbolometer and a CCD element.
  • the invention has several advantages over the prior art. First, it monolithically integrates all parts of the spectrometer, including the detector, thereby making the APPENDIX - - PCT APPLICATION NO. PCT/US99/07781 instrument compact, rugged and alignment-free. Further, by fabricating the entire device on one piece of silicon, using silicon micromachining technology, we take advantage of the thirty years of silicon process development, and the emergence of silicon as a commodity item, to enable the fabrication of the instrument at a very low cost.
  • spectrometer constructed according to the invention include: (a) the ability to sense and identify chemicals i n a variety of applications and environments, (b) the ability to determine chemical concentrations, (c) operation with low-cost, low power, and long-term unattended operation or calibration, (d) uncooled operation, without expensive and unwieldy cooled detector elements, and (e) a rugged, alignment-free instrument.
  • Figure 1 shows a perspective view of a spectrometer system of the invention
  • Figure 2 illustrates optical bench features of the waveguide, optical path, and detector of the system of Figure 1
  • Figure 3 shows a schematic layout of the optical trace of the system of Figure 1.
  • one embodiment of the invention includes a spectrometer system 10 with the following elements: a silicon block waveguide 12, a cylindrical mirror 14 (e.g., gold-coated), a diffraction grating 16 (preferably gold-coated), and a linear detector array 18 (e.g., a microbolometer or microthermopile array).
  • Electronics 20 can couple to the array 14 so as to collect electronic data representative of the spectral characteristics of the light 22 entering the system 10. Control of the system 10 is obtained through user interface 24.
  • a battery 26 can be used to power the system 10.
  • the system of Figures 1 and 2 can further include detector preamps 30, a multiplexer 32, input focusing lenses 34, a microprocessor 36, an LCD display 38, keys 40 on the interface 24, and a protective housing 42.
  • the reflective flat mirror folds the path of the injected light so as to achieve a fiber-to-grating vs. grating-to-detector distance ratio of 2:1 or more. This is important if the input light comes from an infrared fiber with a typical core diameter of 2 00 ⁇ m yet we are focusing the light onto a detector array with 50 ⁇ m wide pixels. To focus the light effectively with only one concave surface, fiber-to-grating vs. grating-to-detector distance ratio is directly proportional to the ratio of entrance and exit spot sizes.
  • the concave grating disperses the light into its component wavelengths (shown in Figure 3 as light path 40) and focuses it onto the detector planel 8.
  • each wavelength has a different focal length. Therefore, the focal points for the various wavelengths will lie along the circumference of a circle that includes the grating. This circle is called the Rowland circle and is well-known by those skilled in the art of optics.
  • chi ⁇ ing the grating i.e., varying the line spacing, flattens out the focal points for the various wavelengths almost completely.
  • the detector plane 19" ( Figure 3) is where the linear detector array 18 is located and the point at which the grating 16 focuses all wavelengths.
  • the linear detector array 18 converts the dispersed photons into electrical signals for each wavelength.
  • Anti-reflection coating(s) should be used on either or both the input plane and the detector array plane. By suppressing reflection at the interface bekveen high index of refraction silicon and the ambient environment, we ensure that the maximum number of photons are available for detection.
  • the invention can be used in applications where infrared analysis for chemical identification and concentration measurement is desired, especially in applications which require low cost, expendable sensors; low power sensors; or uncooled sensors. Particular applications may include detection and measurement of hazardous and pollutant gases, polluted water, chemical analysis of core samples, rocket plumes, smokestacks, biologic and chemical warfare agents, etc. Appendix A shows and describes other features, embodiments, aspects and advantages of the invention.
  • the proposed spectrometer is small, lightweight, low power, and can classify rocks on a large range of spatial scales
  • An integrated infrared illuminator will allow operation as a reflectance instrument for close inspection of small areas of rock or soil Alternatively, larger areas can be analyzed from a distance
  • the signal-to-noise ratio will range from 200 to 4800, depending on the wavelength and mode of operation.
  • the existing breadboard instrument combines three key technologies 1 ) a compact silicon waveguide spectrometer developed by Ion Optics, Inc., 2) high performance, broadband, uncooled thermopile detector linear arrays developed at JPL, and 3) a custom thermopile readout chip developed by Black Forest Enginee ⁇ ng Further improvements in all three areas, combined with compact packaging, will result in a small, rugged instrument having sufficient sensitivity to characterize rock and soil mineralogy and allow sample selection for subsequent return to Earth The enginee ⁇ ng model produced in this effort will be, with minor modifications, ready for flight qualification.
  • Alpha backscatter provides an excellent measurement of elemental composition
  • Infrared spectroscopy is suited for indicating the mineralogy of large areas and the large-scale variance of mineralogy in rocks, but can only provide precise measurements of composition and/or precise classification of mineral types in favorable mineral mixtures
  • Combining the techniques provides a very powerful m-situ tool set ideal for selecting the few very precious samples returned to Earth
  • it is necessary that each technique utilize a minimum amount of resources with respect to cost, mass, energy, volume, and data rate We propose here a micro infrared spectrometer that meets these criteria.
  • Infrared spectroscopy provides a method for readily distinguishing between classes of rocks and mapping variations in mineralogy within rocks This technique is most effective when used in association with techniques that provide definitive composition and mineralogy for individual grains in rocks Depending on the wavelength region, infrared spectroscopy can provide estimates of the S ⁇ 0 2 content, the degree of hydration of minerals, and can distinguish between va ⁇ ous clays and evapo ⁇ tes Infrared measurements also can provide very quantitative information in favorable circumstances, such a when a Christiansen peak is present. The technique is also suitable for estimating particle size both from scatte ⁇ ng properties and from thermal emission measurements Reflectance measurements penetrate to a few wavelengths of light, so are most diagnostic for freshly exposed surfaces Thermal inertia measurements penetrate to the depth of the diumal thermal wave
  • Fig. 1 From the Aster catalog, illustrate that the spectral region this instrument spans is suited for discriminating between a wide range of geological samples which may be postulated for Mars Note that the spectra in Fig. 1 are in reflectance units, and that some of the ordinal axes have been scaled differently. Fig.
  • spectral range, spectral resolution, and signal-to-noise requirements for a spectrometer can be estimated from Fig. 1 (the figures are scaled). There is always a push for higher resolution and sensitivity, but it is clearly possible to distinguish between a wide variety of minerals, and, to a lesser extent, the composition within classes of minerals at a modest resolution of about 0.1 micron. Similarly, a signal-to-notse ratio of 30 is sufficient to discriminate between mineral classes in most cases.
  • the proposed solid state spectrograph has a spectral resolution ranging from 0.05 to 0.1 microns, and signal-to-noise ratios ranging from 200 to 4800.
  • Our miniature solid state spectrograph has several modes of operation. These include 1) a self-illuminated contact mode, suitable for classifying grains in rocks as small as 1x0.1 mm 2) a distant mode suited for characterizing large rock and locations by measuring spectra integrated over areas up to 1-2 meters in diameter.
  • Reflectance and emissivity measurements sample surfaces to a depth which is at best a few wavelengths of the light being measured.
  • the measurements are best performed on fresh rock or soil surfaces, such as those exposed by trenching, scraping, or drilling.
  • the interpretation of measurements made in this wavelength region are complicated by the crossover between reflected and emitted light (for Mars surface temperatures)
  • ground measurements including measu ⁇ ng samples at different temperatures (different times of day), measuring samples in shadow (though scattered light complicates this), and by measuring radiation from a sample with and without illumination.
  • Infrared measurements can be made quickly, using few resources, and are ideally suited for determination of the crystal field and the type of molecules cont ⁇ butmg to an absorption Quantitative measurements of elemental composition are better obtained using other techniques
  • the small size and cost of the proposed infrared spectrograph leave sufficient mass, power, space and budget to include complementary techniques.
  • the instrument will be compact, lightweight, and low power and will cover the spectral range 3-12 ⁇ m.
  • the complete unit will be roughly 4 cm by 3 cm by 2 cm in size, weigh roughly 100 g, and consume about 200 mW of power.
  • the spectral resolution will be 0.05 ⁇ m in the 3-6 ⁇ m range, and 0.1 ⁇ m in the 6-12 ⁇ m range.
  • the device can operate as a reflectance instrument (contact mode) for close inspection of small areas of rock or soil, with a signal-to-noise ratio ranging from 600 to 4800.
  • the spectrometer can operate in survey mode, passively measuring rock and soil spectra in both the solar reflectance and thermal emission spectral regions. In survey mode it will look from a distance at a large area and will have a signal-to-noise ratio ranging from 200 to over 1000.
  • An existing breadboard instrument shown in Fig. 2, combines three key technologies: 1) a compact silicon waveguide spectrometer developed by Ion Optics, Inc., 2) high performance, broadband, uncooled thermopile detector linear arrays developed at JPL, and 3) a custom thermopile readout chip developed by Black Forest Engineering. A spectrum taken with the silicon waveguide is shown in Fig. 3. Further improvements in all three areas, combined with compact packaging, will result in a small, rugged instrument with sensitivity sufficient to characterize and select samples for subsequent return to Earth. The engineering model produced in this effort will be, with minor modifications, ready for flight qualification.
  • Silicon waveguide spectrometer and housing passes through air and a 3.38 ⁇ cut-on filter. developed by Ion Optics with NAS SBIR funding. The midpoint of the filter is at channel 11. Channel 30 shows a C0 2 absorption at 4.26 ⁇ m.
  • Fig. 4 The existing breadboard instrument, which operates in the 3-5.5 ⁇ m range, is shown schematically in Fig. 4. It was developed by Ion Optics with funding from NASA SBIR phase II contract NAS7-1389. At the heart of this breadboard is a D-shaped silicon waveguide 1 mm thick and roughly 5 cm by 3 cm across. The unit uses a conventional Ebert layout, with the optical elements on the edges of the silicon waveguide. Infrared radiation is focused by an external lens onto an entrance slit defined by gold coatings on the silicon edge. Radiation traverses the silicon to a curved mirror on the opposite face, then the light is dispersed by an ion milled diffraction grating and focused by a second concave mirror.
