Methods and systems to record amplitudes of an interference pattern of a source light at successive rows of a focal plane array as an interferometer traverses the source light, while varying an optical path difference of the interferometer. A fixed frame rate of the focal plane array may be selected such that each in-track row of the focal plane array provides a different point along the interferogram, for the same ground location.

BACKGROUND

Imaging systems include scanner systems, staring systems (also known as step-staring systems), and time-delay integration (TDI) systems. Scanning systems include push broom scanners (also known as an along-track scanners), and whisk broom scanners (also known as across-track scanners). Scanning arrays are constructed from linear arrays (or very narrow 2-D arrays), that are rastered across a desired field of view using a rotating or oscillating mirror to construct a 2-D image over time. A TDI imager operates in similar fashion to a scanning array except that it images perpendicularly to the motion of the camera. A staring array is analogous to the film in a typical camera, in that it directly captures a 2-D image projected by a lens at an image plane. A scanning array is analogous to piecing together a 2D image with photos taken through a narrow slit. A TDI imager is analogous to looking through a vertical slit out the side window of a moving car, and building a long, continuous image as the car passes the landscape.

There is a demand for satellite-based hyperspectral imaging data collected over a wide area of the earth at fine spatial resolution. Fourier Transform Spectroscopy (FTS) instruments provide relatively high quality hyperspectral data. Conventional FTS system are operated as a step-stare systems, which require relatively fast focal plane array (FPA) frame rates to collect an interferogram while staring. This tends to the maximum size of the FPA, and thus requires fast step-staring to achieve a wide swath width. The resulting short stare time negatively impacts signal-to-noise ratio (SNR), and increases instrument complexity. As a result, FTS is normally not used for wide swath widths. Instead, dispersive systems are used, which provide inferior data quality, and tend to have higher cooling needs.

In the drawings, the leftmost digit(s) of a reference number identifies the drawing in which the reference number first appears.

DETAILED DESCRIPTION

Disclosed herein are techniques referred to as push-broom Fourier transform spectroscopy.

A satellite-based push-broom Fourier Transform Spectrometer (FTS) effectively stares at nadir (e.g., vertically downward from a satellite toward a terrestrial surface), such that motion of the satellite causes a point on the ground to drift across a FPA of the push-broom FTS, in an in-track direction. As the point drifts across the FPA, an optical path difference (OPD) of the push-broom FTS is varied over a range of OPD (e.g., by sweeping a mirror of the push-broom FTS), to generate an interferogram. A frame rate of the FPA is selected such that each in-track row of the FPA provides a different point along the interferogram, for the same ground location. Once the full FPA has drifted across a given ground point, the FPA has effectively produced a complete interferogram at each cross-track location in the FPA. The interferometer then reverses and begins a new OPD sweep in the opposite direction. Slight overlap in the FPA in-track direction prevents data gaps during reversals.

FIG. 1is a block diagram of a push-broom scanning Fourier Transform Spectrometer (spectrometer)100, to determine a spectrum of a source light102. Source light102may represent, for example and without limitation, light reflected from a portion of the Earth as viewed from a satellite.

Spectrometer100includes an interferometer104, which includes a detector105to provide a hyperspectral interferogram106of source light102. Spectrometer100further includes a Fourier transform module or engine116to convert hyperspectral interferogram106to a frequency domain spectrum (spectrum)118. Fourier transform engine116may be configured to perform a fast Fourier transform (FFT) on a power of 2 samples.

Spectrometer100further includes a controller108to control an optical path difference (OPD) of interferometer104. Controller108may be configured to control the OPD of interferometer104based on a relative velocity110of spectrometer100and/or based on a dimension112of source light102, so as to sweep a predetermined range of OPD. Controller108may also be configured to control FPA frame rate of interferometer104based on a relative velocity110of spectrometer100and/or based on a dimension112of source light102.

Spectrometer100and/or portions thereof, may be configured as described in one or more examples below. Spectrometer100is not, however, limited to the examples below.

