We demonstrate a spectroscope using single-photon counters and a chip-integrated lithium niobate micro-ring filter to measure the atmospheric CO2 absorption spectrum passively. By thermo-optically sweeping the filter over 150 pm and referencing the resulting photon counts to a bypass channel, we sample the absorption spectrum at an ultrahigh-resolution of 6 pm. Incorporating it into a ground-based field system, we characterize the CO2 absorption through the atmosphere by counting the solar photons across the absorption line around 1572.02 nm, which agrees well with its transmission spectrum at standard atmospheric pressure. Our results highlight the potential of adopting integrated photonics and single-photon counting in remote sensing systems for high detection sensitivity, superior resolution, and significantly reduced size, weight, and power.

Spectroscopic measurement and concentration dynamics tracking of atmospheric CO2 gas are essential in many areas of environmental monitoring, atmospheric composition analysis, carbon cycle research, and satellite-based gas remote sensing as they are integral for assessing and search of technological countermeasures on global warming. Thus far, distributed in situ remote sensing stations on the ground and satellite-based observatories provide global measurement of greenhouse gases with a wide terrestrial coverage.1 To this end, active spectroscopic techniques such as differential absorption lidar (DIAL)2 and integrated path differential absorption lidar (IPDA),3,4 despite notable successes, require narrow linewidth lasers of high power or cavity-enhanced optical frequency comb sources with excellent wavelength stability that is locked to a reliable reference.5,6 They are thus less ideal for satellite missions or field deployment due to complicated experimental configurations, considerable operational instability, and restrictions in accessible wavelength range and sensing distance.

In contrast, passive spectroscopic techniques, with sufficient spectral resolution and high detection sensitivity, could empower both ground-based and space-borne atmospheric CO2 column measurements with much less device demands and operation overhead.1,7 In those systems, however, conventional diffractive bulk-optics are subject to a trade-off among the instrument size, throughput, and spectral resolution. They limit the system's end performance for space-borne applications where the device size, weight, and power (SWaP) are at a premium.8 Recently, photonic integrated circuits (PIC) have been gathering pace toward the next-generation optical instrumentation in remote sensing, with substantially reduced SWaP yet boosted cost-effectiveness,8,9 while promising unparalleled performance over a diverse range of functionalities. Nevertheless, the high performance of PIC devices usually relies on diffraction-limited waveguiding in the fundamental mode. Coupling inherently multimode sunlight into such devices is challenging,10,11 usually requires adaptive optics to match the intensity distribution and phase front of the incident beam to the waveguide.12 

This paper reports a passive spectroscope consisting of an external multi-channel single-photon detector and a chip-integrated filter using an add-drop microring resonator (MRR) etched on lithium niobate on insulator (LNOI). LNOI is a rapid progressing platform for integrated photonics propelled by its excellent optical properties on many aspects. Notably, its outstanding electro-optic and efficient thermo-optic effects, wide transparent window ranging from ultraviolet (UV) to mid-infrared (mid-IR), and low propagation losses are ideal for the development of tunable narrow-linewidth MRR filters for sensing of various gas species.13–15 In this demonstration, our 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, we first verify its operation with a CO2 gas cell by comparing it to a direct laser transmission measurement. Then, we apply it to measure the spectrum of sunlight passing through the atmosphere, near the absorption line of the CO2 gas around 1572.02 nm. By differentiating the registered photon counts with those simultaneously recorded in an adjacent bypass channel, we are able to acquire the infrared absorption spectrum with high resolution and accuracy amid the imperfections in sunlight couplings and dynamic atmospheric conditions without the aid of adaptive optics. With fast scanning, single-photon sensitive, high resolution, and ultralow detection noise, the present spectroscopy can prove useful for pervasive deployments in satellite, airborne, and wide-field missions.

Figure 1 illustrates our experiment, where we capture the sunlight using a beam expander into a single-mode fiber via a fiber collimation lens (F280FC-1550), couple it onto an integrated MRR filter, and use a cryogenically cooled superconducting nanowire single-photon detector (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. The MRR is an add-drop filter integrated on chip with silica cladding from a commercial 600-nm thin film of X-cut LiNbO3, as shown in Fig. 2(a). 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 narrow bandwidth. Two pulley bus waveguides are etched on the two sides of MRR for add and drop. To excite only the fundamental TE mode in the MRR while suppressing all other guided modes, the top widths of the MRR and waveguides are 1.5 and 1.0 μm,16 respectively, designed so to fulfill the phase-matching condition for the mode coupling.