  • thermopile infrared detectors After exiting the silicon waveguide on the flat edge, the spectrum is detected by a linear array of thermopile infrared detectors.
  • Figure 4 Schematic drawing of breadboard 3 to 5.5 ⁇ m spectrometer
  • Infrared light from the target is focused onto a slit by an external lens.
  • the angular spread is reduced due to the high index of refraction of silicon.
  • Inside the silicon slab light is reflected from a concave mirror, dispersed by a diffraction grating, and imaged with a second mirror. The dispersed light is detected by an external linear detector array.
  • the components themselves incorporate a number of advanced design features.
  • the slab waveguide is formed to include the other components (stops, mirrors, and grating), including A R coated beam dump facets for unwanted diffraction orders. Because silicon is opaque to photons above its band gap, the slab excludes visible light from the detector. Silicon's high refractive index provides lossless reflections at the polished top and bottom faces of the device, acting as a waveguide to keep the radiation in the plane of the slab. Because of the high index of silicon, the cone of incident illumination is dramatically reduced upon entering the waveguide. This effect provides better optical throughput than can be achieved with an open-air spectrometer and overcomes many of the astigmatism issues associated with conventional designs.
  • Figs. 6a and 6b The modes of operation for this spectrometer, contact mode and survey mode, are shown in Figs. 6a and 6b.
  • contact mode illustrated in Figs. 5 and 6a
  • the rover arm presses the spectrometer against a rock or section of soil.
  • the frame holds the spectrometer at a fixed distance from the sample.
  • the sample is illuminated, and the reflected spectrum recorded.
  • the 3-6 ⁇ m and 6-12 ⁇ m waveguides view different rectangular areas roughly 1.5 mm apart. Each rectangular area is about 1 mm long and 0.1 mm wide, matching the spectrometer entrance slit.
  • survey mode illustrated in Fig. 6b
  • the spectrometer is pointed down or away from the rover, passively sensing reflected solar and thermally emitted radiation.
  • Survey mode analyzes large areas and can help to select interesting areas for further analysis. Long term drifts can be nulled by pointing the spectrometer into space APPENDIX -
  • Proposed instrument configuration shows infrared radiator with lens integrated into the instrument package.
  • the package will be smaller and more robust, with lower power consumption than the existing breadboard spectrometer.
  • the spectrometer can be configured in other ways.
  • the lens can be positioned to provide a fixed focus at longer distances, say 1 meter away. Then the spectrometer will be operated in survey mode while looking at fairly small areas of rock or soil Because of its compact size, this spectrometer can also be configured to fit down a bore hole to analyze composition at various depths, thus eliminating the problem of removing and analyzing intact core samples APPENDIX - - PCT APPLICATION NO. PCT/US99/07781
  • a prototype silicon waveguide spectrometer has been developed by Ion Optics, Inc. with phase II funding from NASA/JPL SBIR NAS7-1389. This waveguide has been integrated with 1 ) a thermopile array developed at JPL and funded by NASA Code S UPN 632-20, and 2) a readout chip developed by Black Forest Engineering as a subcontract to NASA SBIR phase II contract NAS7- 1899. Test results for each component are described below.
  • Fig. 2 shows the D shaped silicon waveguide mounted in a prototype spectrometer housing.
  • Initial tests with a broadband source were very promising.
  • the zero order diffraction spot is roughly the size of the 80 ⁇ m input slit, showing that the imaging components of the spectrometer are optimized. Very little stray light is seen between the zero order spot and the first order diffraction spectrum. Comparison of total light in the first order spectrum to the zero order spot yields a 47% grating efficiency.
  • An uncalibrated spectrum is shown in Fig. 3.
  • Radiation from the source passes through air and a 3.38 ⁇ m cut-on filter.
  • the midpoint of the filter can be seen at channel 11 ; channel 30 shows a C0 2 abso ⁇ tion at 4.26 ⁇ m.
  • the spectral dispersion is 0.46 ⁇ m per pixel, as predicted.
  • the total throughput of the silicon waveguide with input lens is estimated to be 15%.
  • Thermopiles are broadband, uncooled infrared detectors in the same class as bolometers and pyroelectric detectors.
  • a thermopile consists of several series-connected thermocouples running from the substrate to a thermally isolated infrared absorbing structure. Incident infrared radiation creates a temperature difference between the absorber and the substrate, which is measured as a voltage across the thermocouples.
  • Thermopiles offer a simplicity of system requirements that make them ideal for some applications; they typically operate over a broad temperature range and are insensitive to drifts in substrate temperature, requiring no temperature stabilization. They are passive devices, generating a voltage output without bias. They do not require a chopper.
  • thermopile detectors can be supported by more simple, lower power, more reliable systems than either bolometers, pyroelectric or ferroelectric detectors. If thermopiles are read out with high input impedance amplifiers they exhibit negligible 1/f noise since there is no current flow. They typically have high linearity over many orders of magnitude in incident infrared power.
  • thermopile detectors on silicon substrates. These detectors are 1.5 mm long with a pixel pitch of 75 ⁇ m, matching the geometry of the current Ion Optics spectrometer.
  • Each micromachined detector consists of a 0.6 ⁇ m thick suspended-silicon-nitride membrane with 1 1 thermocouples composed of Bi-Te and Bi-Sb-Te.
  • a schematic diagram of a single pixel is shown in Fig. 7, while a photograph of the complete array is given in Fig. 8.
  • Detector responsivity is typically 1100 V/W at dc, and response times are 100 ms.
  • Detector noise has been measured at frequencies as low as 20 mHz, and the only noise source is Johnson noise from the 40 k ⁇ detector resistance.
  • Responsivity and response time have been measured as a function of temperature from room temperature down to 100 K. The total change in these quantities over this temperature range is roughly 20%, indicating that the detectors perform well over the entire range of Martian temperatures.
  • the signal readout must have a low noise figure for the given detector resistance and at the slowest measurement frequency of interest.
  • the total input-referred noise after amplification must be only slightly higher than the detector Johnson noise. Typically this goal is difficult to achieve at low frequency for low resistance detectors.
  • thermopile array is read out with a custom integrated circuit chip designed by Steve Galeema of Black Forest Engineering.
  • This sixty four channel device includes a preamplifier and filter for each channel, a multiplexer and a 12 bit A/D converter. Power draw is 55 mW.
  • the input uses a chopping scheme to dramatically reduce low frequency noise and drift.
  • the output is easily interfaced with a commercial processor chip.
  • Black Forest's chip was originally designed for use with a thermopile array having a detector resistance of 1300 ⁇ . Initial tests indicate that the completed chips increase the noise of a 1300 ⁇ source by a factor of two over the range 0.1-10 Hz. This performance is excellent for a CMOS circuit with such a low source impedance.
  • this readout chip is mated to the 40 k ⁇ JPL detectors. The mismatch between detector and readout resistance causes some degradation in the system noise performance.
  • Ion Optics' core product is thepu/ ⁇ R* radiator, shown in Fig. 9.
  • Ion Optcs supplies infrared radiator components, first demonstrated in a 1994 DOE SBIR program, for the commercial gas sensor OEM industry.
  • Ion Optics puldR is expected to be in nearly 10% of all non- dispersive infrared gas sensors buik in the United States.
  • the key to the operation of the pulstR* radiator is an extremely thin metal ribbon with a proprietary surfece treatment to improve the conversion efficiency of radiator thermal energy to in-band radiation.
  • the pulsTR! sources, with efficient, low thermal mass radiators and appropriate infrared windows (or windowless) convert significantly more electrical energy into useful, in band radiation than standard tungsten filament bulbs.
  • the objectives of this task are to substantially increase performance of the existing MWIR waveguide, and to make a similar waveguide in the 6-12 ⁇ m range.
  • a simultaneous improvement in the resolution and signal-to-noise performance of the existing spectrograph can be made by making the silicon waveguide less than one-half its present size These improvements can be achbved with a Littrow mount for the gradng, folding the spectrometer optbal path on itself and significantly reducing the size of the silbon waveguide (Fig. 10). This design disposes of unwanted diffraction orders; dumps for unwanted diffraction orders occupy much of the flat facet space in the breadboard spectrometer.
  • the shorter optbal path in the Littrow mount design results in smaller losses due to absorption of light in the silbon and its escape from silbon faces. Lessons learned during fabrication of the current waveguide will lead to a more efficient grating with fewer stray refbctions. Combining a Littrow mount design with an improved gradng fabrication process is expected to result in a factor of three improvement in both throughput and stray light rejection.
  • the current linear thermopile detector array will have to be redesigned and fabricated as a bi-linear array with the appropriate geometry for the proposed instrument.
  • thermoelectric films can be improved.
  • Film composition can be optimized by changing the sputter target composition in an iterative fashion. Depositing the films on warm (100 * C) substrates may improve the thermoelectric properties without overheating the photoresist used in lift-off. Optimization of the post- fabrication annealing procedure may also improve material properties.
  • Thickness of the silicon nitride membrane will be reduced from its current value of 6300 A to about 4500 A, decreasing thermal conduction paths to the substrate and improving responsivity and D* values. Response time will not increase because heat capacity is also reduced.
  • the device absorptivity will be increased from its current value of 50-60%.
  • this improvement can be achieved by depositing a reflecting layer of platinum on the underside of the membrane and a platinum layer with about 377 ⁇ /square on the top of the membrane.
  • These metal layers combined with the 4500 A silicon nitride layer between them, form an optical cavity with high abso ⁇ tion in the 3-6 ⁇ m range.