FIG. 2is a diagram of an interferometer204that includes a beam splitter208to split a source light202into first and second portions. The first portion is reflected from beam splitter208to a fixed-position mirror210along a first path216. The second portion is transmitted through beam splitter208to a position-controllable mirror212along a second path218. Mirrors210and212reflect the respective portions of light back to beam splitter208, which re-directs the portions (or fractions thereof), as an interference pattern220, to a detector214. Detector214may include an array (linear or 2-dimensional array), of light-sensing pixels at the focal plane of a lens. Detector214is also referred to herein as a focal plane array (FPA).

Interferometer204may include one or more additional optical elements (e.g., a lens) between beam splitter208and one or more of source light202, mirror210, mirror212, and detector214. Mirror116and/or mirror118may include a flat mirror and/or a corner cube reflector.

Detector214is configured to record amplitudes of interference pattern220, to provide an interferogram206.

Where source light202includes multiple wavelengths of light, interferogram206will be more complex than a single sinusoid, such as described below with reference toFIG. 3.

FIG. 3is a depiction of an example interferogram300. The horizontal or X-axis of interferogram300represents an optical path difference (OPD).

OPD is a measure of an optical path difference between light beams travelling through two arms of an interferometer (e.g., a difference between first and second paths216and218inFIG. 2). InFIG. 2, OPD is a function of a product of the physical distance travelled by mirror212, a multiplier that is a function of a number of reflecting elements, and an index of refraction of a medium of the interferometer arms (e.g., air, nitrogen for purged systems, etc.).

Interferometer204has a natural reference point when mirrors210and212are the same distance from beam splitter208. This condition is called zero path difference (ZPD). The moving mirror displacement, Δ, is measured from the ZPD. InFIG. 2, light reflected from mirror212travels 2Δ further than light reflected from fixed-position mirror210. The relationship between optical path difference, and mirror displacement, Δ, is OPD=2Δn.

InFIG. 3, units of spectral measurement (OPD), are defined as a wavenumber (cm−1). A wavenumber represents the number of full waves of a particular wavelength per centimeter (cm) of length of travel of a mirror of an interferometer (typically in vacuum; index of refraction n=1). An advantage of defining the spectrum in wavenumbers is that the wavenumber are directly related to energy levels. For example, a spectral feature at 4,000 cm-1 spectral location represents a transition between two molecular levels separated by twice the energy of a transition with spectral signature at 2,000 cm−1.

Interferogram300includes a spike or center burst302at 0 cm−1, which is a signature of a broadband source light. Center burst302indicates that all or substantially all wavelengths of a source light are in-phase at ZPD, such that contributions from each wavelength is at maximum. As the optical path difference, OPD, grows (i.e., as mirror212inFIG. 2moves away from ZPD, toward λ or −λ), different wavelengths of the source light produce peak readings at different positions of the movable mirror (e.g., mirror212inFIG. 2). For a broadband source light, the different wavelengths reach their respective peaks at ZPD and, as the movable mirror moves away from ZPD, interferogram300becomes a relatively complex-looking oscillatory signal with decreasing amplitude.

Each individual spectral component of the source light contributes a sinusoid to interferogram300, with a frequency that is inversely proportional to the wavelength of the respective spectral component.

FIG. 4is a conceptual illustration of a focal plane array (FPA)402of an interferometer to capture or record amplitudes of an interference pattern generated from a source light404, as the interferometer traverses (e.g., as FPA402passes or drifts over) source light404, and as the OPD of the interferometer is varied. The interferometer may, for example, reside on a satellite that orbits a terrestrial body, and source light404may represent an area of a surface of the terrestrial body.

FIG. 5is a conceptual illustration of FPA402as a first pixel of FPA402records an amplitude504of the interference pattern, at a first OPD setting OPD_0.

FIG. 6is a conceptual illustration of FPA402as a second pixel of FPA402records an amplitude604of the interference pattern, at a second OPD setting OPD_1.