FIG. 1.

Illustration of the atmospheric CO2 absorption spectroscopy with solar photon counting and LNOI MRR filter.

FIG. 1.

Illustration of the atmospheric CO2 absorption spectroscopy with solar photon counting and LNOI MRR filter.

Close modal
FIG. 2.

(a) Microscope image of the MRR with a platinum micro-heater. (b) Photo showing the fiber-packaged LNOI chip connected to electric probes. (c) The measured transmission spectrum of the MRR filter and CO2 gas cell absorption spectrum around 1572 nm. (d) The MRR filter transmission spectrum red shifted by the applied electric power. (e) The thermo-optical calibration data and their linear fitting. (f) The extended gas cell absorption spectrum from 1568 to 1585 nm.

FIG. 2.

(a) Microscope image of the MRR with a platinum micro-heater. (b) Photo showing the fiber-packaged LNOI chip connected to electric probes. (c) The measured transmission spectrum of the MRR filter and CO2 gas cell absorption spectrum around 1572 nm. (d) The MRR filter transmission spectrum red shifted by the applied electric power. (e) The thermo-optical calibration data and their linear fitting. (f) The extended gas cell absorption spectrum from 1568 to 1585 nm.

Close modal

Thermal induced change in effective refractive index of a mode has been used as a resonance frequency tuning method in micro-cavities. An integrated platinum micro-heater with 10-nm Ti and 100-nm Pt is deposited on the top of the silica cladding for thermal-optical (TO) tuning [see Fig. 2(a)]. Balancing the on-chip mode coupling efficiency, narrow filtering bandwidth, thermal-optical stability, and high extinction ratio of the add-drop MRR, we use over-coupled fundamental TE mode case in our design. The end device has a transmittance spectrum of 6 pm bandwidth (full-width half-maximum, FWHM) and 30 dB extinction ratio around 1572.02 nm, which is the targeted CO2 absorption line to minimize the interference from water vapor lines in atmosphere.17 This wavelength choice has been demonstrated in air-borne atmospheric CO2 remote sensing,3,18,19 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 CO2.18,19 To characterize our device's tunability, we use a current source and a pair of electric probes to supply electric power on the micro-heater via the on-chip Pt electrode pads, as shown in Fig. 2(b). Finally, we package the LNOI chip using UV light exposed epoxy with a high numerical-aperture (NA = 0.4) fiber to stabilize the on-off chip coupling.

The MRR filter calibration is performed using a narrowband, tunable laser (Santec, TSL-550). The result is plotted in Fig. 2(c), 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 pm. These data verify that the filter has adequately narrow bandwidth to measure the CO2 absorption with high resolution by thermal-optical tuning.

The filter center wavelength is shifted by applying current to change the local temperature of the MRR. Figure 2(d) shows the MRR resonance as a function of the applied electric power, ranging from 0 to 40 mW. It results in a red shift of 0.45 nm in the transmission spectrum. Figure 2(e) shows a linear relationship between the shift and electric power, with a slope of 8.75 pm/mW. This good linearity is the key to the proposed spectroscopic measurement. The tuning efficiency can be significantly improved to 53.7 pm/mW by etching tranches20 for improved thermal isolation around the MRR.

We first calibrate the MRR filter against the CO2 gas cell (pressure: 740 Torr, path length: 80 cm). The absorption spectrum between 1568 and 1585 nm is measured using the tunable laser, with the result shown in Fig. 2(f). Out of many absorption peaks, we pick a Lorentzian-shape line at 1572.02 nm by using a bandpass filter to define a ±0.3 nm spectral window. To characterize the MRR, electric current is 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 1571.95to 1572.1 nm at an 8 pm step size. A broadband source from amplified spontaneous emission (ASE) is 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. 3, 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.

FIG. 3.

Absorption spectrum of the CO2 gas cell measured using the MRR filter and by laser transmission, respectively.

FIG. 3.

Absorption spectrum of the CO2 gas cell measured using the MRR filter and by laser transmission, respectively.

Close modal

To measure the atmospheric CO2 absorption, we use the experimental setup outlined in Fig. 4. It consists of 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 ±0.3 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 us to account for the photon count variation 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.

FIG. 4.

The experimental setup for atmospheric CO2 absorption measurement with solar photon counting.