  • a gold or carbon black may be used.
  • a metallized surface in close proximity behind the array will increase abso ⁇ tion by allowing some of the lost light to reflect back onto the detectors.
  • Black Forest Engineering will design and test the readout circuits as a subcontract. Fabrication will be performed on multiproject wafers at a commercial foundry.
  • Thermopile detectors require vacuum for maximum sensitivity. Therefore, a hermetic package with an infrared window will be designed and fabricated to hold the silicon waveguides, detector array, and readout. The cover will be laser welded under vacuum. A surrounding dust- resistant case will enclose this package in addition to the radiation source, lenses and electronics. Within the dust-resistant case will be a commercial digital signal processor (DSP) chip and associated electronics for controlling the array readout chip. Packaging components will be designed with eventual flight qualification in mind.
  • DSP digital signal processor
  • the output from the miniature solid state spectrometer will probably consist of an asynchronous serial interface conforming to the EIA-RS-232 standard.
  • the data format is still to be determined, but the RS-232 standard allows the electrical interface between the instrument and its host to be specified independent of the data format. It will consist of a half-duplex, bi-directional, two wire interface. Power consumption can be minimized by reducing the data rate and terminating the lines with high impedance loads.
  • Silicon waveguide (all parameters characte ⁇ zed as a function of wavelength)
  • the complete system will be carefully characterized and calibrated in the laboratory.
  • the instrument response function and signal-to-noise performance will be characterized with blackbody sources in vacuum.
  • Wavelength characterization and calibration will utilize a second spectrometer to provide monochromatic light as well as calibration standards (such as indene and polystyrene) to verify band positions. Stability of the calibration will be tested over the range of temperature conditions expected on Mars (200-300K).
  • instrument performance Prior to testing on the rover, instrument performance will be validated on mineral and petrologic suites consistent with candidate surface materials expected on Mars. These samples will be measured in Mars-like conditions (C0 2 atmosphere, night and day temperature range) in the laboratory with artificial illumination. They will also be measured in ambient earth conditions with natural illumination. Furthermore, the instrument will be demonstrated in a variety of field sites which have good exposure of a range of petrographic types (such as evaporites, basalts, carbonates). In order to ensure that the instrument is available for rover integration, two or more spectrometers will be fabricated, permitting laboratory characterization and calibration to continue while one spectrometer is used in rover field tests. Making multiple copies of the instrument is expected to be straightforward.
  • the program's performance goals are (a) detector-readout assemblies with a system D * value of 2x10' cmHz ⁇ /W and (b) a total spectrometer throughput of 65% with an f/1 input lens. Section 2 describes performance of the existing instrument, while section 3 describes the tasks required to reach these much more aggressive performance goals.
  • noise equivalent radiance on the target within a spectral band equivalent to one detector (0.05 ⁇ m for 3-6 ⁇ m and 0.1 ⁇ m for 6-12 ⁇ m).
  • the noise equivalent in-band power at the detectors normalized to frequency bandwidth, is 1.68x10 " W/Hz" 2 .
  • Assurning an integration time of 10 seconds, noise equivalent in-band power at the detectors is 5.30xl0 ' 2 W.
  • the noise equivalent in-band input power to the spectrometer is 8.16xl0" 12 W.
  • Two f/l optics will image the target onto the slit with no magnification.
  • the target size is the same as the slit (75 ⁇ m by 1 mm), and the lens acceptance cone ys 0.84 steradians, resulting in a noise equivalent in-band radiance from the target of 1.29x10 W/(cm ster).
  • the signal-to-noise ratio is more than adequate for mineralogical surveys (signal-to-noise requirements are described in section 1 A) Higher signal-to-noise ratios can be obtained at the expense of spectral resolution by adding signals from groups of consecutive pixels. Increasing the integration time is another technique for increasing the signal-to-noise ratio.
  • FIG. 11 Projected signal-to-noise ratios for survey mode, showing sensitivity to reflected sunlight and thermal emission for typical day and night Mars temperatures. Integration time is 10 s.
  • the illuminator is a filament source produced by Ion Optics, which can be made with high emissivity over the entire wavelength range. Assuming 60 mW of electrical power to the illuminator, 60% efficiency in conversion from electrical power to APPENDIX - - PCT APPLICATION NO. PCT/US99/07781
  • the filament temperature will be 500 K
  • the 2 mm by 5 mm filament is imaged onto the target with no magnification by an f/1 lens with 90% transmission
  • the noise equivalent in-band radiance on target calculated above (1 29x10 W/(cm 3 ster)
  • results in the signal-to-noise ratios shown in Fig 12 In this mode of operation the signal to-noise ratio far exceeds the level required to meet the mission science goals (described in section 1A)
  • a higher input power to the source or a longer integration time can increase these values significantly
  • thermopile readout chips 100 mW commercial DSP chip for controlling readout chip 50 W
  • Characterizing the mineralogy and geology of Mars is important for understanding the processes that have shaped Mars.
  • This instrument can be used to differentiate between the mineralogy characteristic of igneous, metamo ⁇ hic, and sedimentary regimes
  • the silica content of basalts can be estimated and used to validate viscosities estimated from the mo ⁇ hology of flows.
  • Soil types and aggregations, as well as C0 2 content of the soil can be estimated using a combination of scattering, thermal, and spectral techniques. Soil layers can be measured (using all three techniques) in trenches carved by a rover arm and in modest size boreholes.
  • this miniature spectrograph are relevant for a large number of NASA's objectives for missions to Mars.
  • Many of the measurements that can be performed with this instrument are of fundamental importance for understanding Mars
  • these measurements can provide confirmation and redundancy for measurements performed by other instruments that may have higher specificity and ⁇ sk.
  • This instrument is particularly ideal because it is rugged enough to survive a range of landing techniques and consumes a small fraction of the resources available to a rover
  • the requested funding is $365 9k for FY '98, $356 5k for FY '99, and $197 Ik for FY '00
  • the approximate cost breakdown for each task is silicon waveguide development - $310k, detector APPENDIX - - PCT APPLICATION NO. PCT/US99/07781
  • phase 2 SBIR program with JPL, Ion Optics, Inc. is developing a tiny, integrated infrared reflection probe.
  • This device will provide fundamental chemical composition and density information which is important in satellite-based exo-biology and ecological studies. Perhaps the most striking application of this instrument is to extract composition information from soil and rock samples, which is critical to the search for evidence of planetary life on the Mars Pathfinder 2003 mission.
  • the phase 2 SBIR program wfll produce working prototype instruments, as an intermediate step towards developing and demonstrating a fully miniaturized, extremely low power flight instrument for the Mars 2003 mission.
  • Figure 1 Schematic of tiny infrared micro-probe arrangement for subsurface exploration on Mars.
  • the miniature solid-state spectrograph and pulsed infrared source allows spectroscopic measurement of spectral lines of specific contaminants as well as local measurements of thermal inertia and soil density.
  • Phase II prototype (inset) is now undergoing final integration and test for delivery in the first quarter of 1998.
  • Infrared spectral information is critical to understanding Martian geology and evidence of conditions for life.
  • Infrared spectrometers are particularly suitable for rapidly characterizing differences in mineralogy of a large number of rocks and extensive areas of soil. They are most powerful, in the in-situ environment, when used in combination with instruments which can unequivocally (though at a slower pace) identify elemental compositions and crystal structures.
  • IR spectrometers are also very useful for micro-thermometry and micro-calorimetry (powerful techniques for differentiating rocks) because a color temperature can be obtained.
  • the infrared region from 1 to 10 microns is particularly useful for detecting evidence of past aqueous environments. This is due to the multitude of bands in this wavelength region related to clays, evaporites, and carbonates.
  • the fabrication of the gratings required three steps, spin coating the photoresist onto the blocked up benches, exposing the hologram into the resist, and then transferring the hologram into me silicon via ion beam etching. All three of these steps required very careful preparation, testing and finally execution.
  • the photoresist spin required the fabrication of a special spin rig to handle the heavy and cumbersome silicon block.
  • the block was carefully cleaned to remove surface contaminant B ⁇ gure 22 Contaminants from the small spaces or from the edges of the pieces.
  • Special adaptations "cracks'' between adjacent waveguides destroyed the were made to the blocking fixture to allow for the original grating mask. differential expansion of the silicon and the aluminum block.
  • the hologram required extensive exposure testing and precise alignment on the optical table m r °!f ,? S ? rc made ° n ⁇ M pieCCS md ion ctch rates determined. Finally the exposure was a d e and the hologram processed. The block was then baked to set the resist At this point
  • the second hologram was successfully etched into the surface measured and dehvered to Computer Optics for final coatings.
  • the finished pieces with the machined gratings appears in figure 2.3. It appears that at least 10 of the waveguides look very good and that there are another 6 or so that are usable. In order to minimize risk, we have decided to coat only 10 of the devices and hold the others in reserve.
  • the coating process involves applying an anti-reflection coating to the flat surface to allow scattered light to easily escape, and to gold coat the diffraction grating and back cylindrical surface. We expect to received ' finished pieces before the end of the year.
  • FIG 23 Photograph of silicon waveguides with the diffraction grating applied. The grating can be seen in the center of the waveguides.
  • micro-thermopile detector developed by JPL under another program. We plan to integrate this detector into our unit and evaluate its performance. There are many advantages to using the micro-thermopile: it is sensitive over a wide range of wavelengths, it does not suffer from 1/f noise and so does not require a chopper, and it does not have to be cooled.
  • thermopiles are not as sensitive as a PbSe detector, however this device has a measured D* of 1.4 x
  • the design of the optical housing and associated components has been finalized and the parts have been received. We are currently in the process of assembling an optics housing using a spare unpolished waveguide (see 2.5).
  • the optical housing has a removable cover that will permit the components to be aligned prior to the cover being applied.
