SYSTEM AND METHODS FOR GAS SPECTROSCOPIC SENSING WITH PHOTON COUNTING AND TUNABLE INTEGRATED PHOTONIC FILTERS

Abstract

A spectroscope using single-photon counters and a chip-integrated lithium niobate micro-ring filter to measure the atmospheric CO2 absorption spectrum passively is disclosed. By thermo-optically sweeping the filter over 150 pm and referencing the resulting photon counts to a bypass channel, the absorption spectrum can be sampled at an ultrahigh-resolution of 6 pm. The spectroscope can be a part of a ground-based field system, wherein the CO2 absorption through the atmosphere can be characterized by counting the solar photons across the absorption line around 1572.02 nm, which agrees well with its transmission spectrum at standard atmospheric pressure.

Claims

1. A method for single-photon spectroscopy, comprising the steps of: collecting photons using a beam expander into a collimated single-mode fiber; letting said photons pass through a long-pass filter to eliminate photons from other bands; using a band-pass filter on said long-pass filtered photons to define a narrow spectral window in order to pick a single gas absorption line-shape and obtain filtered light; utilizing a fiber polarizer to select said filtered light along a certain polarization; splitting said filtered and polarized light into two channels of photon counting, to create a reference channel and a signal channel, with the signal channel containing a calibrated micro-ring resonator filter before the photon counting; tuning the resonance of the resonator filter to sweep through said narrow spectral window; using a field-programmable-gate-array to tune a center wavelength of said micro-ring resonator filter; and acquiring synchronized photon counting data from said reference channel and said signal channel.

2. The method of claim 1, wherein said collected photons are solar photons.

3. The method of claim 1, wherein said collected photons are created by a laser source illuminating on a target through Raman scattering, fluorescence, or Brillouin scattering optical processes.

4. The method of claim 1, wherein said micro-ring resonator filter is an add-drop micro-ring resonator etched on lithium niobate on insulator.

5. The method of claim 4, wherein said micro-ring resonator filter includes an integrated metal heater.

6. A thermal tunable micro-ring resonator filter, comprising: an add-drop micro-ring resonator; and an integrated metal heater.

7. The thermal tunable micro-ring resonator filter of claim 6, wherein said add-drop micro-ring resonator has an ultranarrow transmittance bandwidth that constitutes a high-performance optical filter.

8. The thermal tunable micro-ring resonator filter of claim 7, wherein said add-drop micro-ring resonator has a refractive index which increases when electric heating power is applied thereto.

9. The thermal tunable micro-ring resonator filter of claim 8, wherein said electric heating power is provided from said integrated metal heater.

10. The thermal tunable micro-ring resonator filter of claim 7, wherein said filter is adapted for use with solar photons.

11. The thermal tunable micro-ring resonator filter of claim 7, wherein said filter is adapted for use with laser-generated photons.

12. A passive spectroscope, comprising: an external multi-channel single-photon detector; and a chip integrated filter using an add-drop micro-ring resonator etched on lithium niobate on insulator.

13. The passive spectroscope of claim 12, wherein said single-photon detector is adapted for use with solar photons.

14. The passive spectroscope of claim 12, wherein said single-photon detector is adapted for use with laser-generated photons.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] For a more complete understanding of the present invention, reference is made to the following detailed description of various exemplary embodiments considered in conjunction with the accompanying drawings, in which:

[0017] FIG. 1 shows a micro-ring resonator filter structure;

[0018] FIG. 2 shows a cross-section of a micro-ring resonator filter in a remote sensing system, which includes a waveguide surrounded by a SiO2 cladding layer and a platinum micro-heater deposited atop the cladding layer;

[0019] FIG. 3 is the transmission spectrum of the micro-ring filter and the CO2 gas cell absorption spectrum around 1572 nm;

[0020] FIG. 4. is a microscope image of the MRR with a platinum micro-heater;

[0021] FIG. 5 is a photo showing the fiber packaged LNOI chip connected to electric probes, which includes an MRR filter chip using UV light exposed epoxy with a high numerical-aperture fiber to stabilize the on-off chip coupling;

[0022] FIG. 6 shows the thermo-optical calibration data of the micro-ring resonator filter and their linear fitting;

[0023] FIG. 7 shows the micro-ring resonator filter transmission shifted by the applied electric power, wherein when the applied electric power on the micro-heater is increased, the filtering optical window is right-shifted. Therefore, the gas absorption spectrum can be measured;

[0024] FIG. 8 shows the absorption spectra of a CO2 gas cell measured using the MRR filter and by laser transmission, respectively, wherein the orange curve is the CO2 absorption spectrum measured with ultra-high-resolution sweeping-laser, and the blue points are the measured optical power with MRR filter in corresponding wavelengths;