FIG. 4.

The experimental setup for atmospheric CO2 absorption measurement with solar photon counting.

Close modal
TABLE I.

A glance of the system parameters.

ParameterValue
Central wavelength 1572.02 nm 
Spectral sampling number 20 
Sampling step size 8pm(971.2MHz) 
Optical receiver diameter 2.5cm 
Filter spectral width (FWHM) 6pm(728.4MHz) 
Photon counting integration time 125 ms 
Atmospheric CO2 detection sensitivity 1.2ppm 
SNSPD quantum efficiency 85% 
SNSPD dark counts (signal and reference) 300Hz 
SNSPD counting rate without solar signal 9.6–16 (kHz) 
SNSPD counting rate per sample (signal) 112–256 (kHz) 
SNSPD counting rate per sample (reference) 1.07–1.12 (MHz) 
ParameterValue
Central wavelength 1572.02 nm 
Spectral sampling number 20 
Sampling step size 8pm(971.2MHz) 
Optical receiver diameter 2.5cm 
Filter spectral width (FWHM) 6pm(728.4MHz) 
Photon counting integration time 125 ms 
Atmospheric CO2 detection sensitivity 1.2ppm 
SNSPD quantum efficiency 85% 
SNSPD dark counts (signal and reference) 300Hz 
SNSPD counting rate without solar signal 9.6–16 (kHz) 
SNSPD counting rate per sample (signal) 112–256 (kHz) 
SNSPD counting rate per sample (reference) 1.07–1.12 (MHz) 

The integration time for each photon counting is 125 ms in the current system, rendering the total data acquisition time around 3 s. Here, adequate integration time is crucial to attain statistical significance by suppressing the Poissonian noise with single-photon counts. This ensures the signal-to-noise ratio (SNR), NSolar/NSolar+NDC, to be greater than 145 across all data points.21 Here, NSolar is the registered photon counts in the signal channel. In contrast, NDC is the total noise count of the entire detection system, measured by disconnecting the optical fiber from the beam expander. As shown in Table I, NDC 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.20 An even shorter time is possible by using electro-optical tuning.

To retrieve the atmospheric CO2 absorption spectrum, we plot the normalized photon counts, corrected with the reference channel, as the MRR resonance is tuned. The result is shown in Fig. 5 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).22,23 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

Ndiff=NoffNon,
(1)

with Noff(on) being the off (on) CO2 absorption line photon count. The uncertainty of Ndiff due to shot noise is

ΔNdif=Noff+Non,
(2)

which limits the atmospheric CO2 concentration variation detection sensitivity to be ΔNdiff/Ndiff×412ppm=1.2ppm (parts per million), about 30 times better than a typical photon-counting IPDA lidar.24 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.21 

FIG. 5.

The solar photon counts through the atmosphere around the carbon dioxide absorption spectrum line at 1572.02 nm and the simulated transmittance using HITRAN.

FIG. 5.

The solar photon counts through the atmosphere around the carbon dioxide absorption spectrum line at 1572.02 nm and the simulated transmittance using HITRAN.

Close modal

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 regime25 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.26 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.

In this experiment, the free spectral range (FSR) of our MRR filter is about 2 nm. Its small volume allows to rapidly scan the transmission line at high efficiency.20 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),27 long-pass filters,28 and cascaded micro-ring resonators.29 Also prospective is the integration of SNSPDs on the same chip, for which NbN SNSPDs have been recently demonstrated on thin-film lithium niobate.30,31 By these advances, the presently demonstrated technique could lead to impactful applications in multiple areas of remote sensing.

By single-photon counting and using a narrow-band filter made of an integrated, fast swept MRR on LNOI, we have demonstrated a high-resolution measurement of the CO2 absorption spectrum. 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. Our results show that the LNOI-based integrated photonics and single-photon detection combined promise photon-efficient remote sensing. By using photonic lantern32 for light collection from the telescope into the single-mode devices on LNOI chips with wide transparency window, we envision this technique to find a breadth of applications in satellite remote sensing, monitoring solar-induced fluorescence,33 and exoplanet remote sensing.34 

See the supplementary material for device fabrication processes.

This study was supported by the NASA Langley Research Center's IRAD program grant (No. 80NSSC19K1618). We acknowledge the ASRC nanofabrication facility at CUNY for device fabrication.

The data that support the findings of this study are available from the corresponding authors upon reasonable request.

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