  • the cover contains the entrance window with a replaceable cutoff filter. This filter can be replaced accommodating different spectral bands.
  • the package also incorporates a mount to hold the input lens or a fiber optic input
  • FIG 23 Photograph of ⁇ ie completed optical housing and base.
  • a siUcon waveguide has been mounted to the base with the chopper and beam dump.
  • the I/O connector is visible mounted to the base under the waveguide.
  • Phase I demonstrated the feasibility of a compact, rugged, lightweight spectrograph operating at room temperature in the 2-to-5 ⁇ m wavelength range.
  • the critical innovation is the spectrometer's completely solid state construction — it can be assembled entirely from micro- machined silicon components (even the MWTR detector arrays can be made of silicon).
  • the resulting instrument has the advantages that it can be made very small and very light (about an inch in diameter and weighing a few grams), is permanently aligned for operation over a broad spectral range, and is extremely resistant to damage resulting from harsh environments (launch vibration, aggressive chemicals, radiation).
  • the Phase II instrument will be approximately V* the size of the Phase I instrument with higher spectral resolution, lower noise , and more sophisticated signal processing and control.
  • IR light enters the entrance slit, a thin film deposited A R coated area, and illuminates a cylinder mirror which collimates light
  • the high refractive index of silicon greatly reduces the angle of the cone of light, this permits the use of an F/1 lens at the entrance aperture. It also reduces the size of the cylinder mirror and permits the spectrograph to operate near the diffraction limit
  • the collimated beam is reflected off a gold coated diffraction grating.
  • the grating disperses the IR spectrum which is re-imaged after a second reflection off the cylinder mirror.
  • the light exits the block through an AR coated surface and is imaged onto the detector plane.
  • the detector plane is at a slight angle in order to flatten the field across the image.
  • the air gap permits the detector plane location to be adjusted, allowing for manual focusing. As the rays exit the block, the large cone angle of the rays is restored, fully filling the acceptance angle of the detector.
  • Figure 2.1 Phase 2 optical bench ray trace. This layout has only two optical surfaces, greatly simplifying fabrication and increasing reliability by eliminating tolerance stack up inaccuracies.
  • the flat grating design increases grating efficiency and makes the device easier to manufacture.
  • Its F/1 design allows for twice the through put of the Phase I design, and its air gap at the detectors provide for focusing and permit the use of conventional AR coats.
  • the grating produces a number of lower energy unwanted difiEracfion orders. In our earlier design these orders were trapped in the block and were scattered and added to unwanted background signal on the detector. The new design permits the exit of these unwanted orders out of the flat surface where they will be captured by a light trap.
  • the components themselves incorporate a number of advanced design features.
  • the diffraction grating is made with a variable line density or "chirp" to flatten the Rowland circle and provide a flat focal plane for the array.
  • the slab waveguide is formed to include mounting surfaces for other components and -includes baffling and beam dumps. Since the silicon is opaque to photons above its bandgap, the slab excludes visible light from the detector.
  • Silicon's high refractive index provides lossless reflections at the polished top and bottom faces of the device, wavegwding the radiation in the plane of the device.
  • the cone of incident illumination is dramatically smaller inside the device. This provides better optical throughput than can be achieved with an open-air design.
  • the entire spectrograph can readily be cooled on a single cold-finger, in much the same way that focal plane arrays are now cooled in conventional instruments.
  • Figure 2.2 The central innovation which makes the design of the instrument possible is the high-index slab waveguide. Refraction at the entrance slit sharpens the entrance cone and improved throughput, as shown in the thermal image and line scan data.
  • the mid infrared waveband operating range of the phase 2 instrument should reveal the presence of many of the scientifically important C0 2 and H 2 0 along with Fe minerals, distinguished by Cpx - Opx, and Olivine. Observations of the C-H stretch band at 3.4 micron wavelength and bound water in the 2.5-3.5 micron band should also confirm the likely presence of organic materials and/or hydrated minerals.
  • the ability to resolve characteristic carbonate absorption bands in the 3.2-4.4 micron waveband and sulfates in the 1.3-3.6 micron region should provide the ability to distinguish various clays and micas. This is illustrated in 3.1 by an FTIR spectrum obtained in our laboratory from a calcite sample furnished by JPL.
  • Cylinder The blanks will be sent to Digital Optics Corporation who will stack twenty parts into a monolithic block then shape the block into a cylinder lens. They possessed the interferometric equipment necessary to measure and produce the necessary surface on the cylinder mirror. They will gold coat the mirror and AR coat the flat.
  • the gratings will be built by Diffraction Limited. They will produce a grating in photoresist on the surface and etch it into the silicon. They will also work with the elements ⁇ cicmblH 'n a stack. Once the grating is made they will add gold reflector overcoat and deposit the entrance slit.
  • the part will be returned to Ion Optics where its performance will be measured. This is done by feeding the block with a lab spectrometer and measuring the quality of the output image with an IR camera. Once its performance is verified the device will be integrated into the housing with the detector and focused.
  • the spectrometer design requires the precise alignment of the optical bench, the chopper, the detector array, filter, and input lens. This is all accomplished with a precision sealed package. The package holds the chopper, filter and detector in position and provides for a vacuum seal.
  • Figure 33 77 ⁇ e design of the optical housing and mounts is essentially complete. This drawing shows the assembly drawing for the optical housing.
  • the optical housing has a removable cover that will permit the components to be aligned prior to the cover being applied.
  • the cover contains the entrance window with a replaceable cutoff filter. This filter can be replaced accommodating different spectral bands.
  • the package also incorporates a mount to hold the input lens or a fiber optic input
  • the chopper consists of a two layer piezo-electric modulator.
  • the deflection at the tip of the device depends on the bias voltage applied.
  • a flag at the tip of the device alternately opens and blocks the entrance slit of the silicon waveguide.
  • FIG 3.5 The top trace shows the raw multiplexed data from all pixels during a single frame.
  • the middle trace corresponds to the signal on a single pixels over many frames after the data has been unpacked The modulated input signal is clearly evident.
  • the bottom trace is the power spectral density for the modulated signal.
  • FIG. 3 illustrates the process. Synchronizing the A/D clock with the pixel clock, the serial data output of the multiplexor is sampled. Consecutive samples in time for each pixel are assembled to form a sampled waveform for each and every pixel. The modulation signal can then be identified and measured using Fourier transform analysis techniques.
  • Solid State Infrared Spectrograph the limitations of the data acquisition equipment, we were nonetheless encouraged enough by the performance of the multiplexed array to proceed with the procurement of the detector array with the custom multiplexor. We will work closely with the detector vendor in the design and layout of the multiplexor circuit and design data acquisition for the instrument compatible with its expected performance.
  • Figure 3.6 Pixel to pixel offsets can be extracted and the signal amplified improving the dynamic range of the measurement.
  • thermopiles are not as sensitive as a PbSe detector, however scientists at JPL believe that they can produce a micro machined thermopile, in the configuration we desire, with a D* of l ⁇ ', equivalent to uncooled PbSe. Although this device is experimental, we hope to incorporate this device into a prototype spectrometer and evaluate its performance.
  • DSP56L811 is a cost-effective solution that combines the prodigious number crunching of a digital signal processor with the versatile functionality of a microcontroller.
  • This processor will perform the signal processing necessary for signal extraction and chemical detection and can also control a display and keypad; a serial interface will be used for communication with an external computer.
  • An off-the-shelf development board has been acquired which will facilitate the rapid development of the spectrometer processing unit
  • a compiler has also been acquired for software development in 'C. Benchmark tests performed on the processor at Ion Optics have verified the performance of the processor in our application.
  • the optical front end With the delivery of the silicon waveguides, PbSe detectors, optical housing and mounts expected in October, the optical front end will be assembled and tested. Data will be collected for data processing and system characterization.
  • NASA has a need for low-cost, low-power, low-mass, highly integrated spectral data acquisition instruments that enable space science observations from small and micro-spacecraft.
  • Phase I demonstrated the feasibility of a compact, rugged, lightweight spectrograph operating at room temperature in the 2-to-5 ⁇ m wavelength range.
  • the critical innovation is the spectrometer's completely solid state construction — it can be assembled entirely from micro- machined silicon components (even the MWTR detector arrays can be made of silicon).
  • the resulting instrument has the advantages that it can be made very small and very light (about an inch in diameter and weighing a few grams), is permanently aligned for operation over a broad spectral range, and is extremely resistant to damage resulting from harsh environments (launch vibration, aggressive chemicals, radiation).
  • Figure 2.1 Phase one optical bench and lay out showing grating, detector plane, chopper, and filter.
  • the diffraction grating is made with a variable line density or "chirp" to flatten the Rowland circle and provide a flat focal plane for the array
  • the slab waveguide is formed to include mounting surfaces for other components and includes baffling and beam dumps Since the silicon is opaque to photons above its bandgap, the slab excludes visible light from the detector Silicon's high refractive index provides lossless reflections at the polished top and bottom faces of the device, waveguiding the radiation in the plane of the device.
  • the high index of the silicon the cone of incident illumination is dramatically smaller inside the device This provides better optical throughput then can be achieved with an open-air design
  • the entire spectrograph can readily be cooled on a single cold-finger, in much the same way that focal plane arrays are now cooled in conventional instruments.
  • phase 1 block spectrograph was reevaluated to uncover the lessons learned and to find areas of improvement for the phase two miniaturization redesign. The following lists some of the key points-
  • Phase 1 design is too large, does not provide an advantage over more conventional designs, and is relatively complex and expensive to manufacture.
  • Phase 1 block had 4 active facets, which were at precise odd angles to one another, two of which require AR coating
  • the tuning fork chopper used in the Phase 1 device has magnetic coil drivers, which radiate EM interference. This interference is at the chopping frequency and can not be filtered from the signal. This is an intrinsic short coming of the tuning fork design.