[0025] FIG. 9 shows an illustration of the atmospheric CO2 absorption spectroscopy with solar photon counting and LNOI Micro-ring resonator filter;

[0026] FIG. 10 shows the solar photon counts through the atmosphere around the gas absorption spectrum line and the transmittance spectrum from the HITRAN database (high-resolution transmission molecular absorption database);

[0027] FIG. 11 is a flow-chart showing a fabrication procedure for the microring resonance filter of FIG. 1;

[0028] FIG. 12 is a system diagram for devices made in accordance with the present invention;

[0029] FIG. 13 is a flow diagram illustrating a method of using devices made in accordance with an embodiment of the present invention;

[0030] FIG. 14 is a plot showing the extended gas cell absorption spectrum from 1568 to 1585 nm in connection with an embodiment of the present invention;

[0031] FIG. 15 is an experimental setup for atmospheric CO.sub.2 absorption measurement with solar photon counting, in accordance with an embodiment of the present invention; and

[0032] FIG. 16 is a table of system parameters for a system constructed in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0033] The following disclosure is presented to provide an illustration of the general principles of the present invention and is not meant to limit, in any way, the inventive concepts contained herein. Moreover, the particular features described in this section can be used in combination with the other described features in each of the multitude of possible permutations and combinations contained herein.

[0034] All terms defined herein should be afforded their broadest possible interpretation, including any implied meanings as dictated by a reading of the specification as well as any words that a person having skill in the art and/or a dictionary, treatise, or similar authority would assign thereto.

[0035] Further, it should be noted that, as recited herein, the singular forms “a”, “an”, “the”, and “one” include the plural referents unless otherwise stated. Additionally, the terms “comprises” and “comprising” when used herein specify that certain features are present in that embodiment, however, this phrase should not be interpreted to preclude the presence or addition of additional steps, operations, features, components, and/or groups thereof.

[0036] All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure.

[0037] A thermal tunable micro-ring resonator (MRR) filter in accordance with an embodiment of the present invention is shown in FIG. 1. Such a resonator filter is based on the thermo-optical effect with an add-drop micro-ring resonator and integrated metal heater. The near inferred (NIR) laser or sunlight photons are coupled into the waveguide at the input port and coupled into the micro-ring resonator. The ultranarrow transmittance bandwidth of the micro-ring resonator constitutes a high-performance optical filter. The refractive index of the micro-ring resonator can be increased when electric heating power is applied. With the tuning electric power of the integrated metal heater, the wavelength of the transmittance spectrum is therefore changed. This allows for the design of the filtering optical window.

[0038] An add-drop micro-ring resonator filter with air cladding, etched from a commercial 600-nm thin film of X-cut Lithium Niobate (LiNbO3), as shown in FIG. 1, can be fabricated as illustrated in FIGS. 11 and 12. The thin film is shallowly etched by 350 nm for waveguiding while leaving a 250 nm slab across the chip to ensure a high-quality factor for achieving a narrow bandwidth filter, as shown in FIG. 2. Two pulley bus waveguides can be positioned on the two sides of the micro-ring resonator for input and drop. To singly excite the fundamental transverse electric (TE) mode in the Micro-ring resonator while suppressing all other guided modes, the top widths of the micro-ring resonator and waveguides can be 1500 nm and 1000 nm, respectively, designed so as to fulfill the phase-matching condition for the mode coupling. FIG. 2, more specifically, is the cross section of the micro-ring resonator filter. The lithium niobate micro-ring resonator filter chip includes multiple layers. In an embodiment, the bottom layer is a silicon substrate. The thin-film lithium niobate is 600 nm, with 350 nm shallowed etch and 250 nm remining. In between is a 4.7 um thick silicon dioxide substrate layer. Plasma-enhanced-chemical-vapor-deposition (PECVD) can be employed to make a 1.5 um thick silicon dioxide cladding on the top of the sample to protect the waveguides. Subsequently, a 100 nm thick platinum micro-heater can be deposited on the top of the micro-ring resonator.

[0039] As an example of such proposed thermo-optical micro-ring resonator filters, a ridge waveguide with top height of 350 nm and base height of 250 nm may be employed, as shown in FIG. 4. The width of the waveguide is 1500 nm. The buried SiO2 layer is 4.7 um, and the cladding SiO2 layer is 1500 nm, as shown in FIG. 2. The electrodes may be placed on top of the waveguide cladding with 1 um width and 100 nm thickness. The radius of the micro-ring resonator filter and/or the integrated platinum heater can be 80 um. The signal is applied on one electrode, while the ground is applied on the other electrode, so that the electric power is created on the top of the waveguides and the refractive index change occurs due to the thermal-optical effect. Therefore, the resonance wavelength of the guided modes is changed, as shown in FIG. 1.