FIG. 7is a conceptual illustration of FPA402as a third pixel of FPA402records an amplitude704of the interference pattern, at a third OPD setting OPD_2.

FIG. 8is a conceptual illustration of FPA402as a fourth pixel of FPA402records an amplitude804of the interference pattern, at a fourth OPD setting OPD_3.FIG. 8further illustrates an interferogram806generated or constructed from the recorded amplitudes504,604,704, and804of the interference pattern.

The technique illustrated inFIGS. 4-8may be extended to generate interferograms of multiple source lights, as an interferometer traverses the respective source lights, and as the OPD of the interferometer is varied, such as described below with reference toFIGS. 9-13.

FIG. 9is a conceptual illustration of a focal plane array (FPA)902of an interferometer to capture or record amplitudes of interference patterns generated from multiple source lights, as the interferometer traverses the respective source lights, and as the OPD of the interferometer is varied. In the example ofFIG. 9, the source lights are illustrated as including grid positions904,906,908, and910, which may correspond to respective portions or areas of a terrestrial surface.

FIG. 10is a conceptual illustration of FPA902as a first pixel of FPA902records an amplitude1002of the interference pattern of grid point904, at a first OPD setting OPD_0.

FIG. 11is a conceptual illustration of FPA902as a second pixel of FPA902records an amplitude1002of the interference pattern of grid point904, and as the first pixel of FPA902records an amplitude1004of the interference pattern of grid point906, at a second OPD setting OPD_1.

FIG. 12is a conceptual illustration of FPA902as a third pixel of FPA902records an amplitude1202of the interference pattern of grid point904, the second pixel of FPA902records an amplitude1204of the interference pattern of grid point906, and the first pixel of FPA902records an amplitude1206of the interference pattern of grid point908, at a third OPD setting OPD_2.

FIG. 13is a conceptual illustration of FPA902as a fourth pixel of FPA902records an amplitude1302of the interference pattern of grid point904, the third pixel of FPA902records an amplitude1304of the interference pattern of grid point906, the second pixel of FPA902records an amplitude1306of the interference pattern of grid point908, and the first pixel of FPA902records an amplitude1308of the interference pattern of grid point910, at an OPD setting OPD_3.

Additional amplitudes may be recorded for subsequent positions of FPA902and corresponding OPDs, until there are sufficient data points to provide an interferogram for the respective grid points. This may include reversing a direction of movement, or re-setting a position of an OPD control mechanism, such as mirror212inFIG. 2. This may facilitate collection of amplitude data1320through1330inFIG. 13.

Techniques disclosed herein may be further extended to a 2-dimensional FPA, such as to map a relatively broad swath of a terrestrial surface, such as described below with reference toFIG. 14.

FIG. 14is a diagram of a 2-dimensional FPA1400that includes n columns of m rows of detectors or pixels. Each of them columns is configured to record amplitudes of interference patterns as an interferometer traverses a light source (e.g., a terrestrial surface), to provide a stream of interferograms for a corresponding sequence of grid areas or points, such as described above with reference toFIGS. 9-13.

FIG. 15. is a block diagram of a push-broom scanning Fourier transform spectrometer (spectrometer)1500. Spectrometer1500includes an interferometer1504, an OPD controller1508, and a Fourier transform (FT) engine1516, such as described above with reference toFIG. 1.

Spectrometer1500further includes an interferogram constructor1520to construct an interferogram1510for each of multiple grid coordinates1522of a source light (e.g., for each of multiple grid areas or points of a terrestrial surface). For each grid coordinate1522, interferogram constructor1520is configured to receive/collect pixel amplitudes1524for a range of OPD values1526, and arrange the pixel amplitudes into interferograms1510. This may be useful in situations where pixel amplitudes that are collected out of order (e.g., amplitudes1320-1330inFIG. 13).

A push-broom scanning Fourier transform spectrometer, as disclosed herein, may be configured to modify an interferogram in a spatial domain and/or to modify a spectrum of the interferogram in a spectral domain, such as described below with respect toFIG. 16.