  • a system model was created to determine the detectivity of the system.
  • the basic question to be answered was how much radiance must the target provide in order to be detected by the spectrographic system.
  • the intuitive answer is to make the system with the highest detectivity.
  • the first is to determine what temperature of the target (a measure of radiance) is required to get a signal to noise of 1 out of the detectors at each wavelength. This assumes that the sample radiates like a black body.
  • the second aspect is what spectral features are we looking for in the sample gasses, what resolution is required to resolve them, and are they within our spectral sensitivity.
  • FIGURE 2.3 This figure shows the detectivity of the spectrometer system, as modeled The horizontal axes indicate the spectral band and the thermal temperature (radiant intensity) the vertical axis indicates the signal to noise ratio. Not surprisingly, this analysis shows that at longer wavelengths the signal to noise is higher (system will detect better).
  • the mid infrared waveband operating range of the phase 2 instrument should reveal the presence of many of the scientifically important C0 2 and H 2 0 along with Fe' + minerals, distinguished by Cpx - Opx, and Olivine. Observations of the C-H stretch band at 3.4 micron wavelength and bound water in the 2.5-3.5 micron band should also confirm the likely presence of. organic materials and/or hydrated minerals.
  • the ability to resolve characteristic carbonate absorption bands in the 3.2-4.4 micron waveband and sulfates in the 1.3-3.6 micron region should provide the ability to distinguish various clays and micas. This is illustrated in Figure 2.4 by an FTIR spectrum obtained in our laboratory from a calcite sample furnished by JPL.
  • FIG. 2.5 Phase 2 optical bench raytrace.
  • This layout has only two optical surfaces, greatly simplifying fabrication and increasing reliability by eliminating tolerance stack up inaccuracies.
  • the flat grating design increases grating efficiency and makes the device easier to manufacture.
  • Its F/1 design allows for twice the through put of the Phase 1 design, and its air gap at the detectors provide for focusing and permit the use of conventional AR coats.
  • LR light enters the entrance slit, a thin film deposited A R coated area, and illuminates a cylinder mirror which collimates light
  • the high refractive index of silicon greatly reduces the angle of the cone of light, this permits the use of an F/1 lens at the entrance apearature. It also reduces the size of the cylinder mirror and permits the spectrograph to operate near the diffraction limit.
  • the collimated beam is reflected off a gold coated drffraction grating.
  • the grating disperses the IR spectrum which is re-imaged after a second reflection off die cylinder mirror.
  • the light exits the block through an AR coated surface and is imaged onto the detector plane.
  • the grating produces a number of lower energy unwanted diffraction orders. In our earlier design these orders were trapped in the block and were scatterred and added to unwanted background signal on the detector. The new design permits the exit of these unwanted orders out of the flat surface where they will be captured by a light trap.
  • the following table addresses all the pros and cons of this new "D" shaped design.
  • Optical blanks "D” shaped optical blanks are purchased from VA Optical, a silicon supplier. Their main business is to produce substrates for the " micro-chip industry. For this reason they can produce superior flat polish on both sides of the optic. The near net shape will include all the polished flats but not the cylindrical surface.
  • Cylinder The blanks will be sent to Digital Optics Corporation who will stack twenty parts into a monolithic block than shape the block into a cylinder lens. They possessed the interferometric equipment necessary to measure and produce the necessary surface on the cylinder mirror. They will gold coat the mirror and AR coat the flat.
  • the gratings will be built by Diffraction Limited. They will produce a grating in photoresist on the surface and etch it into the silicon. They will also work with the elements assembled in a stack. Once the grating is made they will add gold reflector overcoat and deposit the entrance slit.
  • the electronic read-out will fit into the vacuum package so the signals are amplified and multiplexed .before leaving the package. This provides for the minimum number of electrical feed throughs. It also provides a modularity of the design so the spectrometer optics package can be traded out with little effort.
  • the detector mount will hold the backside of the TE cooler to which the detector array is bonded.
  • the mount will allow the detector to be precisely positioned with micrometer screw adjusts and then locked into place. Heat from the TE cooler will be conducted through the mount and into the base of the package.
  • the package will have a removable cover that will permit the components to be aligned prior to the cover being applied.
  • the cover will contain the entrance window that will be coated to act as a cutoff filter in the system
  • the package will also incorporate a mount to hold the input lens or a fiber optic input.
  • thermoelectrically cooled PbSe array provides the best performance with moderate cooling requirements in our region of interest. When cooled to approximately 145 to 250 K, low NEP performance extends well beyond 5.5 micrometers. Thermoelectric coolers are rugged low cost devices that can provide adequate cooling for the PbSe device. We anticipate a requirement for a two stage TE cooler.
  • Figure 2.7 This is the package design that utilized the Optoelectronics/Textron integrated multiplexer and PbSe detector array.
  • NEP New England Photoconductor
  • thermopiles are not as sensitive as a PbSe detector, however scientists at JPL believe that they can produce a micro machined thermopile, in the configuration we desire with a D* of 10 5 , equivalent to uncooled PbSe. Although this device is experimental, we hope to incorporate this device into a prototype spectrometer and evaluate its performance. Although we believe we need the sensitivity of a PbSe detector at this time, the micro-thermopile is a very attractive alternative, that could be used in spectrographs that range from, 1 - 20 urn (the full IR spectrum).
  • a modulated or chopped detector signal is recovered using a lock-in amplifier or synchronous detection scheme.
  • the large number of pixels and the use of a multiplexor complicates the extraction of the modulated signal from our PbSe detector.
  • the serial output of the multiplexor will be digitized with a high speed, high resolution A/D converter.
  • the sampled outputs from each pixel will be collected from multiple scans and processed digitally. For example, by performing a Fast Fourier Transform (FFT) on the data collected for each one pixel, the energy content at the modulation frequency can be determined.
  • the FFT acts as a narrowband filter at the modulation frequency. This energy can be averaged over time to further increase signal to noise.
  • FFT Fast Fourier Transform
  • the silicon wafer and optical substrate fabricator will produce the silicon optic without the cylindrical optical surface and optical coatings.
  • the lens fabricator will finish the cylindrical surface and AR coat the opposite flat.
  • the grating fabricator will etch and gold coat the grating
  • the goal of these three steps is to define a manufacturing process to maximize yield and operating characteristics.
  • a vendor has been selected and a quote obtained for five chopper assemblies based on the proof of concept prototype.
  • the purchase order will be executed as soon as the final drawing package for the optical bench is released. This is anticipated to occur in June, 1997.
  • Phase I demonstrated the feasibility of a compact, rugged, lightweight spectrograph operating at room temperature in the 2-to-5 ⁇ m wavelength range
  • the critical innovation is the spectrometer's completely solid state construction -- it can be assembled entirely from micromachined silicon components (even the MWTR detector arrays can be made of silicon)
  • the resulting instrument has the advantages that it can be made very small and very light (about an inch in diameter and weighing a few grams), is permanently aligned for operation over a broad spectral range, and is extremely resistant to damage resulting from harsh environments (launch vibration, aggressive chemicals, radiation)
  • Phase ITs objective is to extrapolate this prototype to a rugged, lightweight, uncooled MWTR spectrograph applicable to ongoing and upcoming NASA/JPL program needs
  • These needs include remote sensing from airborne and satellite platforms, and planetary programs to measure water vapor (H,0) and carbon dioxide (C0 2 ) in clouds, sulfur dioxide (SO.) and hydrogen sulfide (H..S) in volcano plumes, and frozen gases and organic compounds in borehole cores.
  • Phase II focus will be upon (1) improving the Phase I spectrometer design for better resolution, sensitivity, manufacturability, and smaller .size, and (2) extensive testing and evaluation in cooperation with .scientists at JPL, yielding a miniature, mass-producible device with better resolution and lower noise than its Phase I parent. Beyond its usefulness to NASA, the fully developed spectrometer is expected to excite extensive '..commercial interest for applications requiring a low-£ost-_ compact chemical sensor.
  • Figure 1 In the Phase I proof-of-concepl device, light enters the block, reflects off a flat fold mirror and is dispersed (and focused) onto the detector plane by a diffraction grating.
  • the holographic grating (lying next to the block in this picture) is built on the surface of a silicon plano-convex lens. It has a variable line density, or chirp, to overcome the Rowland circle problem and flatten the focal plane APPENDIX - - PCT APPLICATION NO. PCT/US99/07781
  • the silicon micro-spectrograph is a slab of high punty, optical grade silicon, with a holographic IR grating and linear detector array
  • This system will be lightweight, compact, rugged, alignment-free, calibration-free, and low-cost Dunng the third quarter of this project, we completed integration and test of a 3" prototype device with a 16 channel PbSe linear detector array
  • the package includes an optical bench, detector array, chopper and chopper d ⁇ ver, input optics and filter, 16 channel lock-in amplifier, high voltage drive batte ⁇ es, and stand alone bar graph display Detector output, previously fed into a PC via an A D card, the digitally demodulated and displayed, is now demodulated via a dedicated 16 channel analog lock-in circuit and viewed on a stand alone bar graph display
  • the total spectrograph runs on two batteries and occupies the volume of a shoe box Its compactness allows it to be easily carried and used in the field This is emphasized by Ion Optics' recent demonstration of the spectrometer for Exxon/Chemical in Houston Texas, and its exhibition at the Boston SPJE conference
  • Figure 9 A major internal milestone for the phase 2 program is an optimized 3 " system to address stray-light and throughput issues identified in the phase I test results.
  • 3 r ⁇ * quarter we built a room temperature benchtop demonstration with an integrated 16 channel linear detector array, signal processing electronics, and an LED bar graph display.
  • Figure 2 The heart of the system is the all solid-state spectrometer-on-a-chip.