[0040] One consideration in the implementation of the present invention is balancing the on-chip mode coupling efficiency, narrow filtering bandwidth, thermal-optical stability, and high extinction ratio of the add-drop micro-ring resonator. In an embodiment, the over-coupled fundamental TE mode case can be used. In an exemplary embodiment, the end device has a transmittance spectrum of 6 pm bandwidth (full-width half-maximum, FWHM) and a 30 dB extinction ratio around 1572.02 nm, which is the targeted CO2 absorption line, to minimize the interference from water vapor lines in the atmosphere (see FIG. 3). This wavelength choice has been demonstrated in air-borne atmospheric CO2 remote sensing, as the differential—absorption cross-section by the water vapors at 1572.02 nm is more than four orders of magnitude smaller than that of CO2.

[0041] The MRR filter calibration can be performed using a narrowband, tunable laser (Santec, TSL-550). The result is plotted in FIG. 3, showing a transmission FWHM of 6 pm. Also plotted in the same figure is the absorption spectrum of the fiber-coupled CO2 gas cell, measured by the same laser, which shows an FWHM bandwidth around 50 μm. These data verify that the filter has adequately narrow bandwidth to measure the CO2 absorption with high resolution by thermal-optical tuning.

[0042] To characterize the tunability of devices made in accordance with the present invention, a current source can be employed, along with a pair of electric probes to supply electric power on the microheater via the on-chip platinum electrode pads, as shown in FIG. 5. Finally, the LNOI chip is packaged using UV light exposed epoxy with a high numerical-aperture (NA=0.4) fiber to stabilize the on-off chip coupling. In such a manner, the filter center wavelength is shifted by applying current to change the local temperature of the MRR. FIG. 6 shows the MRR resonance as a function of the applied electric power, ranging from 0 mW to 40 mW. This results in a red shift of 0.45 nm in the transmission spectrum. As shown in FIG. 7, there is a linear relationship between the shift and electric power, with a slope of 8.75 pm/mW. This good linearity facilitates the inventive spectroscopic measurement. The tuning efficiency can be significantly improved to 53.7 pm/mW by etching tranches for improved thermal isolation around the MRR.

[0043] In some embodiments, the relationship is not linear, in which case a lookup table may be generated during the device calibration to obtain the shift for a given electric power.

[0044] In some embodiments, the cavity resonance shift is realized by electro-optical effects, wherein the filter center wavelength is shifted by applying a voltage to change the refractive index of the MRR material, and thus the cavity resonance. Compared with the thermo-optical effect, the electro-optical tuning can achieve higher speed, such as a full spectrum sweeping over a nanosecond or less. However, it only applies to MRR materials having adequate electro-optical responses, such as lithium niobate.

[0045] To calibrate the performance of the filter, the filter can be operated against a CO.sub.2 gas cell and compared to direct laser transmission, as shown in FIG. 8. Then, the filter may be applied to measure the spectrum of sunlight passing through the atmosphere, near the absorption line of the atmospheric gas around a specific absorption spectrum line-shape. By differentiating the registered photon counts from those simultaneously recorded in an adjacent bypass channel, an infrared absorption line shape can be acquired with high resolution and accuracy amid the imperfections in sunlight couplings, and without the aid of adaptive optics (see FIG. 10). To minimize the interference of other optical effects in the measurement results, the bypass channel is to be chosen close to the targeted absorption line. In some embodiments, the center spectrum of the bypass channel is 1 nm or less away from the absorption line.

[0046] FIG. 9 illustrates an exemplary experimental setup, wherein the sunlight is captured using a beam expander into a single-mode fiber via a fiber collimation lens (F280FC-1550), coupled onto an integrated MRR filter, and used with a cryogenically cooled superconducting nanowire single-photon detector (i.e., SNSPD; 4 channels SNSPDs, ID281, ID Quantique) for photon counting. Not shown in this figure is a reference channel that simultaneously detects photons at a close-by wavelength as needed to account for the atmosphere dynamics and variations in the coupling collection.