FIG. 16is a block diagram of a push-broom scanning Fourier transform spectrometer (spectrometer)1600. Spectrometer1600includes an interferometer1604, an OPD controller1608, and a Fourier transform (FT) engine1516, such as described above with reference toFIG. 1.

Spectrometer1600further includes a digitizer1620to digitize an interferogram1610. Digitizer1620may be configured to sample interferogram1610at a fixed rate.

Spectrometer1600further includes a spatial domain process engine1622to modify the digitized interferogram in a spatial domain. Modifications may include, without limitation, re-sampling and/or compensation.

One or more features disclosed herein may be implemented in, without limitation, circuitry, a machine, a computer system, a processor and memory, a computer program encoded within a computer-readable medium, and/or combinations thereof. Circuitry may include discrete and/or integrated circuitry, application specific integrated circuitry (ASIC), field programmable gate array (FPGA), a system-on-a-chip (SOC), and combinations thereof.

FIG. 17is a flowchart of a method1700of extracting a spectrum of a source light with a push-broom scanning Fourier transform technique. Method1700is described below with reference to one or more preceding examples. Method1700is not, however, limited to any of the preceding examples.

At1702, an OPD of an interferometer is controlled to generate an interference pattern of a light source as the interferometer traverses the light source, such as described in one or more examples above.

At1704, amplitudes of the interference pattern are recorded at successive rows of a focal plane array as the interferometer traverses the light source, and as the OPD is varied, such as described in one or more examples above.

At1706, an interferogram is generated from the recorded amplitudes, such as described in one or more examples above.

At1708, a spectrum of the source light is extracted from the interferogram, such as described in one or more examples above.

FIG. 18is a block diagram of a computer system1800, configured to extract a spectrum of a source light with a push-broom scanning Fourier transform technique.

Computer system1800includes one or more processors, illustrated here as a processor1802, to execute instructions of a computer program1806encoded within a computer-readable medium1804. Medium1804may include a transitory or non-transitory computer-readable medium.

Computer-readable medium1804further includes data1808, which may be used by processor1802during execution of computer program1806, and/or generated by processor1802during execution of computer program1806.

Processor1802may include one or more instruction processors and/or processor cores, and a control unit to interface between the instruction processor(s)/core(s) and computer readable medium1804. Processor1802may include, without limitation, a microprocessor, a graphics processor, a physics processor, a digital signal processor, a network processor, a front-end communications processor, a co-processor, a management engine (ME), a controller or microcontroller, a central processing unit (CPU), a general purpose instruction processor, and/or an application-specific processor.

In the example ofFIG. 18, computer program1806includes OPD instructions1810to cause processor1802to vary an OPD of an interferometer1850as interferometer1850traverses a source light, such as described in one or more examples above. OPD instructions1810may include instructions to cause processor1802to synchronize a rate of change of the OPD with a relative velocity of interferometer1850, such as described in one or more examples above.

Computer program1806further includes FPA readout instructions1812to cause processor1802to read (e.g., sample), amplitudes of interferogram recorded by an FPA of interferometer1850. FPA readout instructions1812may include instruction to processor1802to output or sample the amplitudes at a fixed rate.

Computer program1806further includes Fourier transform (FT) instructions1814to cause processor1802to extract spectrums from interferograms, such as described in one or more examples above.

Computer program1806may further include spatial domain modification instructions1816to cause processor1802to modify interferograms in a spatial domain, such as described in one or more examples above.

Computer program1806may further include spectral domain modification instructions1818to cause processor1802to modify extracted spectra in a spectral domain, such as described in one or more examples above.

Methods and systems are disclosed herein with the aid of functional building blocks illustrating functions, features, and relationships thereof. At least some of the boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries may be defined so long as the specified functions and relationships thereof are appropriately performed. While various embodiments are disclosed herein, it should be understood that they are presented as examples. The scope of the claims should not be limited by any of the example embodiments disclosed herein.