  • Figure 2 uses a modified "Ebert" layout , fabricated entirely from micromachined silicon It consists of a roughly "D” shaped silicon die with an entrance slit defined by thin film coatings. IR light crosses the slab (with the silicon acting as a waveguide) and is focused toward a diffraction grating by the cylindrical edge of the die. The chirped (variable line spacing), grating disperses the spectrum and images the source onto a flat focal plane where it is detected by a linear infrared detector array.
  • FIG. 3 Mechanical layout (left) and (CODE-V) ray trace (right) illustrate the operation of the miniature solid state optical bench Tfie entire assembly will be small enough to Jit in a standard microprocessor chip carrier.
  • the new design has fewer milled facets and incorporates built-in beam dumps for disposal of unwanted diffraction orders The importance of these beam dumps was confirmed by our test results with the 3 " prototype. They are implemented as AR-coated exit windows on the flat spectrometer entrance/exit facet. Dimensions are in mm.
  • Figure 1 This instrument will provide complete spectral characterization of the surrounding soil media by allowing stratigraphic traces of specific infrared lines as a function of depth down the borehole.
  • the combination of a pulsed blackbody source with the spectrometer has the advantage that, in addition to making reflection mode measurements of contaminant composition and concentration, it can also be used to measure thermal inertia of the surrounding media. Thermal inertia, the rate of heating and cooling following a measured thermal pulse, provides an important measure of the density of the surrounding media
  • Figure 1 Schematic of tiny infrared micro spectrophotometer arrangement for contaminant monitoring in boreholes.
  • the miniature solid-state FTIR and pulsed infrared source allows spectroscopic measurement of spectral lines from specific contaminants as well as local measurements of thermal mertia and soil density.
  • the manufacturing technique for the 1 inch spectrograph is being developed to facilitate quantity production.
  • the raw silicon is purchased as silicon wafers from an electronic chip supplier, the advantage is that their standard electronic product is to a much higher surface finish then is normally available from optical manufacturers.
  • Special optical grade silicon is used to meet our requirements (micro chip grade silicon is opaque in our wavebands). They will slice the wafers to near net shape with all the flat sides polished to the required finish. The wafers are shipped to the lens grinder, (who owns a cylinder generating machine).
  • phase 2 we will develop signal processing, analysis, and packaging required to build our sensor into a fieldable instrument. We expect that it will become a standard tool for propulsion research, not only for rockets but for high performance aircraft engines and for industrial process monitoring.
  • T T?he new instrument will be about the size of a video palmcorder, containing a miniature infrared spectrograph, simple sighting and focusing optics, a control panel, and a flat panel display.
  • a visible spotting scope boresighted with the spectrogaph is used to train light from the plume onto the entrance window of the block spectrograph. The spectrum is dispersed onto a linear detector array covering the mid infrared gas emission band (2.5 - 5 ⁇ m wavelength).
  • Signals from the array are preamplified, conditioned, and digitized so that a microprocessor with embedded data reduction software can convert the signal into a spectrum and extract concentrations of compounds in the plume.
  • the result is then displayed on the instrument's LCD flat panel display.
  • FIG. 1 Schematic illustration of compact, IR plume spectrometer (left). The phase I breadboard device (right) exceeded our performance predictions and successfully measured spectral lines from a I cm flame in our laboratory testing.
  • phase 2 brassboard prototype will, itself, be a useful diagnostic tool for engine work at SSC It will also lead to a revolutionary new product for propulsion research and for on-line monitoring of industrial combustion and chemical processes
  • the diffraction grating is made with a variable line density or "chirp" to flatten the Rowland circle and provide a flat focal plane for the array.
  • the slab waveguide is formed to i nclude mounting surfaces for other components and includes baffling and beam dumps. Since the silicon is opaque to photons above its bandgap, the slab excludes visible light from the detector. Silicon's high refractive index provides lossless reflections at the polished top and bottom faces of the device, waveguiding the radiation in the plane of the device. However, more important, because of the high index of the silicon, the cone of incident illumination is dramatically smaller inside the device. This provides better optical throughput then can be achieved with an open-air design.
  • This instrument fills a gap in available field instrumentation for plume measurements - a handheld spectrometer capable of measuring the concentration of chemical species (CO2, hydrocarbons, NO2, N 2 0, and HCl) present in rocket engine exhaust plumes. This is important to assure environmental compliance during test firings (for example, the Space Shuttle Main Engine (SSME)). Because the combustible mixtures in rocket engines are typically fuel rich, most of the active IR signature is caused by afterburning of the plume in the atmosphere.
  • CO2 chemical species
  • nitrogen oxides are a natural by-product of high temperature combustion in rockets and in high performance jet engines. These compounds can be harmful to the ozone layer, so minimizing their presence in engine exhausts is a concern in the design of new engines for high altitude aircraft.
  • the nitrogen oxides have strong infrared emission lines, but unfortunately, these are coincident with other prominent infrared emission bands, notably water and hydrocarbons.
  • the spectrometer will also be useful for engine performance testing (and environmental monitoring) of liquid propellant engines and hybrid motors burning hydrocarbon fuels.
  • Typical ' fuel/oxidizer combinations of interest to SSC include RPl-JP/liquid oxygen (LOX) and polybutadiene hybrid engines.
  • LOX RPl-JP/liquid oxygen
  • the resulting exhaust plumes have blackbody-like characteristics and contain CH * , CO* and NO x .
  • These species are readily detectable by emission spectroscopy in the 2 to 8 micron range, thus the proposed spectrometer will function as a performance diagnostic for next-generation engines and motors employed on NASA's orbital and interplanetary flight vehicles, and perhaps for high performance air-breathing aircraft as well.
  • phase I project consisted of three major related activities — spectrograph optimization, spectrograph modeling/performance predictions, and breadboard device integration and test. Because of the possibility of detecting nitrogen oxides in the presence of interferences from other engine plume constituents, we have paid special attention to the stray light and optical signal-to-noise performance of the spectrograph in Phase 1. Stray light analysis and testing is important because a critical design feature of the plume spectrometer is its reliability and accuracy. To quantify the likely errors and evaluate potential self-referencing schemes, it is vital to understand the stray light performance of the instrument itself. To study the light budget, APPENDIX - - PCT APPLICATION NO. PCT/US99/07781
  • the SNR is plotted on a 3-D surface plot to show SNR as a function of both wavelength and source temperature, and the projection of the surface on the temperature wavelength plane, with color used to show the regions within the SNR is within specific bounds.
  • FIG. 4.8 A thermal imaging camera was used to check out the gratings under back illumination.
  • torches were collected through an external optical train with a 400 Hz tuning fork chopper. Preamplified signals were brought off the device on a ribbon cable to a computer A D card. A simple synchronous detection scheme in software, for direct digital lock-in analysis of the PbSe signals was developed. Data acquisition software will command the board to sample each channel at roughly 20 kHz (depending on the number of channels configured for a given measurement.) Monitoring the trigger signal from the tuning fork chopper (which, itself, has a programmable phase delay), the software computes the HI and LOW integrals at pre-set delay times and average these over a pre-set integration time (currently baselined as 1 sec)
  • Figure 4.9 Computer screen shows a typical image and signal-to-noise acheived from the digital lock-in program. Notice that the digital lock-in technique is rejecting both low-frequency (60 Hz pickup) and high frequency (1.5 kHz ringing) noise from the 400 Hz chopped signal.
  • An IR imaging camera be.hind the mirror location was used to align entrance radiation with the mirror.
  • the mirror was installed and butt coupled to the block with an aluminum clamp.
  • the grating was installed and the zero order reflection was seen at the beam dump location adjacent to the mirror.
  • Imagery of the detector plane showed the spectrally dispersed image (test 4).
  • Installation of the 4 um peak filter shows the image at location center left of the open spectrum, (test 7).
  • We mapped the position of these peaks by scanning the PbSe detector across the detector face with a micrometer stage and calibrating the micrometer positions to the known (from corresponding FTIR traces) transmission wavelengths of these filters.
  • phase 2 project The goal of the phase 2 project is to make the successful phase 1 sensor into a successful plume monitoring instrument and ultimately into Ion Optics' next commercial product.
  • N2O, NO, NO z have absorption peaks which may overlap those of water vapor, carbon dioxide, and hydrocarbons.
  • Water vapor in particular presents problems because its abso ⁇ tion peaks are broad and because it dominates the composition of the exhaust plume from the SSME.
  • both NO and N0 2 have strong multiple abso ⁇ tions in the range 1200-1800 cm ' , a region of strong water vapor abso ⁇ tion. Each also has a weak overtone abso ⁇ tion in the 2900 cm * region where water vapor has no abso ⁇ tion.
  • the phase 2 project will explore the practical lower limit of size for the silicon block waveguide spectrograph.
  • the first consideration is the size of the detector array or, more accurately, the size of the detector pixels.
  • a typical lower limit on pixel size is 50 ⁇ m for the silicon microthermopile arrays. If we want to spread the wavelengths 3-5 ⁇ m over the detector array with a resolution of 0.01 ⁇ m, (10 nm) then we need 200 array elements. At 50 ⁇ m per element, the array would be at least 1 cm long.
  • the block diameter must be at least 1 4 times the length of the detector a ⁇ ay, which is just over y -inch for a 1 cm long a ⁇ ay
  • the focusing power of the grating is limited to x2 by aberration and that therefore the focused spot size for a 250 ⁇ m fiber will be -130 ⁇ , covering several pixels and limiting resolution to about 0 03 ⁇ m for 3-5 ⁇ m spread over 200 pixels.
  • PCA Data Compression - Principal component analysis
  • coordinates x > and ⁇ indicate two zero-centered measurement variables with sampled data represented by the black points.
  • the projections of the standard deviation of the data, ⁇ ⁇ and ⁇ - onto vectors v ⁇ and v ⁇ provide information regarding the natural coordinates of the data.