[0047] FIG. 9 also illustrates an innovative passive spectroscope comprising an external multi-channel single photon detector and a chip integrated filter using an add-drop micro-ring resonator etched on lithium niobate on insulator. In this example, the filter's linewidth is only 6 pm, with its center wavelength rapidly swept by applying a bias voltage to shift the MRR's resonance. To test the system's performance, its operation was first verified with a CO2 gas cell by comparing it to a direct laser transmission measurement. Then, it was applied to measure the spectrum of sunlight passing through the atmosphere, near the absorption line of the CO.sub.2 gas around 1572.02 nm. By differentiating the registered photon counts from those simultaneously recorded in an adjacent bypass channel, the infrared absorption spectrum can be acquired with high resolution and accuracy amid the imperfections in sunlight couplings and dynamic atmospheric conditions without the aid of adaptive optics. A general outline of this process can be seen in FIG. 13.

Example 1 Atmospheric CO2 Absorption Measurement with Solar Photon Counting:

[0048] The MRR filter was first calibrated against the CO2 gas cell (pressure: 740 Torr, path length: 80 cm). The absorption spectrum between 1568 and 1585 nm was measured using the tunable laser, with the result shown in FIG. 14. Out of many absorption peaks, a Lorentzian-shape line at 1572.02 nm was chosen by using a bandpass filter to define a 60.3 nm spectral window. To characterize the MRR, electric current was applied to the Pt micro-heater from an external power supply (GPD-4303S, Instek), whose power is increased from 24 to 38.4 mW at a 0.8 mW interval. It red shifts the filter's center wavelength from 1571.95 to 1572.1 nm at an 8 pm step size. A broadband source from amplified spontaneous emission (ASE) was passed through the gas cell, filtered by the MRR filter, and detected using an optical power meter. The recorded power as the MRR is red-shifted is shown in FIG. 8, which resolves the CO2 absorption around 1572.02 nm. The extinction ratio is 0.83. As a comparison, the laser transmittance through the same optical path is also plotted in the same figure, showing good agreement.

[0049] To measure the atmospheric CO2 absorption, the experimental setup outlined in FIG. 15 can be used. It includes a free-space to fiber beam reducer (Thorlabs-GBE10-C) to collect the solar radiation into a multi-mode fiber (50-μm diameter) that is spliced to a single-mode fiber with 2 to 3 dB loss. A long-pass filter (cut off wavelength: 1200 nm) is used to eliminate sunlight's visible spectrum from saturating the SNSPD. Another bandpass filter is used to further reduce the transmitted solar radiation to a 60.3 nm spectral window centered at 1572.02 nm. After a fiber polarizer, the filtered light is split into two channels with a 99:1 coupler, with 99% of the light coupled into the MRR filter on the LNOI chip and the remaining 1% directly into the reference channel of SNSPD. This configuration allows for the photon count variation to be accounted in real-time due to the fluctuations in the fiber coupling and the dynamic atmosphere attenuation induced by, for example, cloud coverage. The MRR filter's output is fiber coupled into the signal channel of SNSPD to count the photon numbers while the filter is swept from 1571.95 to 1572.1 nm. Finally, a field-programmable-gate-array (FPGA, Zynq-7000, Xilinx) is employed as the central processor for (i) tuning the center wavelength of the MRR filter by varying the electric power on the Pt micro-heater and (ii) acquiring synchronized photon counting data from the two SNSPD channels. Table I lists the parameters of each part of the system.

[0050] The integration time for each photon counting is 125 ms in the example system, rendering the total data acquisition time around 3s. Here, adequate integration time is needed to attain statistical significance by suppressing the Poissonian noise with single-photon counts. This ensures the signal-to-noise ratio (SNR), N.sub.solar/√{square root over (N.sub.solar+N.sub.DC)} to be greater than 145 across all data points. Here, N.sub.solar is the registered photon counts in the signal channel. In contrast, N.sub.DC is the total noise count of the entire detection system, measured by disconnecting the optical fiber from the beam expander. As shown in the table of FIG. 15, N.sub.DC fluctuates between 9.6 and 16 kHz, much higher than the detector dark count rates (300 Hz). This phenomenon is due to the coupling fiber's thermal photons from the MRR and single-photon detector. Hence, further noise reduction is achievable by shortening the connecting optical fiber in a compact experiment setup. A shorter integration time is needed to achieve the same SNR with a lower noise level or reduced insertion loss of the MRR filter. With the thermal-optical tuning, the shortest thermo-optic response time is 60 μs. An even shorter time is possible by using electro-optical tuning.