  • an orthogonal space is found where the primary axes align along the directions of maximum variance.
  • the experimental data indicates a linear relationship between the two measured variables x > and ** .
  • the two measured variables could be well- represented by one variable v ⁇ .
  • PCA provides the direction of the maximum variance in the data and compresses the dimensionality of the data from 2 to 1.
  • the grating line widths were about 5 ⁇ m
  • the radii for the zone plate are given by m ⁇ _ / ( 1 I
  • phase 1 optical breadboard For a single species, it is straightforward to determine the concentration of gas in a plume by measuring either emission (or equivalently abso ⁇ tion over some known path). We demonstrated that our phase 1 optical breadboard can do this, without any additional signal processing. In phase 2 we will address the real-life situation with multiple species and the consequent need to both recognize which species are present and then determine the absolute concentration of each species. This is also a general problem faced by IR spectroscopists and instrument designers working • with multiple sensor or transducer inputs. Following earlier work in the literature, References [Hashem, 1994; Lewis, 1995], we will team with the neural network experts at Neurodyne to apply neural network processing to solve the deconvolution problem.
  • Spectral information can be very useful for characterizing multi-species environments. However this information is usually buried in thousands of variables captured over a wide range of wavelength. Chemometric methods attempt to separate the useful information from noise and redundancy by determining relevant correlations in the data. There are two fundamental steps in these approaches. The first is the compression of data and the second is the extraction of interesting characteristics. APPENDIX PCT APPLICATION NO. PCT/US99/07781
  • the r x n matrix of raw data L is formatted such that each element ⁇ ' is theyth measurement over n total variables measured during the rth run over r total runs
  • the r x n raw data matrix L is mean-centered to produce the r x n data matrix M where the elements
  • M is - (L v - ) for 1 ⁇ ⁇ r and I ⁇ j ⁇ n
  • the eigenvectors ofthe covariance matrix are linear combinations of the original variables that represent orthogonal components of the variance in the data and are referred to as principal components.
  • the right eigenvectors of the covariance matrix can be calculated by solving the equation:
  • each n dimensional eigenvector ' corresponds to the/th eigenvalue ⁇ .
  • the associated eigenvalues and eigenvectors are ordered in a descending manner such that ⁇ J ⁇ ⁇ j ⁇ ... ⁇ ⁇ till Therefore the eigenvector v ⁇ associated with the largest eigenvalue ⁇ ! of C indicates the direction of maximum variance and the eigenvector associated with the smallest eigenvalue indicates the direction of minimum variance.
  • the variance in any direction v t may be measured by dividing the associated eigenvalue J by the sum ofthe n eigenvalues or the trace of covariance matrix C.
  • n of matrix V can be reduced to a more manageable number while capturing as much variance as possible.
  • the rows of T correspond to experimental runs 1 through r.
  • the vectors of scores 1 through y represent the majority of variance in the data while reducing the column dimension by (n-y).
  • Feature Extraction The feature extraction methods vary from simple linear regression [Wold, Kowalski] (e.g. PCR, ridge regression) to nonlinear function approximation [White, McAvoy] (e.g. neural networks).
  • Wold, Kowalski e.g. PCR, ridge regression
  • White, McAvoy e.g. neural networks
  • Our plan is to investigate three basic approaches; PCR, PLS and neural network based learning algorithms.
  • PCR Principal Component Regression
  • PLS Partial Least Squares,
  • Both PCR and PLS utilize the decomposition of a matrix of input data X and the linear regression of output data matrix Y upon X.
  • X is transformed by the orthogonal matrix of eigenvectors of X ' to a matrix of scores T (as shown in equation 6 above).
  • the matrix output matrix Y is then regression upon the reduced subset of scores, T .
  • PLS weights the covariance matrix by the addition of positive definite matrix YY to X, providing a new covariance matrix X YY X . in this new form, those elements of Y that are close to zero reduce the effect of associated rows ofX and thus we do not try to predict noise [6].
  • the addition of Y containing elements that are close to zero results in the associated rows of X to not be used [6].
  • the weighted covariance matrix is decomposed into a matrix of eigenvectors, R and eigenvalues, D.
  • the matrix X is then transformed using the eigenvector matrix R to an orthogonal matrix of scores, T as described in (2) above.
  • the output matrix Y is then regressed upon T , as performed in PCR
  • PLS In terms of the physical , inte ⁇ retation of PCA, PLS involves the rotation of the components of X and Y such that the resulting distance is mii ⁇ nized. This is accomplished by iteratively regressing on the selected components of X and Tand stopping when the components of X are no longer near those of Y.
  • Nonliuear Relationships Between Measurements and Concentrations There are often nonlinear relationships between spectral data and characteristics reflected in species concentrations, the influence of plume temperature noted above is a good example.
  • a nonlinear function approximation method e.g. locally weighted regression, polynomial networks, multi-layer perceptron networks
  • these methods are lumped into the general class of neural network or learning algorithms.
  • the most applicable learning algorithm to the application described herein are spatially localized learning algorithms.
  • the most generic form of training a multilayer perceptron (MLP) network uses gradient descent where the weights are adjusted proportional to the derivative of error between the actual output and target values. This approach can best be visualized as a ball traveling along a bowl shaped surface along the steepest direction until the minimum is achieved.
  • model parameters are solved once for all future queries where local regression is more expensive because new weights and a new solution for beta is required for each query There are methods of making this computation efficient, however [Deng and Moore, 1995]
  • Multi-Layer Perceptron Neural Network Although there exists several nonlinear function approximation approaches to select from, several neural network approaches have demonstrated benefits over traditional regression techniques in several semiconductor applications [3,5,9,14].
  • this method uses a multilayer perceptron connectionist network with a backpropagation learning algorithm to fit multivariable data
  • a multilayer perceptron (MLP) network uses gradient descent where the weights are adjusted proportional to the derivative of error between the actual output and target values. This approach can best be visualized as a ball traveling along a rough bowl shaped surface along the steepest direction until a minimum is achieved.
  • This Task will be to solidify system and sub-system designs and fabrication procedures with respect to produ biliry or adaptability to cost-effective production. While a large part of the design analysis • will have been performed in previous tasks to optimize system performance, this task will re-evaluate the system with emphasis on how readily the design lends itself to high volume fabrication, manufacturing, and assembly. It is entirely possible that the best design from a performance standpoint will be compatible with volume manufacturing, while it is also possible that compromises will be required to make an effective and producible system. This evaluation will be carried out in conjunction with industrial partners, potential customers, and finally with the relevant outside vendors.
  • NIST traceablity for the instrument will be a critical stage in the development and acceptance of our plume spectrometer.
  • NIST's Radiometric Physics Division in keeping with its charter to promote accurate and useful radiation measurement technology in the UV, visible, and infrared spectral regions, takes an active role in pyrometry and spectral radiance calibrations and the preparation of calibration protocols.
  • Ion Optics will coordinate with NIST to produce a calibration plan whose objective is to develop the pulsIR flat-pack as transfer standard. This will require working with personnel in the NIST laboratories, revising the plan to inco ⁇ orate their recommendations, and finally resubmitting it to NIST for final review and approval. The approved certification plan will then be carried out in the appropriate facilities in the Physics Division's laboratories.
  • HACR cryogenic radiometer
  • LBIR low background infrared
  • the LBIR provides accurate calibrations of IR sources, spectrophotometers, and low background detectors over the range from 0.2 to 25 ⁇ m.
  • Figure 5.5 A cross-sectional view of the low background infrared facility, its blackbody, and its absolute cryogenic radiometer.
  • the assembled system will be subjected to a series of tests to validate its accuracy and reliability. To the extent possible, we will work with other organizations to achieve an absolute wavelength and intensity calibration of the system against secondary sources and NIST-traceable references. Stability tests will consist of measuring the system's response to known samples under controlled laboratory conditions and then subjecting the system to vibration, thermal cycling, and pressure changes. Throughout these environmental variations the systems response to the constant sample will be recorded and compared to the initial indication.
  • the refined prototype system will be demonstrated in an actual field test. This test will consist of installing the instrument at SSC. By mounting the prototype system next to estabUshed monitoring equipment, we will be able to verify its reliability and accuracy for use as a sole source of information in plume testing on actual engines at SSC.
  • NASA has a need for low-cost, low-power, low-mass, highly integrated spectral data acquisition instruments that enable data acquisition and feed back control from both ground based testing facilities and on spacecraft.
  • We building a novel, monolithically integrated, infrared micro-spectrograph. It is fabricated from silicon to address this need.
  • This device will incorporate the silicon micro spectrograph, simple focusing optics, and an electrical interface for power input and remote readout.
  • the silicon micro spectrograph is a slab of high purity, optical grade silicon, with a holographic diffraction grating and linear detector array monolithically integrated right on the surface. .
  • This system will be lightweight, compact, rugged, alignment-free, calibration-free, and low-cost.
  • Figure 3.1 Phase I optical bench and lay out showing input lens, silicon slab, grating, detector plane, chopper, and filter.
  • Phase I design is too large, does not provide an advantage over more conventional designs, and is relatively complex and expensive to manufacture.
  • Phase I block had 4 active facets, which were at precise odd angles to one another, two of which require AR coating.
  • the tuning fork chopper used in the Phase I device has magnetic coil drivers, which radiate EM interference. This interference is at the chopping frequency and can not be filtered from the signal. This is an intrinsic short coming of the tuning fork design.
  • a system model was created to determine the detectivity of the system.
  • the basic question ' to be answered was how much radiance must the target provide in order to be detected by the spectrographic system.
  • the intuitive answer Is to make the system with the highest detectivity.
  • the first is to determine what temperature of the target (a measure of radiance) is required to get a signal to noise of 1 out of the detectors at each wavelength. This assumes that the sample radiates like a black body.
  • the second aspect is what spectral features are we looking for in the sample gasses, what resolution is required to resolve them, and are they within our spectral sensitivity.
  • FIGURE 3.2 This figure snows the detectivity of the spectrometer system.
  • the horizontal axis indicate the spectral band and the thermal temperature (radiant intensity), the vedical axis indicates the signal to noise ratio. It shows that at longer wavelengths and higher temperatures the signal to noise is higher (system will detect better).
  • the effort in the next quarter of Phase II will include estimates of S/N over the regions of interest and relative to the detectors being purchased under Phase II. This effort will be coupled with a determination as to what the spectral features are within the sample gasses, and what species are within our spectral sensitivity.
  • NDI Neurodyne, Inc.
  • a wavelength range of 3.17 to 5.42 um at a resolution of 0.034 um is acceptable for measuring hydrocarbons, CO, CO 2 , and NO. This range will allow us to see the hydrocarbon major peaks from 3.2 to 3.6 microns, the CO 2 peaks from 4.2 to 4..4 microns, the CO peaks from 4.5 to 4.8 microns and the NO peaks from 5.1 to 5.4 microns
  • the cooling will require a more complex housing to seal out moisture, a driver for the TE cooler with thermal feed back control, and a heat sink.
  • NDI has utilized auto exhaust emissions. These emission contain similar gasses to that of the plume, are readily available, and can be provided in measured mixture by NDl's engine lab facility in West Virginia. Emission samples were taken from a diesel engine operated over a number of engine load conditions. The samples were evaluated using a MIDAC FTIR system. Macros were written to pull out key peaks in various ranges which would help deconvolve NO x from water vapor. From their preliminary analysis, the water vapor is the most difficult problem, overlapping and masking the other gas constituents. They have been examining combustion equations to look for correlations that can be exploited in analyzing the spectra. One solution would be to correlate the CO 2 line with water vapor since their production is strongly associated within the combustion process.
  • NDI has requested information on rocket plume spectral data. We are in the process of trying to collect that data from sources such as NASA and the Air Force. We plan to submit a formal request for that data shortly.
  • FIG. 3.4 Phase II optical bench raytrace.
  • This layout has only two optical surfaces, greatly simplifying fabrication and increasing reliability by eliminating tolerance stack up inaccuracies.
  • the flat grating design increases the grating's efficiency and makes the device easier to manufacture.
  • Its F/1 design allows for twice the throughput of the Phase I design, and its air gap at the detectors provide for focusing and permits the use of conventional AR coats. Dimensions are in mm.
  • IR light enters the entrance slit, an AR coated area defined by thin film deposited layer.
  • the radiation illuminates a cylinder mirror which collimates the light.
  • the high refractive index of silicon greatly reduces the angle of the cone of light, this permits the use of an F/1 lens at the entrance aperture. It also reduces the size of the cylinder mirror, and permits the spectrograph to operate near the diffraction limit.
  • the collimated beam is reflected off a gold coated diffraction grating.
  • the grating disperses the IR spectrum, which is re-imaged after a second reflection off the cylinder mirror.
  • the light exits the block through an AR coated surface and is imaged onto the detector plane.
  • the detector plane is at a slight angle in order to flatten the field across the image.
  • the air gap permits the detector plane location to be adjusted, allowing for manual focusing. As the rays exit the block the large cone angle of the rays is restored, more fully filling the acceptance angle of the detector.
  • Detector/block interface Contact coupled Air gap exchange of detectors. hree standing to Deposited slit is finer, less
  • Optical blanks "D" shaped optical blanks are purchase from VA Optical, a silicon supplier. Their main business is to produce substrates for the micro-chip industry. For this reason they can produce superior flat polish on both sides of the optic. The near net shape will include all the polished flats but not the cylinder surface.
  • Cylinder The blanks will be sent to Digital Optics Corporation who will stack 20 parts into a monolithic block, then shape the block into a cylinder lens. They possess the interferometric equipment necessary to measure and produce the necessary surface on the cylinder mirror. They will gold coat the mirror and AR coat the flat.
  • the gratings will be built by Diffraction Limited. They will produce a grating in photoresist on the surface and etch it into the silicon. They will also work with the elements assembled in a stack. Once the grating is made they will add gold reflector overcoat and deposit the entrance slit.
  • the part will be returned to Ion Optics where its performance will be measured. This is done by feeding the block with a lab spectrometer and measuring the quality of the output image with an IR camera. Once its performance is verified the device will be integrated into the housing with the detector and focused.
  • the design requires the precise alignment of the optical bench, the chopper, the detector array and the filter. This is all accomplished with a precision sealed package.
  • the package holds the chopper, filter and detector in position and provides for a vacuum seal.
  • the miniature chopper we developed consists of a piezo-electric (PZT) bimorph strip with a flag on the end.
  • the bimorph strip is a sandwich of PZT and stainless steel bonded to together.
  • the bimorph curls like a bimetallic, as a voltage is applied across the PZT causing it to change length. This provides very fast chopping of the optical beam with minimum volume and no EMI.
  • the electronic read out will be fit into the vacuum package so the signals are amplified and multiplexed before leaving the package. This provides for a minimum number of APPENDIX - - PCT APPLICATION NO. PCT/US99/07781
  • the detector mount will hold the back side of the TE cooler to which the detector array is bonded.
  • the mount will allow the detector to be precisely positioned with micrometer screw adjusts and then locked into place. Heat from the TE cooler will be conducted through the mount and into the base of the package.
  • the package will have a removable cover that will permit the components to be aligned prior to the cover being applied.
  • the cover will contain the entrance window that will be coated to act as a cut off filter in the system.
  • the package will also incorporate a mount to hold the input tens or an input fiber optic.
  • FIGURE 3.5 The preliminary design of housing, heat sinks, chopper, detector mount, and alignment fixtures have been completed. These will be further refined and detailed once the detector configuration has been finalized.
  • thermoelectrically cooled PbSe array provides the best performance with moderate cooling requirements in our spectral region of interest. When cooled to between 145 and 250 K, low noise performance is obtained.
  • solid state thermoelectric coolers which are rugged, low cost devices that can provide sufficient cooling for the PbSe device.
  • Figure 3.6 This is the package design that utilizes the Optoelectronics/Textron integrated multiplexor and PbSe detector array.
  • NEP New England Photoconductor
  • thermopiles are not as sensitive as a PbSe detector, however a source at JPL believes that they can produce a micro machined thermopile in the configuration we desire with a D* of 109, equivalent to PbSe. Although this device is experimental, we hope to incorporate this device into a prototype spectrometer and evaluate its performance. Although we believe we need the sensitivity of a PbSe detector at this time, the micro-thermopile is a very attractive alternative, that could be used in spectrographs that range from 1 - 20 um (the full IR spectrum).
  • a modulated or chopped detector signal is recovered using a lock-in amplifier or synchronous detection scheme.
  • the large number of pixels and the use of a multiplexor complicates the extraction of the modulated signal from our PbSe detector.
  • the serial output of the multiplexor will be digitized with a high speed, high resolution A/D converter.
  • the sampled outputs from each pixel will be collected from multiple scans and processed digitally. For example, by performing a Fast Fourier Transform (FFT) on the data collected for each one pixel, the energy content at the modulation frequency can be determined.
  • the FFT acts as a narrowband filter at the modulation frequency. This energy can be averaged over time to further increase signal to noise.
  • FFT Fast Fourier Transform
  • DSP56L81 1 from Motorola.
  • This processor is a cost-effective solution that combines the prodigious number crunching of a digital signal processor with the versatile functionality of a microcontroller.
  • This processor could perform the signal processing necessary for signal extraction and chemical detection and also control a display and keypad; a serial interface will be used for communication with an external computer.
  • An off-the-shelf development board will facilitate the rapid prototyping of the final spectrometer unit.
  • Figure 4.1 Detailed drawing of optical bench ready for fabrication.
  • the drawing package has been sent out for quotation. We intend to contract three vendors to build the assembly in stages.
  • the silicon wafer and optical substrate fabricator will produce the silicon optic without the cylindrical optical surface and optical coatings.
  • the grating fabricator will etch and gold coat the grating
  • the goal of these three steps is to define a manufacturing process to maximize yield and operating characteristics.
  • a vendor has been selected and a quote obtained for five chopper assemblies based on the proof of concept prototype.
  • the purchase order will be executed as soon as the final drawing package for the optical bench is released. This is anticipated to occur early June, 1997.
  • a monolithic spectrometer system comprising: a slab waveguide having an input surface for accepting optical radiation and an exit surface; a diffraction grating disposed with a third surface ofthe waveguide; a detector array aligned adjacent to the second surface; a first reflective mirror coated on the waveguide such that the radiation transmitted within the waveguide is reflected and collimated to the grating, wherein light energy diffracted from the grating is dispersed to the mirror for subsequent refocus through the second surface and onto the array.
  • detector array is selected from the group consisting essentially of microbolometers, PbSe, PbS, and CCD.
  • a system according to claim 1 further comprising input optics to focus optical radiation onto the first surface.
  • a system according to claim 1 further comprising an electronics subsystem for collecting signals from the array, for processing the signals, and for correlating the signals to reference data so that a chemical species ofthe radiation spectrum is determined.
  • the invention includes a solid optical grade waveguide coupled to a line array of detectors.
  • Light from a source e.g., earth emissions transmitted through gases
  • a source e.g., earth emissions transmitted through gases
  • the reflector refocusses the diffracted light onto the array at a second surface ofthe waveguide.
  • the detectors are preferably microbolometers or made from PbS or PbSe material.
  • the waveguide is typically silicon to transmit IR radiation with relatively high index of refraction. Other materials can be used.
  • Electronics responsive to user input, processes detector signals and determines spectral characteristics ofthe light such as to indicate the presence of a chemical species.

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