[0051] To retrieve the atmospheric CO2 absorption spectrum, the normalized photon counts, corrected with the reference channel were plotted, as the MRR resonance is tuned. The result is shown in FIG. 10 along with the calculated dry-air CO2 absorption spectrum from the HITRAN 2016 (high-resolution transmission molecular absorption database, temperature: 296 k, pressure: 1 atm). An excellent agreement is seen, both on the absorption linewidth and depth. Specifically the measured FWHM is about 49 pm, compared with 52 pm as extracted from the HITRAN database. This small discrepancy is within the error caused by the 6 pm resolution of the MRR filter. The depth extinction is 0.26 by fitting the measurement results with a Lorentzian function, which agrees with the HITRAN database value of 0.25, too. With the current ground-based measurement system, the CO2 concentration can be calculated from the differential photon counts proportional to atmospheric CO2 concentration, as

N.sub.diff=N.sub.off−N.sub.on  (1)

With N.sub.off(on) being the off (on) CO2 absorption line photon count. The uncertainty of N.sub.diff due to shot noise is

ΔN.sub.diff=√{square root over (N.sub.off−N.sub.on)}  (2)

which limits the atmospheric CO2 concentration variation detection sensitivity to be ΔN.sub.diff=√{square root over (N.sub.off−N.sub.on)}×412 ppm=1:2 ppm (parts per million), about 30 times better than a typical photon-counting IPDA lidar. Note that the CO2 detection sensitivity of the current system can be improved significantly by reducing the system dark count and coupling efficiency of solar photons.

[0052] Benefiting from the high efficiency and low dark count level of the SNSPD, only a few tens of thousands of photons per spectral point need to be counted for a precise absorption with high SNR. This photon receiving level ensures that the SNSPD operates in the linear counting regime for accurate and bias-free spectroscopic measurement. It also relaxes the requirement for high-end signal collection apparatuses, like a telescope with a wide aperture and adaptive optics for enhanced coupling. Furthermore, a commercial SNSPD with active quenching can already operate in the counting regime up to a few tens of MHz with high linearity Therefore, real-time, dynamic monitoring of the atmospheric CO2 concentration can be realized by using dual MRR filters on a single nanophotonic chip, with one tuned into the absorption line (i.e., on-line) and the other off (i.e., off-line) while recording the ratio of their photon counts using SNSPD. This approach is similar to the IPDA lidar but without restriction in laser wavelength. Moreover, monitoring and spectroscopic measurement of multi-species of gases are possible by leveraging the photonics chip's scalability and broadband nature of solar radiation.

[0053] In this experiment, the free spectral range (FSR) of the MRR filter is about 2 nm. Its small volume allows it to rapidly scan the transmission line at high efficiency. The external long-pass filter is inserted to prevent solar photons from saturating the signal photon-sensitive SNSPD, while the external bandpass filter is employed to eliminate mode-order ambiguity beyond a FSR. In the future, those external filters can potentially be integrated on the same chip, by using array waveguide gratings (AWGs) long-pass filters, and cascaded microring resonators. Also prospective is the integration of SNSPDs on the same chip, for which NbN SNSPDs have been recently demonstrated on thin-film lithium niobate. By these advances, the presently demonstrated technique could lead to impactful applications in multiple areas of remote sensing.

[0054] By single-photon counting and using a narrow-band filter made of an integrated, fast swept MRR on LNOI, a high-resolution measurement of the CO2 absorption spectrum is demonstrated. Thanks to the ultralow system noise, only several tens of thousands of photon counts were needed per spectral point to retrieve the fine line shape of the atmospheric CO2 vibrational-rotational band around 1572.02 nm. These results show that the LNOI-based integrated photonics and single-photon detection combined promise photon-efficient remote sensing. By using photonic lantern for light collection from the telescope into the single-mode devices on LNOI chips with wide transparency window, the present technique may find a breadth of applications in satellite remote sensing, monitoring solar-induced fluorescence, and exoplanet remote sensing.

[0055] Further features, details and embodiments of the present invention can be found in the publication by Jiuyi Zhang et al. entitled “Carbon-Dioxide Absorption Spectroscopy with Solar Photon Counting and Integrated Lithium Niobate Micro-Ring Resonator,” Applied Physics Letters 118 (2021), 171103, the entire contents of which are incorporated herein by reference and made a part of the present Specification for all purposes.

[0056] In some embodiments, instead of measuring solar photons, the disclosed systems and their variants or derivatives are used for remote measurements of Raman spectroscopy, fluorescent spectroscopy, and Brillouin scattering. In those applications, the targets of interest are illuminated by narrowband lasers, and the generated photons at a wavelength shifted from that of the lasers are collected and their spectral properties are measured by using the disclosed systems.

[0057] It will be understood that the embodiments described herein and in the incorporated paper are merely exemplary and that a person skilled in the art may make many variations and modifications without departing from the spirit and scope of the invention. All such variations and modifications are intended to be included within the scope of the invention.